# verilog-to-routing/vtr-verilog-to-routing

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# Architecture Reference

This section provides a detailed reference for the FPGA Architecture description used by VTR. The Architecture description uses XML as its representation format.

As a convention, curly brackets { } represents an option with each option separated by |. For example, a={1 | 2 | open} means field a can take a value of 1, 2, or open.

## Top Level Tags

The first tag in all architecture files is the <architecture> tag. This tag contains all other tags in the architecture file. The architecture tag contains the following tags:

• <models>
• <layout>
• <device>
• <switchlist>
• <segmentlist>
• <directlist>
• <complexblocklist>

## Recognized BLIF Models (<models>)

The <models> tag contains <model name="string"> tags. Each <model> tag describes the BLIF .subckt model names that are accepted by the FPGA architecture. The name of the model must match the corresponding name of the BLIF model.

Note

Standard blif structures (.names, .latch, .input, .output) are accepted by default, so these models should not be described in the <models> tag.

Each model tag must contain 2 tags: <input_ports> and <output_ports>. Each of these contains <port> tags:

.. arch:tag:: <port name="string" is_clock="{0 | 1} clock="string" combinational_sink_ports="string1 string2 ..."/>

:req_param name: The port name.
:opt_param is_clock: Indicates if the port is a clock. Default: 0
:opt_param clock: Indicates the port is sequential and controlled by the specified clock (which must be another port on the model marked with is_clock=1). Default: port is treated as combinational (if unspecified)
:opt_param combinational_sink_ports: A space-separated list of output ports which are combinationally connected to the current input port. Default: No combinational connections (if unspecified)

Defines the port for a model.



An example models section containing a combinational primitive adder and a sequential primitive single_port_ram follows:

<models>
<model name="single_port_ram">
<input_ports>
<port name="we" clock="clk" />
<port name="data" clock="clk" combinational_sink_ports="out"/>
<port name="clk" is_clock="1"/>
</input_ports>
<output_ports>
<port name="out" clock="clk"/>
</output_ports>
</model>

<input_ports>
<port name="a" combinational_sink_ports="cout sumout"/>
<port name="b" combinational_sink_ports="cout sumout"/>
<port name="cin" combinational_sink_ports="cout sumout"/>
</input_ports>
<output_ports>
<port name="cout"/>
<port name="sumout"/>
</output_ports>
</model>
</models>

Note that for single_port_ram above, the ports we, addr, data, and out are sequential since they have a clock specified. Additionally addr and data are shown to be combinationally connected to out; this corresponds to an internal timing path between the addr and data input registers, and the out output registers.

For the adder the input ports a, b and cin are each combinationally connected to the output ports cout and sumout (the adder is a purely combinational primitive).

.. seealso:: For more examples of primitive timing modeling specifications see the :ref:arch_model_timing_tutorial



## Global FPGA Information

.. arch:tag:: <layout/>

Content inside this tag specifies device grid layout.

.. seealso:: :ref:arch_grid_layout


.. arch:tag:: <device>content</device>

Content inside this tag specifies device information.

.. seealso:: :ref:arch_device_info


.. arch:tag:: <switchlist>content</switchlist>

Content inside this tag contains a group of <switch> tags that specify the types of switches and their properties.


.. arch:tag:: <segmentlist>content</segmentlist>

Content inside this tag contains a group of <segment> tags that specify the types of wire segments and their properties.


.. arch:tag:: <complexblocklist>content</complexblocklist>

Content inside this tag contains a group of <pb_type> tags that specify the types of functional blocks and their properties.



## FPGA Grid Layout

The valid tags within the <layout> tag are:

.. arch:tag:: <auto_layout aspect_ratio="float">

:opt_param aspect_ratio:
The device grid's target aspect ratio (:math:width / height)

**Default**: 1.0

Defines a scalable device grid layout which can be automatically scaled to a desired size.

.. note:: At most one <auto_layout> can be specified.


.. arch:tag:: <fixed_layout name="string" width="int" height="int">

:req_param name:
The unique name identifying this device grid layout.

:req_param width:
The device grid width

:req_param height:
The device grid height

Defines a device grid layout with fixed dimensions.

.. note:: Multiple <fixed_layout> tags can be specified.



Each <auto_layout> or <fixed_layout> tag should contain a set of grid location tags.

### Grid Location Priorities

Each grid location specification has an associated numeric priority. Larger priority location specifications override those with lower priority.

Note

If a grid block is partially overlapped by another block with higher priority the entire lower priority block is removed from the grid.

### Empty Grid Locations

Empty grid locations can be specified using the special block type EMPTY.

Note

All grid locations default to EMPTY unless otherwise specified.

### Grid Location Expressions

Some grid location tags have attributes (e.g. startx) which take an expression as their argument. An expression can be an integer constant, or simple mathematical formula evaluated when constructing the device grid.

Supported operators include: +, -, *, /, along with ( and ) to override the default evaluation order. Expressions may contain numeric constants (e.g. 7) and the following special variables:

• W: The width of the device
• H: The height of the device
• w: The width of the current block type
• h: The height of the current block type

Warning

All expressions are evaluated as integers, so operations such as division may have their result truncated.

As an example consider the expression W/2 - w/2. For a device width of 10 and a block type of width 3, this would be evaluated as \lfloor \frac{W}{2} \rfloor - \lfloor \frac{w}{2} \rfloor = \lfloor \frac{10}{2} \rfloor - \lfloor \frac{3}{2} \rfloor = 5 - 1 = 4.

### Grid Location Tags

.. arch:tag:: <fill type="string" priority="int"/>

:req_param type:
The name of the top-level complex block type (i.e. <pb_type>) being specified.

:req_param priority:
The priority of this layout specification.
Tags with higher priority override those with lower priority.

Fills the device grid with the specified block type.

Example:

.. code-block:: xml

<!-- Fill the device with CLB blocks -->
<fill type="CLB" priority="1"/>

.. figure:: fill_fpga_grid.*

<fill> CLB example


.. arch:tag:: <perimeter type="string" priority="int"/>

:req_param type:
The name of the top-level complex block type (i.e. <pb_type>) being specified.

:req_param priority:
The priority of this layout specification.
Tags with higher priority override those with lower priority.

Sets the perimeter of the device (i.e. edges) to the specified block type.

.. note:: The perimeter includes the corners

Example:

.. code-block:: xml

<!-- Create io blocks around the device perimeter -->
<perimeter type="io" priority="10"/>

.. figure:: perimeter_fpga_grid.*

<perimeter> io example


.. arch:tag:: <corners type="string" priority="int"/>

:req_param type:
The name of the top-level complex block type (i.e. <pb_type>) being specified.

:req_param priority:
The priority of this layout specification.
Tags with higher priority override those with lower priority.

Sets the corners of the device to the specified block type.

Example:

.. code-block:: xml

<!-- Create PLL blocks at all corners -->
<corners type="PLL" priority="20"/>

.. figure:: corners_fpga_grid.*

<corners> PLL example


.. arch:tag:: <single type="string" priority="int" x="expr" y="expr"/>

:req_param type:
The name of the top-level complex block type (i.e. <pb_type>) being specified.

:req_param priority:
The priority of this layout specification.
Tags with higher priority override those with lower priority.

:req_param x:
The horizontal position of the block type instance.

:req_param y:
The vertical position of the block type instance.

Specifies a single instance of the block type at a single grid location.

Example:

.. code-block:: xml

<!-- Create a single instance of a PCIE block (width 3, height 5)
at location (1,1)-->
<single type="PCIE" x="1" y="1" priority="20"/>

.. figure:: single_fpga_grid.*

<single> PCIE example


.. arch:tag:: <col type="string" priority="int" startx="expr" repeatx="expr" starty="expr" incry="expr"/>

:req_param type:
The name of the top-level complex block type (i.e. <pb_type>) being specified.

:req_param priority:
The priority of this layout specification.
Tags with higher priority override those with lower priority.

:req_param startx:
An expression specifying the horizontal starting position of the column.

:opt_param repeatx:
An expression specifying the horizontal repeat factor of the column.

:opt_param starty:
An expression specifying the vertical starting offset of the column.

**Default:** 0

:opt_param incry:
An expression specifying the vertical increment between block instantiations within the region.

**Default:** h

Creates a column of the specified block type at startx.

If repeatx is specified the column will be repeated wherever :math:x = startx + k \cdot repeatx, is satisfied for any positive integer :math:k.

A non-zero starty is typically used if a <perimeter> tag is specified to adjust the starting position of blocks with height > 1.

