config_protocol.rst

Configuration Protocol

Configuration protocol is the circuitry designed to program an FPGA. As an interface, configuration protocol could be really different in FPGAs, depending on the application context. OpenFPGA supports versatile configuration protocol, providing different trade-offs between speed and area.

Template

<configuration_protocol>
  <organization type="<string>" circuit_model_name="<string>" num_regions="<int>"/>
</configuration_protocol>

type="scan_chainstandaloneql_memory_bank"

Specify the type of configuration circuits.

OpenFPGA supports different types of configuration protocols to program FPGA fabrics:

• scan_chain: configurable memories are connected in a chain. Bitstream is loaded serially to program a FPGA
• frame_based: configurable memories are organized by frames. Each module of a FPGA fabric, e.g., Configurable Logic Block (CLB), Switch Block (SB) and Connection Block (CB), is considered as a frame of configurable memories. Inside each frame, all the memory banks are accessed through an address decoder. Users can write each memory cell with a specific address. Note that the frame-based memory organization is applid hierarchically. Each frame may consists of a number of sub frames, each of which follows the similar organization.
• memory_bank: configurable memories are organized in an array, where each element can be accessed by an unique address to the BL/WL decoders
• ql_memory_bank: configurable memories are organized in an array, where each element can be accessed by an unique address to the BL/WL decoders. This is a physical design friendly memory bank organization, where BL/WLs are efficiently shared by programmable blocks per column and row
• standalone: configurable memories are directly accessed through ports of FPGA fabrics. In other words, there are no protocol to control the memories. This allows full customization on the configuration protocol for hardware engineers.

Note

Avoid to use standalone when designing an FPGA chip. It will causes a huge number of I/Os required, far beyond any package size. It is well applicable to eFPGAs, where designers do need customized protocols between FPGA and processors.

Warning

Currently FPGA-SPICE only supports standalone memory organization.

Warning

Currently RRAM-based FPGA only supports memory-bank organization for Verilog Generator.

circuit_model_name="<string>"

Specify the name of circuit model to be used as configurable memory.

• scan_chain requires a circuit model type of ccff
• frame_based requires a circuit model type of sram
• memory_bank requires a circuit model type of sram
• ql_memory_bank requires a circuit model type of sram
• standalone requires a circuit model type of sram

num_regions="<int>"

Specify the number of configuration regions to be used across the fabrics. By default, it will be only 1 configuration region. Each configuration region contains independent configuration protocols, but the whole fabric should employ the same type of configuration protocols. For example, an FPGA fabric consists of 4 configuration regions, each of which includes a configuration chain. The more configuration chain to be used, the fast configuration runtime will be, but at the cost of more I/Os in the FPGA fabrics. The organization of each configurable region can be customized through the fabric key (see details in fabric_key).

Warning

Currently, multiple configuration regions is not applicable to

• standalone configuration protocol.
• ql_memory_bank configuration protocol when BL/WL protocol flatten is selected

Note

For ql_memory_bank configuration protocol when BL/WL protocol shift_register is selected, different configuration regions cannot share any WLs on the same row! In such case, the default fabric key may not work. Strongly recommend to craft your own fabric key based on your configuration region plannning!

Configuration Chain Example

The following XML code describes a scan-chain circuitry to configure the core logic of FPGA, as illustrated in fig_ccff_fpga. It will use the circuit model defined in fig_ccff_config_chain.

<configuration_protocol>
  <organization type="scan_chain" circuit_model_name="ccff" num_regions="<int>"/>
</configuration_protocol>

Example of a configuration chain to program core logic of a FPGA

Examples of single- and multiple- region configuration chains

Frame-based Example

The following XML code describes frame-based memory banks to configure the core logic of FPGA. It will use the circuit model defined in fig_config_latch.

<configuration_protocol>
  <organization type="frame_based" circuit_model_name="config_latch"/>
</configuration_protocol>

Through frame-based configuration protocol, each memory cell can be accessed with an unique address given to decoders. fig_frame_config_protocol_example illustrates an example about how the configurable memories are organizaed inside a Logic Element (LE) shown in fig_k4n4_arch. The decoder inside the LE will enable the decoders of the Look-Up Table (LUT) and the routing multiplexer, based on the given address at address[2:2]. When the decoder of sub block, e.g., the LUT, is enabled, each memory cells can be accessed throught the address[1:0] and the data to write is provided at data_in.

Example of a frame-based memory organization inside a Logic Element

fig_frame_config_protocol shows a hierarchical view on how the frame-based decoders across a FPGA fabric.

Frame-based memory organization in a hierarchical view

Note

Frame-based decoders does require a memory cell to have

• two outputs (one regular and another inverted)
• a Bit-Line input to load the data
• a Word-Line input to enable data write

Warning

Please do NOT add inverted Bit-Line and Word-Line inputs. It is not supported yet!