Example:

.. code-block:: xml

<!-- Create a column of RAMs starting at column 2, and
repeating every 3 columns -->
<col type="RAM" startx="2" repeatx="3" priority="3"/>

.. figure:: col_fpga_grid.*

<col> RAM example

Example:

.. code-block:: xml

<!-- Create IO's around the device perimeter -->
<perimeter type="io" priority=10"/>

<!-- Create a column of RAMs starting at column 2, and
repeating every 3 columns. Note that a vertical offset
of 1 is needed to avoid overlapping the IOs-->
<col type="RAM" startx="2" repeatx="3" starty="1" priority="3"/>

.. figure:: col_perim_fpga_grid.*

<col> RAM and <perimeter> io example


.. arch:tag:: <row type="string" priority="int" starty="expr" repeaty="expr" startx="expr"/>

:req_param type:
The name of the top-level complex block type (i.e. <pb_type>) being specified.

:req_param priority:
The priority of this layout specification.
Tags with higher priority override those with lower priority.

:req_param starty:
An expression specifying the vertical starting position of the row.

:opt_param repeaty:
An expression specifying the vertical repeat factor of the row.

:opt_param startx:
An expression specifying the horizontal starting offset of the row.

**Default:** 0

:opt_param incry:
An expression specifying the horizontal increment between block instantiations within the region.

**Default:** w

Creates a row of the specified block type at starty.

If repeaty is specified the column will be repeated wherever :math:y = starty + k \cdot repeaty, is satisfied for any positive integer :math:k.

A non-zero startx is typically used if a <perimeter> tag is specified to adjust the starting position of blocks with width > 1.

Example:

.. code-block:: xml

<!-- Create a row of DSPs (width 1, height 3) at
row 1 and repeating every 7th row -->
<row type="DSP" starty="1" repeaty="7" priority="3"/>

.. figure:: row_fpga_grid.*

<row> DSP example


.. arch:tag:: <region type="string" priority="int" startx="expr" endx="expr repeatx="expr" incrx="expr" starty="expr" endy="expr" repeaty="expr" incry="expr"/>

:req_param type:
The name of the top-level complex block type (i.e. <pb_type>) being specified.

:req_param priority:
The priority of this layout specification.
Tags with higher priority override those with lower priority.

:opt_param startx:
An expression specifying the horizontal starting position of the region (inclusive).

**Default:** 0

:opt_param endx:
An expression specifying the horizontal ending position of the region (inclusive).

**Default:** W - 1

:opt_param repeatx:
An expression specifying the horizontal repeat factor of the column.

:opt_param incrx:
An expression specifying the horizontal increment between block instantiations within the region.

**Default:** w

:opt_param starty:
An expression specifying the vertical starting position of the region (inclusive).

**Default:** 0

:opt_param endy:
An expression specifying the vertical ending position of the region (inclusive).

**Default:** H - 1

:opt_param repeaty:
An expression specifying the vertical repeat factor of the column.

:opt_param incry:
An expression specifying the horizontal increment between block instantiations within the region.

**Default:** h

Fills the rectangular region defined by (startx, starty) and (endx, endy) with the specified block type.

.. note:: endx and endy are included in the region

If repeatx is specified the region will be repeated wherever :math:x = startx + k_1*repeatx, is satisified for any positive integer :math:k_1.

If repeaty is specified the region will be repeated wherever :math:y = starty + k_2*repeaty, is satisified for any positive integer :math:k_2.

Example:

.. code-block:: xml

<!-- Fill RAMs withing the rectangular region bounded by (1,1) and (5,4) -->
<region type="RAM" startx="1" endx="5" starty="1" endy="4" priority="4"/>

.. figure:: region_single_fpga_grid.*

<region> RAM example

Example:

.. code-block:: xml

<!-- Create RAMs every 2nd column withing the rectangular region bounded
by (1,1) and (5,4) -->
<region type="RAM" startx="1" endx="5" starty="1" endy="4" incrx="2" priority="4"/>

.. figure:: region_incr_fpga_grid.*

<region> RAM increment example

Example:

.. code-block:: xml

<!-- Fill RAMs within a rectangular 2x4 region and repeat every 3 horizontal
and 5 vertical units -->
<region type="RAM" startx="1" endx="2" starty="1" endy="4" repeatx="3" repeaty="5" priority="4"/>

.. figure:: region_repeat_fpga_grid.*

<region> RAM repeat example

Example:

.. code-block:: xml

<!-- Create a 3x3 mesh of NoC routers (width 2, height 2) whose relative positions
will scale with the device dimensions -->
<region type="NoC" startx="W/4 - w/2" starty="W/4 - w/2" incrx="W/4" incry="W/4" priority="3"/>

.. figure:: region_incr_mesh_fpga_grid.*

<region> NoC mesh example



### Grid Layout Example

<layout>
<!-- Specifies an auto-scaling square FPGA floorplan -->
<auto_layout aspect_ratio="1.0">
<!-- Create I/Os around the device perimeter -->
<perimeter type="io" priority=10"/>

<!-- Nothing in the corners -->
<corners type="EMPTY" priority="100"/>

<!-- Create a column of RAMs starting at column 2, and
repeating every 3 columns. Note that a vertical offset (starty)
of 1 is needed to avoid overlapping the IOs-->
<col type="RAM" startx="2" repeatx="3" starty="1" priority="3"/>

<!-- Create a single PCIE block along the bottom, overriding
I/O and RAM slots -->
<single type="PCIE" x="3" y="0" priority="20"/>

<!-- Create an additional row of I/Os just above the PCIE,
which will not override RAMs -->
<row type="io" starty="5" priority="2"/>

<!-- Fill remaining with CLBs -->
<fill type="CLB" priority="1"/>
</auto_layout>
</layout>

Example FPGA grid

## FPGA Device Information

The tags within the <device> tag are:

.. arch:tag:: <sizing R_minW_nmos="float" R_minW_pmos="float"/>

:req_param R_minW_nmos:
The resistance of minimum-width nmos transistor.
This data is used only by the area model built into VPR.

:req_param R_minW_pmos:
The resistance of minimum-width pmos transistor.
This data is used only by the area model built into VPR.

:required: Yes

Specifies parameters used by the area model built into VPR.


.. arch:tag:: <connection_block input_switch_name="string"/>

.. figure:: ipin_diagram.*

Input Pin Diagram.

:req_param switch_name:
Specifies the name of the <switch> in the <switchlist> used to connect routing tracks to block input pins (i.e. the input connection block switch).

:required: Yes


.. arch:tag:: <area grid_logic_tile_area="float"/>

:required: Yes

Specifies the default area used by each 1x1 grid logic tile (in :term:MWTAs<MWTA>), *excluding routing*.

Used for an area estimate of the amount of area taken by all the functional blocks.

.. note:: This value can be overriden for specific <pb_type>s with the area attribute.


.. arch:tag:: <switch_block type="{wilton | subset | universal | custom}" fs="int"/>

:req_param type: The type of switch block to use.
:req_param fs: The value of :math:F_s

:required: Yes

This parameter controls the pattern of switches used to connect the (inter-cluster) routing segments. Three fairly simple patterns can be specified with a single keyword each, or more complex custom patterns can be specified.

**Non-Custom Switch Blocks:**

When using bidirectional segments, all the switch blocks have :math:F_s = 3 :cite:brown_fpgas.
That is, whenever horizontal and vertical channels intersect, each wire segment can connect to three other wire segments.
The exact topology of which wire segment connects to which can be one of three choices.
The subset switch box is the planar or domain-based switch box used in the Xilinx 4000 FPGAs -- a wire segment in track 0 can only connect to other wire segments in track 0 and so on.
The wilton switch box is described in :cite:wilton_phd, while the universal switch box is described in :cite:chang_universal_switch_modules.
To see the topology of a switch box, simply hit the "Toggle RR" button when a completed routing is on screen in VPR.
In general the wilton switch box is the best of these three topologies and leads to the most routable FPGAs.

When using unidirectional segments, one can specify an :math:F_s that is any multiple of 3.
We use a modified wilton switch block pattern regardless of the specified switch_block_type.
For all segments that start/end at that switch block, we follow the wilton switch block pattern.
For segments that pass through the switch block that can also turn there, we cannot use the wilton pattern because a unidirectional segment cannot be driven at an intermediate point, so we assign connections to starting segments following a round robin scheme (to balance mux size).

.. note:: The round robin scheme is not tileable.

**Custom Switch Blocks:**

Specifying custom allows custom switch blocks to be described under the <switchblocklist> XML node, the format for which is described in :ref:custom_switch_blocks.
If the switch block is specified as custom, the fs field does not have to be specified, and will be ignored if present.


.. arch:tag:: <chan_width_distr>content</chan_width_distr>

Content inside this tag is only used when VPR is in global routing mode.
The contents of this tag are described in :ref:global_routing_info.


.. arch:tag:: <default_fc in_type="{frac|abs}" in_val="{int|float}" out_type="{frac|abs}" out_val="{int|float}"/>

This defines the default Fc specification, if it is not specified within a <fc> tag inside a top-level complex block.
The attributes have the same meaning as the :ref:\<fc\> tag attributes <arch_fc>.



## Switches

The tags within the <switchlist> tag specifies the switches used to connect wires and pins together.

.. arch:tag::
<switch type="{mux|tristate|pass_gate|short|buffer}" name="string" R="float" Cin="float" Cout="float" Tdel="float" buf_size="{auto|float}" mux_trans_size="float", power_buf_size="int"/>

Describes a switch in the routing architecture.

**Example:**

.. code-block:: xml

<switch type="mux" name="my_awsome_mux" R="551" Cin=".77e-15" Cout="4e-15" Tdel="58e-12" mux_trans_size="2.630740" buf_size="27.645901"/>

:req_param type:

The type of switch:

* mux: An isolating, configurable multiplexer

* tristate: An isolating, configurable tristate-able buffer

* pass_gate: A *non-isolating*, configurable pass gate

* short: A *non-isolating*, *non-configurable* electrical short (e.g. between two segments).

* buffer: An isolating, *non-configurable* non-tristate-able buffer (e.g. in-line along a segment).

**Isolation**

Isolating switches include a buffer which partition their input and output into separate DC-connected sub-circuits.
This helps reduce RC wire delays.

*Non-isolating* switch do **not** isolate their input and output, which can increase RC wire delays.

**Configurablity**

Configurable switches can be turned on/off at configuration time.

*Non-configurable* switches can **not** be controlled at configuration time.
These are typically used to model non-optional connections such as electrical shorts and in-line buffers.

:req_param name: A unique name identifying the switch
:req_param R: Resistance of the switch.
:req_param Cin:  Input capacitance of the switch.
:req_param Cout:  Output capacitance of the switch.

:opt_param Tdel:

Intrinsic delay through the switch.
If this switch was driven by a zero resistance source, and drove a zero capacitance load, its delay would be: :math:T_{del} + R \cdot C_{out}.

The ‘switch’ includes both the mux and buffer mux type switches.

.. note:: Required if no <Tdel> tags are specified

.. note:: A <switch>'s resistance (R) and output capacitance (Cout) have no effect on delay when used for the input connection block, since VPR does not model the resistance/capacitance of block internal wires.

:opt_param buf_size:

Specifies the buffer size in minimum-width transistor area (:termMWTA) units.

If set to auto, sized automatically from the R value.
This allows you to use timing models without R’s and C’s and still be able to measure area.

.. note:: Required for all **isolating** switch types.

**Default:** auto

:opt_param mux_trans_size:
Specifies the size (in minimum width transistors) of each transistor in the two-level mux used by mux type switches.

.. note:: Valid only for mux type switches.

:opt_param power_buf_size: *Used for power estimation.* The size is the drive strength of the buffer, relative to a minimum-sized inverter.

.. arch:tag:: <Tdel num_inputs="int" delay="float"/>

Instead of specifying a single Tdel value, a list of Tdel values may be specified for different values of switch fan-in.
Delay is linearly extrapolated/interpolated for any unspecified fanins based on the two closest fanins.

:req_param num_inputs: The number of switch inputs (fan-in)
:req_param delay: The intrinsic switch delay when the switch topology has the specified number of switch inputs

**Example:**

.. code-block:: xml

<switch type="mux" name="my_mux" R="522" Cin="3.1e-15" Cout="3e-15" mux_trans_size="1.7" buf_size="23">
<Tdel num_inputs="12" delay="8.00e-11"/>
<Tdel num_inputs="15" delay="8.4e-11"/>
<Tdel num_inputs="20" delay="9.4e-11"/>
</switch>



### Global Routing Information

If global routing is to be performed, channels in different directions and in different parts of the FPGA can be set to different relative widths. This is specified in the content within the <chan_width_distr> tag.

Note

If detailed routing is to be performed, all the channels in the FPGA must have the same width.

.. arch:tag:: <x distr="{gaussian|uniform|pulse|delta}" peak="float" width=" float" xpeak=" float" dc=" float"/>

:req_param distr: The channel width distribution function
:req_param peak: The peak value of the distribution
:opt_param width: The width of the distribution. Required for pulse and gaussian.
:opt_param xpeak: Peak location horizontally. Required for pulse, gaussian and delta.
:opt_param dc: The DC level of the distribution. Required for pulse, gaussian and delta.

Sets the distribution of tracks for the x-directed channels -- the channels that run horizontally.

Most values are from 0 to 1.

If uniform is specified, you simply specify one argument, peak.
This value (by convention between 0 and 1) sets the width of the x-directed core channels relative to the y-directed channels and the channels between the pads and core.
:numref:fig_arch_channel_distribution should clarify the specification of uniform (dashed line) and pulse (solid line) channel widths.
The gaussian keyword takes the same four parameters as the pulse keyword, and they are all interpreted in exactly the same manner except that in the gaussian case width is the standard deviation of the function.

.. _fig_arch_channel_distribution:

.. figure:: channel_distribution.*

Channel Distribution

The delta function is used to specify a channel width distribution in which all the channels have the same width except one.
The syntax is chan_width_x delta peak xpeak dc.
Peak is the extra width of the single wide channel.
Xpeak is between 0 and 1 and specifies the location within the FPGA of the extra-wide channel -- it is the fractional distance across the FPGA at which this extra-wide channel lies.
Finally, dc specifies the width of all the other channels.
For example, the statement chan_width_x delta 3 0.5 1 specifies that the horizontal channel in the middle of the FPGA is four times as wide as the other channels.

Examples::

<x distr="uniform" peak="1"/>
<x distr="gaussian" width="0.5" peak="0.8" xpeak="0.6" dc="0.2"/>


.. arch:tag:: <y distr="{gaussian|uniform|pulse|delta}" peak=" float" width=" float" xpeak=" float" dc=" float"/>

Sets the distribution of tracks for the y-directed channels.

.. seealso:: <x distr>



## Complex Blocks

.. seealso:: For a step-by-step walkthrough on building a complex block see :ref:arch_tutorial.



The content within the <complexblocklist> describes the complex blocks found within the FPGA. Each type of complex block is specified with a top-level <pb_type> tag within the <complexblocklist> tag.

### PB Type

.. arch:tag:: <pb_type name="string" num_pb="int" blif_model="string" capacity="int" width="int" height="int" area="float" class="{lut|filpflop|memory}"/>

Specifies a top-level complex block, or a complex block's internal components (sub-blocks).
Which attributes are applicable depends on where the <pb_type> tag falls within the hierarchy:

* Top Level: A child of the <complexblocklist>
* Intermediate: A child of another <pb_type>
* Primitive/Leaf: Contains no <pb_type> children

For example:

.. code-block:: xml

<complexblocklist>
<pb_type name="CLB"/> <!-- Top level -->
...
<pb_type name="ble"/> <!-- Intermediate -->
...
<pb_type name="lut"/> <!-- Primitive -->
...
</pb_type>
<pb_type name="ff"/> <!-- Primitive -->
...
</pb_type>
...
</pb_type>
...
</pb_type>
...
</complexblocklist>

.. note: Intermediate pb_types can contain other intermediate or primitive pb_types so arbitrary hierarchies can be specified.

**General:**

:req_param name: The name of this pb_type.

The name must be unique with respect to any parent, sibling, or child <pb_type>.

**Top Level Only:**

:opt_param capacity: The number of instances of this block type at each grid location

**Default:** 1

For example:

.. code-block:: xml

<pb_type name="IO" capacity="2"/>
...
</pb_type>

specifies there are two instances of the block type IO at each of its grid locations.

:opt_param width: The width of the block type in grid tiles

**Default:** 1

:opt_param height: The height of the block type in grid tiles

**Default:** 1

:opt_param area: The logic area (in :term:MWTA) of the block type

**Default:** from the <area> tag

**Intermediate or Primitive:**

:opt_param num_pb: The number of instances of this pb_type at the current hierarchy level.

**Default:** 1

For example:

.. code-block:: xml

<pb_type name="CLB">
...
<pb_type name="ble" num_pb="10"/>
...
</pb_type>
...
</pb_type>

would specify that the pb_type CLB contains 10 instances of the ble pb_type.

**Primitive Only:**

:req_param blif_model: Specifies the netlist primitive which can be implemented by this pb_type.

Accepted values:

* .input: A BLIF netlist input

* .output: A BLIF netlist output

* .names: A BLIF .names (LUT) primitive

* .latch: A BLIF .latch (DFF) primitive

* .subckt <custom_type>: A user defined black-box primitive.

For example:

.. code-block:: xml

...
</pb_type>

would specify that the pb_type my_adder can implement a black-box BLIF primitive named adder.