When multiple configuration region is applied, the configuration frames will be grouped into different configuration regions. Each region has a separated data input bus and dedicated address decoders. As such, the configuration frame groups can be programmed in parallel.

Memory bank Example

The following XML code describes a memory-bank circuitry to configure the core logic of FPGA, as illustrated in fig_memory_bank. It will use the circuit model defined in fig_sram_blwl. Users can customized the number of memory banks to be used across the fabrics. By default, it will be only 1 memory bank. fig_memory_bank shows an example where 4 memory banks are defined. The more memory bank to be used, the fast configuration runtime will be, but at the cost of more I/Os in the FPGA fabrics. The organization of each configurable region can be customized through the fabric key (see details in fabric_key).

<configuration_protocol>
  <organization type="memory_bank" circuit_model_name="sram_blwl"/>
</configuration_protocol>

Example of (a) a memory organization using memory decoders; (b) single memory bank across the fabric; and (c) multiple memory banks across the fabric.

Note

Memory-bank decoders does require a memory cell to have

• two outputs (one regular and another inverted)
• a Bit-Line input to load the data
• a Word-Line input to enable data write

Warning

Please do NOT add inverted Bit-Line and Word-Line inputs. It is not supported yet!

QuickLogic Memory bank Example

The following XML code describes a physical design friendly memory-bank circuitry to configure the core logic of FPGA, as illustrated in fig_memory_bank. It will use the circuit model defined in fig_sram_blwl.

The BL and WL protocols can be customized through the XML syntax bl and wl.

Note

If not specified, the BL/WL protocols will use decoders.

<configuration_protocol>
  <organization type="ql_memory_bank" circuit_model_name="sram_blwl">
    <bl protocol="<string>" num_banks="<int>"/>
    <wl protocol="<string>" num_banks="<int>"/>
  </organization>
</configuration_protocol>

protocol="decodershift_register"

• decoder: BLs or WLs are controlled by decoders with address lines. For BLs, the decoder includes an enable signal as well as a data input signal. This is the default option if not specified. See an illustrative example in fig_memory_bank_decoder_based.
• flatten: BLs or WLs are directly available at the FPGA fabric. In this way, all the configurable memorys on the same WL can be written through the BL signals in one clock cycle. See an illustrative example in fig_memory_bank_flatten.
• shift_register: BLs or WLs are controlled by shift register chains. The BL/WLs are programming each time the shift register chains are fully loaded. See an illustrative example in fig_memory_bank_shift_register.

Example of (a) a memory organization using address decoders; (b) single memory bank across the fabric; and (c) multiple memory banks across the fabric.

Example of (a) a memory organization with direct access to BL/WL signals; (b) single memory bank across the fabric; and (c) multiple memory banks across the fabric.

Example of (a) a memory organization using shift register chains to control BL/WLs; (b) single memory bank across the fabric; and (c) multiple memory banks across the fabric.

num_banks="<int>"

Specify the number of shift register banks (i.e., independent shift register chains) to be used in each configuration region. When enabled, the length of each shift register chain will be sized by OpenFPGA automatically based on the number of BL/WLs in each configuration region. OpenFPGA will try to create similar sizes for the shift register chains, in order to minimize the number of HDL modules. If not specified, the default number of banks will be 1.

Note

This is available applicable to shift-register-based BL/WL protocols

Note

More customization on the shift register chains can be enabled through fabric_key

Note

The flip-flop for WL shift register requires an enable signal to gate WL signals when loading WL shift registers

Note

Memory-bank decoders does require a memory cell to have

• two outputs (one regular and another inverted)
• a Bit-Line input to load the data
• a Word-Line input to enable data write
• (optional) a Word-Line read input to enabe data readback

Warning

Please do NOT add inverted Bit-Line and Word-Line inputs. It is not supported yet!

Standalone SRAM Example

In the standalone configuration protocol, every memory cell of the core logic of a FPGA fabric can be directly accessed at the top-level module, as illustrated in fig_vanilla_config_protocol.

Vanilla (standalone) memory organization in a hierarchical view

The following XML code shows an example where we use the circuit model defined in fig_sram_blwl.

<configuration_protocol>
  <organization type="standalone" circuit_model_name="sram_blwl"/>
</configuration_protocol>

Note

The standalone protocol does require a memory cell to have

• two outputs (one regular and another inverted)
• a Bit-Line input to load the data
• a Word-Line input to enable data write

Warning

Please do NOT add inverted Bit-Line and Word-Line inputs. It is not supported yet!

Warning

This is a vanilla configuration method, which allow users to build their own configuration protocol on top of it.