.. note:: The input/output/clock ports for primitive pb_types must match the ports specified in the <models> section.

:opt_param class: Specifies that this primitive is of a specialized type which should be treated specially.

.. seealso:: :ref:arch_classes for more details.



The following tags are common to all <pb_type> tags:

.. arch:tag:: <input name="string" num_pins="int" equivalent="{none|full}" is_non_clock_global="{true|false}"/>

Defines an input port.
Multple input ports are described using multiple <input> tags.

:req_param name: Name of the input port.
:req_param num_pins: Number of pins the input port has.

:opt_param equivalent:

.. note:: Applies only to top-level pb_type.

Describes if the pins of the port are logically equivalent.
Input logical equivalence means that the pin order can be swapped without changing functionality.
For example, an AND gate has logically equivalent inputs because you can swap the order of the inputs and it’s still correct; an adder, on the otherhand, is not logically equivalent because if you swap the MSB with the LSB, the results are completely wrong.
LUTs are also considered logically equivalent since the logic function (LUT mask) can be rotated to account for pin swapping.

* none: No input pins are logically equivalent.

Input pins can not be swapped by the router. (Generates a unique SINK rr-node for each block input port pin.)

* full: All input pins are considered logically equivalent (e.g. due to logical equivalance or a full-crossbar within the cluster).

All input pins can be swapped without limitation by the router. (Generates a single SINK rr-node shared by each input port pin.)

**default:** none

:opt_param is_non_clock_global:

.. note:: Applies only to top-level pb_type.

Describes if this input pin is a global signal that is not a clock.
Very useful for signals such as FPGA-wide asynchronous resets.
These signals have their own dedicated routing channels and so should not use the general interconnect fabric on the FPGA.


.. arch:tag:: <output name="string" num_pins="int" equivalent="{none|full|instance}"/>

Defines an output port.
Multple output ports are described using multiple <output> tags

:req_param name: Name of the output port.
:req_param num_pins: Number of pins the output port has.

:opt_param equivalent:

.. note:: Applies only to top-level pb_type.

Describes if the pins of the output port are logically equivalent:

* none: No output pins are logically equivalent.

Output pins can not be swapped by the router. (Generates a unique SRC rr-node for each block output port pin.)

* full: All output pins are considered logically equivalent.

All output pins can be swapped without limitation by the router. For example, this option would be appropriate to model an output port which has a full crossbar between it and the logic within the block that drives it. (Generates a single SRC rr-node shared by each output port pin.)

* instance: Models that sub-instances within a block (e.g. LUTs/BLEs) can be swapped to achieve a limited form of output pin logical equivalence.

Like full, this generates a single SRC rr-node shared by each output port pin. However, each net originating from this source can use only one output pin from the equivalence group. This can be useful in modeling more complex forms of equivalence in which you can swap which BLE implements which function to gain access to different inputs.

.. warning:: When using instance equivalence you must be careful to ensure output swapping would not make the cluster internal routing (previously computed by the clusterer) illegal; the tool does not update the cluster internal routing due to output pin swapping.

**Default:** none


.. arch:tag:: <clock name="string" num_pins="int" equivalent="{none|full}"/>

Describes a clock port.
Multple clock ports are described using multiple <clock> tags.
*See above descriptions on inputs*


.. arch:tag:: <mode name="string">

:req_param name:
Name for this mode.
Must be unique compared to other modes.

Specifies a mode of operation for the <pb_type>.
Each child mode tag denotes a different mode of operation for the <pb_type>.
Each mode tag may contains other <pb_type> and <interconnect> tags.

.. note:: Modes within the same parent <pb_type> are mutually exclusive.

.. note:: If a <pb_type> has only one mode of operation the mode tag can be omitted.

For example:

.. code-block:: xml

<!--A fracturable 6-input LUT-->
<pb_type name="lut">
...
<mode name="lut6">
<!--Can be used as a single 6-LUT-->
<pb_type name="lut6" num_pb="1">
...
</pb_type>
...
</mode>
...
<mode name="lut5x2">
<!--Or as two 5-LUTs-->
<pb_type name="lut5" num_pb="2">
...
</pb_type>
...
</mode>
</pb_type>

specifies the lut pb_type can be used as either a single 6-input LUT, or as two 5-input LUTs (but not both).



The following tags are unique to the top level <pb_type> of a complex logic block. They describe how a complex block interfaces with the inter-block world.

.. arch:tag:: <fc in_type="{frac|abs}" in_val="{int|float}" out_type="{frac|abs}" out_val="{int|float}">

:req_param in_type:
Indicates how the :math:F_c values for input pins should be interpreted.

frac: The fraction of tracks of each wire/segment type.

abs: The absolute number of tracks of each wire/segment type.

:req_param in_val:
Fraction or absolute number of tracks to which each input pin is connected.

:req_param out_type:
Indicates how the :math:F_c values for output pins should be interpreted.

frac: The fraction of tracks of each wire/segment type.

abs: The absolute number of tracks of each wire/segment type.

:req_param out_val:
Fraction or absolute number of wires/segments to which each output pin connects.

Sets the number of tracks/wires to which each logic block pin connects in each channel bordering the pin.

The :math:F_c value :cite:brown_fpgas is interpreted as applying to each wire/segment type *individually* (see example).

When generating the FPGA routing architecture VPR will try to make 'good' choices about how pins and wires interconnect; for more details on the criteria and methods used see :cite:betz_automatic_generation_of_fpga_routing.

.. note:: If <fc> is not specified for a complex block, the architecture's <default_fc> is used.

.. note:: For unidirection routing architectures absolute :math:F_c values must be a multiple of 2.

**Example:**

Consider a routing architecture with 200 length 4 (L4) wires and 50 length 16 (L16) wires per channel, and the following Fc specification:

.. code-block:: xml

<fc in_type="frac" in_val="0.1" out_type="abs" out_val="25">

The above specifies that each:

* input pin connects to 20 L4 tracks (10% of the 200 L4s) and 5 L16 tracks (10% of the 50 L16s), and

* output pin connects to 25 L4 tracks and 25 L16 tracks.

**Overriding Values:**

.. arch:tag:: <fc_override fc_type="{frac|abs}" fc_val="{int|float}", port_name="{string}" segment_name="{string}">

Allows :math:F_c values to be overriden on a port or wire/segment type basis.

:req_param fc_type:
Indicates how the override :math:F_c value should be interpreted.

frac: The fraction of tracks of each wire/segment type.

abs: The absolute number of tracks of each wire/segment type.

:req_param fc_val:
Fraction or absolute number of tracks in a channel.

:opt_param port_name:
The name of the port to which this override applies.
If left unspecified this override applies to all ports.

:opt_param segment_name:
The name of the segment (defined under <segmentlist>) to which this override applies.
If left unspecified this override applies to all segments.

.. note:: At least one of port_name or segment_name must be specified.

**Port Override Example: Carry Chains**

If you have complex block pins that do not connect to general interconnect (eg. carry chains), you would use the <fc_override> tag, within the <fc> tag, to specify them:

.. code-block:: xml

<fc_override fc_type="frac" fc_val="0" port_name="cin"/>
<fc_override fc_type="frac" fc_val="0" port_name="cout"/>

Where the attribute port_name is the name of the pin (cin and cout in this example).

**Segment Override Example:**

It is also possible to specify per <segment> (i.e. routing wire) overrides:

.. code-block:: xml

<fc_override fc_type="frac" fc_val="0.1" segment_name="L4"/>

Where the above would cause all pins (both inputs and outputs) to use a fractional :math:F_c of 0.1 when connecting to segments of type L4.

**Combined Port and Segment Override Example:**

The port_name and segment_name attributes can be used together.
For example:

.. code-block:: xml

<fc_override fc_type="frac" fc_val="0.1" port_name="my_input" segment_name="L4"/>
<fc_override fc_type="frac" fc_val="0.2" port_name="my_output" segment_name="L4"/>

specifies that port my_input use a fractional :math:F_c of 0.1 when connecting to segments of type L4, while the port my_output uses a fractional :math:F_c of 0.2 when connecting to segments of type L4.
All other port/segment combinations would use the default :math:F_c values.


.. arch:tag:: <pinlocations pattern="{spread|perimeter|custom}">

:req_param pattern:
* spread denotes that the pins are to be spread evenly on all sides of the complex block.

.. note:: *Includes* internal sides of blocks with width > 1 and/or height > 1.

* perimeter denotes that the pins are to be spread evenly on perimeter sides of the complex block.

.. note:: *Excludes* the internal sides of blocks with width > 1 and/or height > 1.

* spread_inputs_perimeter_outputs denotes that inputs pins are to be spread on all sides of the complex block, but output pins are to be spread only on perimeter sides of the block.

.. note:: This is useful for ensuring outputs do not connect to wires which fly-over a width > 1 and height > 1 block (e.g. if using short or buffer connections instead of a fully configurable switch block within the block).

* custom allows the architect to specify specifically where the pins are to be placed using <loc> tags.

Describes the locations where the input, output, and clock pins are distributed in a complex logic block.

.. arch:tag:: <loc side="{left|right|bottom|top}" xoffset="int" yoffset="int">name_of_complex_logic_block.port_name[int:int] ... </loc>

.. note:: ... represents repeat as needed. Do not put ... in the architecture file.

:req_param side: Specifies which of the four sides of a grid location the pins in the contents are located.

:opt_param xoffset:
Specifies the horizontal offset (in grid units) from block origin (bottom left corner).
The offset value must be less than the width of the block.

**Default:** 0

:opt_param yoffset:
Specifies the vertical offset (in grid units) from block origin (bottom left corner).
The offset value must be less than the height of the block.

**Default:** 0

Physical equivalence for a pin is specified by listing a pin more than once for different locations.
For example, a LUT whose output can exit from the top and bottom of a block will have its output pin specified twice: once for the top and once for the bottom.

.. note:: If the <pinlocations> tag is missing, a spread pattern is assumed.


.. arch:tag:: <switchblock_locations pattern="{external_full_internal_straight|all|external|internal|none|custom}" internal_switch="string">

Describes where global routing switchblocks are created in relation to the complex block.

.. note:: If the <switchblock_locations> tag is left unspecified the default pattern is assumed.

:opt_param pattern:

* external_full_internal_straight: creates *full* switchblocks outside and *straight* switchblocks inside the complex block

* all: creates switchblocks wherever routing channels cross

* external: creates switchblocks wherever routing channels cross *outside* the complex block

* internal: creates switchblocks wherever routing channels cross *inside* the complex block

* none: denotes that no switchblocks are created for the complex block

* custom: allows the architect to specify custom switchblock locations and types using <sb_loc> tags

**Default:** external_full_internal_straight

.. _fig_sb_locations:

.. figure:: sb_locations.*

Switchblock Location Patterns for a width = 2, height = 3 complex block

:opt_param internal_switch:

The name of a switch (from <switchlist>) which should be used for internal switch blocks.

**Default:** The default switch for the wire <segment>

.. note:: This is typically used to specify that internal wire segments are electrically shorted together using a short type <switch>.

**Example: Electrically Shorted Internal Straight Connections**

In some architectures there are no switch blocks located 'within' a block, and the wires crossing over the block are instead electrcially shorted to their 'straight-through' connections.

To model this we first define a special short type switch to electrically short such segments together:

.. code-block:: xml

<switchlist>
<switch type="short" name="electrical_short" R="0" Cin="0" Tdel="0"/>
</switchlist>

Next, we use the pre-defined external_full_internal_straight pattern, and that such connections should use our electrical_short switch.

.. code-block:: xml

<switchblock_locations pattern="external_full_internal_straight" internal_switch="electrical_short"/>

.. arch:tag:: <sb_loc type="{full|straight|turns|none}" xoffset="int" yoffset="int", switch_override="string">

Specifies the type of switchblock to create at a particular location relative to a complex block for the custom switchblock location pattern.

:req_param type:
Specifies the type of switchblock to be created at this location:

* full: denotes that a full switchblock will be created (i.e. both staight and turns)
* straight: denotes that a switchblock with only straight-through connections will be created (i.e. no turns)
* turns: denotes that a switchblock with only turning connections will be created (i.e. no straight)
* none: denotes that no switchblock will be created

**Default:** full

.. figure:: sb_types.*

Switchblock Types

:opt_param xoffset:
Specifies the horizontal offset (in grid units) from block origin (bottom left corner).
The offset value must be less than the width of the block.

**Default:** 0

:opt_param yoffset:
Specifies the vertical offset (in grid units) from block origin (bottom left corner).
The offset value must be less than the height of the block.

**Default:** 0

:opt_param switch_override:
The name of a switch (from <switchlist>) which should be used to construct the switch block at this location.

**Default:** The default switch for the wire <segment>

.. note:: The switchblock associated with a grid tile is located to the top-right of the grid tile

**Example: Custom Description of Electrically Shorted Internal Straight Connections**

If we assume a width=2, height=3 block (e.g. :numref:fig_sb_locations), we can use a custom pattern to specify an architecture equivalent to the 'Electrically Shorted Internal Straight Connections' example:

.. code-block:: xml

<switchblock_locations pattern="custom">
<!-- Internal: using straight electrical shorts -->
<sb_loc type="straight" xoffset="0" yoffset="0" switch_override="electrical_short">
<sb_loc type="straight" xoffset="0" yoffset="1" switch_override="electrical_short">

<!-- External: using default switches -->
<sb_loc type="full" xoffset="0" yoffset="2"> <!-- Top edge -->
<sb_loc type="full" xoffset="1" yoffset="0"> <!-- Right edge -->
<sb_loc type="full" xoffset="1" yoffset="1"> <!-- Right edge -->
<sb_loc type="full" xoffset="1" yoffset="2"> <!-- Top Right -->
<switchblock_locations/>



### Interconnect

As mentioned earlier, the mode tag contains <pb_type> tags and an <interconnect> tag. The following describes the tags that are accepted in the <interconnect> tag.

.. arch:tag:: <complete name="string" input="string" output="string"/>

:req_param name: Identifier for the interconnect.
:req_param input: Pins that are inputs to this interconnect.
:req_param output: Pins that are outputs of this interconnect.

Describes a fully connected crossbar.
Any pin in the inputs can connect to any pin at the output.

**Example:**

.. code-block:: xml

<complete input="Top.in" output="Child.in"/>

.. figure:: complete_example.*

Complete interconnect example.


.. arch:tag:: <direct name="string" input="string" output="string"/>

:req_param name: Identifier for the interconnect.
:req_param input: Pins that are inputs to this interconnect.
:req_param output: Pins that are outputs of this interconnect.

Describes a 1-to-1 mapping between input pins and output pins.

**Example:**

.. code-block:: xml

<direct input="Top.in[2:1]" output="Child[1].in"/>

.. figure:: direct_example.*

Direct interconnect example.


.. arch:tag:: <mux name="string" input="string" output="string"/>

:req_param name: Identifier for the interconnect.
:req_param input: Pins that are inputs to this interconnect. Different data lines are separated by a space.
:req_param output: Pins that are outputs of this interconnect.

Describes a bus-based multiplexer.

.. note:: Buses are not yet supported so all muxes must use one bit wide data only!

**Example:**

.. code-block:: xml

<mux input="Top.A Top.B" output="Child.in"/>

.. figure:: mux_example.*

Mux interconnect example.



A <complete>, <direct>, or <mux> tag may take an additional, optional, tag called <pack_pattern> that is used to describe molecules. A pack pattern is a power user feature directing that the CAD tool should group certain netlist atoms (eg. LUTs, FFs, carry chains) together during the CAD flow. This allows the architect to help the CAD tool recognize structures that have limited flexibility so that netlist atoms that fit those structures be kept together as though they are one unit. This tag impacts the CAD tool only, there is no architectural impact from defining molecules.

.. arch:tag:: <pack_pattern name="string" in_port="string" out_port="string"/>

.. warning:: This is a power user option. Unless you know why you need it, you probably shouldn't specify it.

:req_param name: The name of the pattern.
:req_param in_port: The input pins of the edges for this pattern.
:req_param out_port: Which output pins of the edges for this pattern.

This tag gives a hint to the CAD tool that certain architectural structures should stay together during packing.
The tag labels interconnect edges with a pack pattern name.
All primitives connected by the same pack pattern name becomes a single pack pattern.
Any group of atoms in the user netlist that matches a pack pattern are grouped together by VPR to form a molecule.
Molecules are kept together as one unit in VPR.
This is useful because it allows the architect to help the CAD tool assign atoms to complex logic blocks that have interconnect with very limited flexibility.
Examples of architectural structures where pack patterns are appropriate include: optionally registered inputs/outputs, carry chains, multiply-add blocks, etc.

There is a priority order when VPR groups molecules.
Pack patterns with more primitives take priority over pack patterns with less primitives.
In the event that the number of primitives is the same, the pack pattern with less inputs takes priority over pack patterns with more inputs.

**Special Case:**

To specify carry chains, we use a special case of a pack pattern.
If a pack pattern has exactly one connection to a logic block input pin and exactly one connection to a logic block output pin, then that pack pattern takes on special properties.
The prepacker will assume that this pack pattern represents a structure that spans multiple logic blocks using the logic block input/output pins as connection points.
For example, lets assume that a logic block has two, 1-bit adders with a carry chain that links adjacent logic blocks.
The architect would specify those two adders as a pack pattern with links to the logic block cin and cout pins.
Lets assume the netlist has a group of 1-bit adder atoms chained together to form a 5-bit adder.
VPR will break that 5-bit adder into 3 molecules: two 2-bit adders and one 1-bit adder connected in order by a the carry links.

**Example:**

Consider a classic basic logic element (BLE) that consists of a LUT with an optionally registered flip-flop.
If a LUT is followed by a flip-flop in the netlist, the architect would want the flip-flop to be packed with the LUT in the same BLE in VPR.
To give VPR a hint that these blocks should be connected together, the architect would label the interconnect connecting the LUT and flip-flop pair as a pack_pattern:

.. code-block:: xml

<pack_pattern name="ble" in_port="lut.out" out_port="ff.D"/>

.. figure:: pack_pattern_example.*

Pack Pattern Example.



### Classes

Using these structures, we believe that one can describe any digital complex logic block. However, we believe that certain kinds of logic structures are common enough in FPGAs that special shortcuts should be available to make their specification easier. These logic structures are: flip-flops, LUTs, and memories. These structures are described using a class=string attribute in the <pb_type> primitive. The classes we offer are:

.. arch:tag:: class="lut"

Describes a K-input lookup table.

The unique characteristic of a lookup table is that all inputs to the lookup table are logically equivalent.
When this class is used, the input port must have a port_class="lut_in" attribute and the output port must have a port_class="lut_out" attribute.


.. arch:tag:: class="flipflop"

Describes a flipflop.

Input port must have a port_class="D" attribute added.
Output port must have a port_class="Q" attribute added.
Clock port must have a port_class="clock" attribute added.


.. arch:tag:: class="memory"

Describes a memory.

Memories are unique in that a single memory physical primitive can hold multiple, smaller, logical memories as long as:

#. The address, clock, and control inputs are identical and
#. There exists sufficient physical data pins to satisfy the netlist memories when the different netlist memories are merged together into one physical memory.

Different types of memories require different attributes.

**Single Port Memories Require:**

* An input port with port_class="address" attribute
* An input port with port_class="data_in" attribute
* An input port with port_class="write_en" attribute
* An output port with port_class="data_out" attribute
* A clock port with port_class="clock" attribute

**Dual Port Memories Require:**

* An input port with port_class="address1" attribute
* An input port with port_class="data_in1" attribute
* An input port with port_class="write_en1" attribute
* An input port with port_class="address2" attribute
* An input port with port_class="data_in2" attribute
* An input port with port_class="write_en2" attribute
* An output port with port_class="data_out1" attribute
* An output port with port_class="data_out2" attribute
* A clock port with port_class="clock" attribute



### Timing

.. seealso:: For examples of primitive timing modeling specifications see the :ref:arch_model_timing_tutorial



Timing is specified through tags contained with in pb_type, complete, direct, or mux tags as follows:

.. arch:tag:: <delay_constant max="float" min="float" in_port="string" out_port="string"/>

:opt_param max: The maximum delay value.
:opt_param min: The minimum delay value.
:req_param in_port: The input port name.
:req_param out_port: The output port name.

Specifies a maximum and/or minimum delay from in_port to out_port.

* If in_port and out_port are non-sequential (i.e combinational) inputs specifies the combinational path delay between them.
* If in_port and out_port are sequential (i.e. have T_setup and/or T_clock_to_Q tags) specifies the combinational delay between the primitive's input and/or output registers.

.. note:: At least one of the max or min attributes must be specified

.. note:: If only one of max or min are specified the unspecified value is implicitly set to the same value


.. arch:tag:: <delay_matrix type="{max | min}" in_port="string" out_port="string"> matrix </delay>

:req_param type: Specifies the delay type.
:req_param in_port: The input port name.
:req_param out_port: The output port name.
:req_param matrix: The delay matrix.

Describe a timing matrix for all edges going from in_port to out_port.
Number of rows of matrix should equal the number of inputs, number of columns should equal the number of outputs.

* If in_port and out_port are non-sequential (i.e combinational) inputs specifies the combinational path delay between them.
* If in_port and out_port are sequential (i.e. have T_setup and/or T_clock_to_Q tags) specifies the combinational delay between the primitive's input and/or output registers.

**Example:**
The following defines a delay matrix for a 4-bit input port in, and 3-bit output port out:

.. code-block:: xml

<delay_matrix type="max" in_port="in" out_port="out">
1.2e-10 1.4e-10 3.2e-10
4.6e-10 1.9e-10 2.2e-10
4.5e-10 6.7e-10 3.5e-10
7.1e-10 2.9e-10 8.7e-10
</delay>

.. note:: To specify both max and min delays two <delay_matrix> should be used.


.. arch:tag:: <T_setup value="float" port="string" clock="string"/>

:req_param value: The setup time value.
:req_param port: The port name the setup constraint applies to.
:req_param clock: The port name of the clock the setup constraint is specified relative to.

Specifies a port's setup constraint.

* If port is an input specifies the external setup time of the primitive's input register (i.e. for paths terminating at the input register).
* If port is an output specifies the internal setup time of the primitive's output register (i.e. for paths terminating at the output register) .

.. note:: Applies only to primitive <pb_type> tags


.. arch:tag:: <T_hold value="float" port="string" clock="string"/>

:req_param value: The hold time value.
:req_param port: The port name the setup constraint applies to.
:req_param clock: The port name of the clock the setup constraint is specified relative to.

Specifies a port's hold constraint.

* If port is an input specifies the external hold time of the primitive's input register (i.e. for paths terminating at the input register).
* If port is an output specifies the internal hold time of the primitive's output register (i.e. for paths terminating at the output register) .

.. note:: Applies only to primitive <pb_type> tags


.. arch:tag:: <T_clock_to_Q max="float" min="float" port="string" clock="string"/>

:opt_param max: The maximum clock-to-Q delay value.
:opt_param min: The minimum clock-to-Q delay value.
:req_param port: The port name the delay value applies to.
:req_param clock: The port name of the clock the clock-to-Q delay is specified relative to.

Specifies a port's clock-to-Q delay.

* If port is an input specifies the internal clock-to-Q delay of the primitive's input register (i.e. for paths starting at the input register).
* If port is an output specifies the external clock-to-Q delay of the primitive's output register (i.e. for paths starting at the output register) .

.. note:: At least one of the max or min attributes must be specified

.. note:: If only one of max or min are specified the unspecified value is implicitly set to the same value

.. note:: Applies only to primitive <pb_type> tags



#### Modeling Sequential Primitive Internal Timing Paths

.. seealso:: For examples of primitive timing modeling specifications see the :ref:arch_model_timing_tutorial



By default, if only <T_setup> and <T_clock_to_Q> are specified on a primitive pb_type no internal timing paths are modeled. However, such paths can be modeled by using <delay_constant> and/or <delay_matrix> can be used in conjunction with <T_setup> and <T_clock_to_Q>. This is useful for modeling the speed-limiting path of an FPGA hard block like a RAM or DSP.

As an example, consider a sequential black-box primitive named seq_foo which has an input port in, output port out, and clock clk:

<pb_type name="seq_foo" blif_model=".subckt seq_foo" num_pb="1">
<input name="in" num_pins="4"/>
<output name="out" num_pins="1"/>
<clock name="clk" num_pins="1"/>

<!-- external -->
<T_setup value="80e-12" port="seq_foo.in" clock="clk"/>
<T_clock_to_Q max="20e-12" port="seq_foo.out" clock="clk"/>

<!-- internal -->
<T_clock_to_Q max="10e-12" port="seq_foo.in" clock="clk"/>
<delay_constant max="0.9e-9" in_port="seq_foo.in" out_port="seq_foo.out"/>
<T_setup value="90e-12" port="seq_foo.out" clock="clk"/>
</pb_type>

To model an internal critical path delay, we specify the internal clock-to-Q delay of the input register (10ps), the internal combinational delay (0.9ns) and the output register's setup time (90ps). The sum of these delays corresponds to a 1ns critical path delay.

Note

Primitive timing paths with only one stage of registers can be modeled by specifying <T_setup> and <T_clock_to_Q> on only one of the ports.

### Power

.. seealso:: :ref:power_estimation, for the complete list of options, their descriptions, and required sub-fields.


.. arch:tag:: <power method="string">contents</power>

:opt_param method:

Indicates the method of power estimation used for the given pb_type.

Must be one of:

* specify-size
* auto-size
* pin-toggle
* C-internal
* absolute
* ignore
* sum-of-children

**Default:** auto-size.

.. seealso:: :ref:Power Architecture Modelling <power_arch_modeling> for a detailed description of the various power estimation methods.

The contents of the tag can consist of the following tags:

* <dynamic_power>
* <static_power>
* <pin>


.. arch:tag:: <dynamic_power power_per_instance="float" C_internal="float"/>

:opt_param power_per_instance: Absolute power in Watts.
:opt_param C_internal: Block capacitance in Farads.


.. arch:tag:: <static_power power_per_instance="float"/>

:opt_param power_per_instance: Absolute power in Watts.


.. arch:tag:: <port name="string" energy_per_toggle="float" scaled_by_static_prob="string" scaled_by_static_prob_n="string"/>

:req_param name: Name of the port.
:req_param energy_per_toggle: Energy consumed by a toggle on the port specified in name.
:opt_param scaled_by_static_prob: Port name by which to scale energy_per_toggle based on its logic high probability.
:opt_param scaled_by_static_prob_n: Port name by which to scale energy_per_toggle based on its logic low probability.



## Wire Segments

The content within the <segmentlist> tag consists of a group of <segment> tags. The <segment> tag and its contents are described below.

.. arch:tag:: <segment name="unique_name" length="int" type="{bidir|unidir}" freq="float" Rmetal="float" Cmetal="float">content</segment>

:req_param name:
A unique alphanumeric name to identify this segment type.

:req_param length:
Either the number of logic blocks spanned by each segment, or the keyword longline.
Longline means segments of this type span the entire FPGA array.

.. note:: longline is only supported on with bidir routing

:req_param freq:
The supply of routing tracks composed of this type of segment.
VPR automatically determines the percentage of tracks for each segment type by taking the frequency for the type specified and dividing with the sum of all frequencies.
It is recommended that the sum of all segment frequencies be in the range 1 to 100.

:req_param Rmetal:
Resistance per unit length (in terms of logic blocks) of this wiring track, in Ohms.
For example, a segment of length 5 with Rmetal = 10 Ohms / logic block would have an end-to-end resistance of 50 Ohms.

:req_param Cmetal:
Capacitance per unit length (in terms of logic blocks) of this wiring track, in Farads.
For example, a segment of length 5 with Cmetal = 2e-14 F / logic block would have a total metal capacitance of 10e-13F.

:req_param directionality:
This is either unidirectional or bidirectional and indicates whether a segment has multiple drive points (bidirectional), or a single driver at one end of the wire segment (unidirectional).
All segments must have the same directionality value.
See :cite:lemieux_directional_and_singale_driver_wires for a description of unidirectional single-driver wire segments.

:req_param content:
The switch names and the depopulation pattern as described below.



Switch block and connection block pattern example with four tracks per channel

.. arch:tag:: <sb type="pattern">int list</sb>

This tag describes the switch block depopulation (as illustrated in :numref:fig_sb_pattern) for this particular wire segment.
For example, the first length 6 wire in the figure below has an sb pattern of 1 0 1 0 1 0 1.
The second wire has a pattern of 0 1 0 1 0 1 0.
A 1 indicates the existence of a switch block and a 0 indicates that there is no switch box at that point.
Note that there a 7 entries in the integer list for a length 6 wire.
For a length L wire there must be L+1 entries separated by spaces.

.. note:: Can not be specified for longline segments (which assume full switch block population)


.. arch:tag:: <cb type="pattern">int list</cb>

This tag describes the connection block depopulation (as illustrated by the circles in :numref:fig_sb_pattern) for this particular wire segment.
For example, the first length 6 wire in the figure below has an sb pattern of 1 1 1 1 1 1.
The third wire has a pattern of 1 0 0 1 1 0.
A 1 indicates the existence of a connection block and a 0 indicates that there is no connection box at that point.
Note that there a 6 entries in the integer list for a length 6 wire.
For a length L wire there must be L entries separated by spaces.

.. note:: Can not be specified for longline segments (which assume full connection block population)


.. arch:tag:: <mux name="string"/>

:req_param name: Name of the mux switch type used to drive this type of segment.

.. note:: For UNIDIRECTIONAL only.

Tag must be included and name must be the same as the name you give in <switch type="mux" name="...


.. arch:tag:: <wire_switch name="string"/>

:req_param name: Name of the switch type used by other wires to drive this type of segment.

.. note:: For BIDIRECTIONAL only.

Tag must be included and the name must be the same as the name you give in <switch type="tristate|pass_gate" name="... for the switch which represents the wire switch in your architecture.


.. arch:tag:: <opin_switch name="string"/>

.. note:: For BIDIRECTIONAL only.

:req_param name: Name of the switch type used by block pins to drive this type of segment.

Tag must be included and name must be the same as the name you give in <switch type="tristate|pass_gate" name="... for the switch which represents the output pin switch in your architecture.

.. note::
In unidirectional segment mode, there is only a single buffer on the segment.
Its type is specified by assigning the same switch index to both wire_switch and opin_switch.
VPR will error out if these two are not the same.

.. note::
The switch used in unidirectional segment mode must be buffered.



## Clocks

The clocking configuration is specified with <clock> tags within the <clocks> section.

Note

Currently the information in the <clocks> section is only used for power estimation.

.. seealso:: :ref:power_estimation for more details.


.. arch:tag:: <clock C_wire="float" C_wire_per_m="float" buffer_size={"float"|"auto"}/>

:opt_param C_wire: The absolute capacitance, in Farads, of the wire between each clock buffer.
:opt_param C_wire_per_m: The wire capacitance, in Farads per Meter.
:opt_param buffer_size: The size of each clock buffer.



## Power

Additional power options are specified within the <architecture> level <power> section.

.. seealso:: See :ref:power_estimation for full documentation on how to perform power estimation.


.. arch:tag:: <local_interconnect C_wire="float" factor="float"/>

:req_param C_wire: The local interconnect capacitance in Farads/Meter.
:opt_param factor: The local interconnect scaling factor. **Default:** 0.5.


.. arch:tag:: <buffers logical_effort_factor="float"/>

:req_param logical_effort_factor: **Default:** 4.



## Direct Inter-block Connections

The content within the <directlist> tag consists of a group of <direct> tags. The <direct> tag and its contents are described below.

.. arch:tag:: <direct name="string" from_pin="string" to_pin="string" x_offset="int" y_offset="int" z_offset="int" switch_name="string"/>

:req_param name: is a unique alphanumeric string to name the connection.
:req_param from_pin: pin of complex block that drives the connection.
:req_param to_pin: pin of complex block that receives the connection.
:req_param x_offset:  The x location of the receiving CLB relative to the driving CLB.
:req_param y_offset: The y location of the receiving CLB relative to the driving CLB.
:req_param z_offset: The z location of the receiving CLB relative to the driving CLB.
:req_param switch_name: [Optional, defaults to delay-less switch if not specified] The name of the <switch> from <switchlist> to be used for this direct connection.

Describes a dedicated connection between two complex block pins that skips general interconnect.
This is useful for describing structures such as carry chains as well as adjacent neighbour connections.

**Example:**
Consider a carry chain where the cout of each CLB drives the cin of the CLB immediately below it, using the delay-less switch one would enter the following:

.. code-block:: xml

<direct name="adder_carry" from_pin="clb.cout" to_pin="clb.cin" x_offset="0" y_offset="-1" z_offset="0"/>



## Custom Switch Blocks

The content under the <switchblocklist> tag consists of one or more <switchblock> tags that are used to specify connections between different segment types. An example is shown below:

<switchblocklist>
<switchblock name="my_switchblock" type="unidir">
<switchblock_location type="EVERYWHERE"/>
<switchfuncs>
<func type="lr" formula="t"/>
<func type="lt" formula="W-t"/>
<func type="lb" formula="W+t-1"/>
<func type="rt" formula="W+t-1"/>
<func type="br" formula="W-t-2"/>
<func type="bt" formula="t"/>
<func type="rl" formula="t"/>
<func type="tl" formula="W-t"/>
<func type="bl" formula="W+t-1"/>
<func type="tr" formula="W+t-1"/>
<func type="rb" formula="W-t-2"/>
<func type="tb" formula="t"/>
</switchfuncs>
<wireconn from_type="l4" to_type="l4" from_switchpoint="0,1,2,3" to_switchpoint="0"/>
<wireconn from_type="l8_global" to_type="l8_global" from_switchpoint="0,4"
to_switchpoint="0"/>
<wireconn from_type="l8_global" to_type="l4" from_switchpoint="0,4"
to_switchpoint="0"/>
</switchblock>

<switchblock name="another_switch_block" type="unidir">
... another switch block description ...
</switchblock>
</switchblocklist>

This switch block format allows a user to specify mathematical permutation functions that describe how different types of segments (defined in the architecture file under the <segmentlist> tag) will connect to each other at different switch points. The concept of a switch point is illustrated below for a length-4 unidirectional wire heading in the "left" direction. The switch point at the start of the wire is given an index of 0 and is incremented by 1 at each subsequent switch block until the last switch point. The last switch point has an index of 0 because it is shared between the end of the current segment and the start of the next one (similarly to how switch point 3 is shared by the two wire subsegments on each side).

Switch point diagram.

A collection of wire types and switch points defines a set of wires which will be connected to another set of wires with the specified permutation functions (the ‘sets’ of wires are defined using the <wireconn> tags). This format allows for an abstract but very flexible way of specifying different switch block patterns. For additional discussion on interconnect modeling see :cite:petelin_masc. The full format is documented below.

Overall Notes:

1. The <sb type="pattern"> tag on a wire segment (described under <segmentlist>) is applied as a mask on the patterns created by this switch block format; anywhere along a wire’s length where a switch block has not been requested (set to 0 in this tag), no switches will be added.
2. You can specify multiple switchblock tags, and the switches described by the union of all those switch blocks will be created.
.. arch:tag:: <switchblock name="string" type="string">

:req_param name: A unique alphanumeric string
:req_param type: unidir or bidir.
A bidirectional switch block will implicitly mirror the specified permutation functions – e.g. if a permutation function of type lr (function used to connect wires from the left to the right side of a switch block) has been specified, a reverse permutation function of type rl (right-to-left) is automatically assumed.
A unidirectional switch block makes no such implicit assumptions.
The type of switch block must match the directionality of the segments defined under the <segmentlist> node.

<switchblock> is the top-level XML node used to describe connections between different segment types.


.. arch:tag:: <switchblock_location type="string"/>

:req_param type:
Can be one of the following strings:

* EVERYWHERE – at each switch block of the FPGA
* PERIMETER – at each perimeter switch block (x-directed and/or y-directed channel segments may terminate here)
* CORNER – only at the corner switch blocks (both x and y-directed channels terminate here)
* FRINGE – same as PERIMETER but excludes corners
* CORE – everywhere but the perimeter

Sets the location on the FPGA where the connections described by this switch block will be instantiated.


.. arch:tag:: <switchfuncs>

The switchfuncs XML node contains one or more entries that specify the permutation functions with which different switch block sides should be connected, as described below.


.. arch:tag:: <func type="string" formula="string"/>

:req_param type:
Specifies which switch block sides this function should connect.
With the switch block sides being left, top, right and bottom, the allowed entries are one of {lt, lr, lb, tr, tb, tl, rb, rl, rt, bl, bt, br} where lt means that the specified permutation formula will be used to connect the left-top sides of the switch block.

.. note:: In a bidirectional architecture the reverse connection is implicit.

:req_param formula:
Specifies the mathematical permutation function that determines the pattern with which the source/destination sets of wires (defined using the <wireconn> entries) at the two switch block sides will be connected.
For example, W-t specifies a connection where the t’th wire in the source set will connect to the W-t wire in the destination set where W is the number of wires in the destination set and the formula is implicitly treated as modulo W.

Special characters that can be used in a formula are:

* t -- the index of a wire in the source set
* W -- the number of wires in the destination set (which is not necessarily the total number of wires in the channel)

The operators that can be used in the formula are:

* Addition (+)
* Subtraction (-)
* Multiplication (*)
* Division (/)
* Brackets ( and ) are allowed and spaces are ignored.

Defined under the <switchfuncs> XML node, one or more <func...> entries is used to specify permutation functions that connect different sides of a switch block.


.. arch:tag:: <wireconn num_conns="expr" from_type="string, string, string, ..." to_type="string, string, string, ..." from_switchpoint="int, int, int, ..." to_switchpoint="int, int, int, ..." from_order="{fixed | shuffled}" to_order="{fixed | shuffled}"/>

:req_param num_conns:
Specifies how many connections should be created between the from_type/from_switchpoint set and the to_type/to_switchpoint set.
The value of this parameter is an expression which is evaluated when the switch block is constructed.

The expression can be a single number or formula using the variables:

* from -- The number of switchblock edges equal to the 'from' set size.

This ensures that each element in the 'from' set is connected to an element of the 'to' set.
However it may leave some elements of the 'to' set either multiply-connected or disconnected.

.. figure:: wireconn_num_conns_type_from.*
:width: 100%

* to -- The number of switchblock edges equal to the 'to' set size size.

This ensures that each element of the 'to' set is connected to precisely one element of the 'from' set.
However it may leave some elements of the 'from' set either multiply-connected or disconnected.

.. figure:: wireconn_num_conns_type_to.*
:width: 100%

Examples:

* min(from,to) --  Creates number of switchblock edges equal to the minimum of the 'from' and 'to' set sizes.

This ensures *no* element of the 'from' or 'to' sets is connected to multiple elements in the opposing set.
However it may leave some elements in the larger set disconnected.

.. figure:: wireconn_num_conns_type_min.*
:width: 100%

* max(from,to) -- Creates number of switchblock edges equal to the maximum of the 'from' and 'to' set sizes.

This ensures *all* elements of the 'from' or 'to' sets are connected to at least one element in the opposing set.
However some elements in the smaller set may be multiply-connected.

.. figure:: wireconn_num_conns_type_max.*
:width: 100%

* 3*to -- Creates number of switchblock edges equal to three times the 'to' set sizes.

:req_param from_type:
A comma-separated list segment names that defines which segment types will be a source of a connection.
The segment names specified must match the names of the segments defined under the <segmentlist> XML node.
Required if no <from> or <to> nodes are specified within the <wireconn>.

:req_param to_type:
A comma-separated list of segment names that defines which segment types will be the destination of the connections specified.
Each segment name must match an entry in the <segmentlist> XML node.
Required if no <from> or <to> nodes are specified within the <wireconn>.

:req_param from_switchpoint:
A comma-separated list of integers that defines which switchpoints will be a source of a connection.
Required if no <from> or <to> nodes are specified within the <wireconn>.

:req_param to_switchpoint:
A comma-separated list of integers that defines which switchpoints will be the destination of the connections specified.
Required if no <from> or <to> nodes are specified within the <wireconn>.

.. note:: In a unidirectional architecture wires can only be driven at their start point so to_switchpoint="0" is the only legal specification in this case.

:opt_param from_order:
Specifies the order in which from_switchpoints are selected when creating edges.

* fixed -- Switchpoints are selected in the order specified

This is useful to specify a preference for connecting to specific switchpoints.
For example,

.. code-block:: xml

<wireconn num_conns="1*to" from_type="L16" from_switchpoint="0,12,8,4" from_order="fixed" to_type="L4" to_switchpoint="0"/>

specifies L4 wires should be connected first to L16 at switchpoint 0, then at switchpoints 12, 8, and 4.
This is primarily useful when we want to ensure that some switchpoints are 'used-up' first.

* shuffled -- Switchpoints are selected in a (randomly) shuffled order

This is useful to ensure a diverse set of switchpoints are used.
For example,

.. code-block:: xml

<wireconn num_conns="1*to" from_type="L4" from_switchpoint="0,1,2,3" from_order="shuffled" to_type="L4" to_switchpoint="0"/>

specifies L4 wires should be connected to other L4 wires at any of switchpoints 0, 1, 2, or 3.
Shuffling the switchpoints is useful if one of the sets (e.g. from L4's) is much larger than the other (e.g. to L4's), and we wish to ensure a variety of switchpoints from the larger set are used.

**Default:** shuffled

:opt_param to_order:
Specifies the order in which to_switchpoints are selected when creating edges.

.. note:: See from_switchpoint_order for value descritpions.

.. arch:tag:: <from type="string" switchpoint="int, int, int, ..."/>

:req_param type:

The name of a segment specified in the <segmentlist>.

:req_param switchpoint:

A comma-separated list of integers that defines switchpoints.

.. note:: In a unidirectional architecture wires can only be driven at their start point so to_switchpoint="0" is the only legal specification in this case.

Specifies a subset of *source* wire switchpoints.

This tag can be specified multiple times.
The surrounding <wireconn>'s source set is the union of all contained <from> tags.

.. arch:tag:: <to type="string" switchpoint="int, int, int, ..."/>

Specifies a subset of *destination* wire switchpoints.

This tag can be specified multiple times.
The surrounding <wireconn>'s destination set is the union of all contained <to> tags.

.. seealso:: <from> for attribute descriptions.

As an example, consider the following <wireconn> specification:

.. code-block:: xml

<wireconn num_conns_type="to"/>
<from type="L4" switchpoint="0,1,2,3"/>
<from type="L16" switchpoint="0,4,8,12"/>
<to type="L4" switchpoint="0/>
</wireconn>

This specifies that the 'from' set is the union of L4 switchpoints 0, 1, 2 and 3; and L16 switchpoints 0, 4, 8 and 12.
The 'to' set is all L4 switchpoint 0's.
Note that since different switchpoints are selected from different segment types it is not possible to specify this without using <from> sub-tags.