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AWS Shell Interface Specification

Revision History

2016/11/28 - Initial public release with HDK release version

2016/12/06 - Added capability to remove DDR controllers in the CL through parameters in

2017/02/02 - Major updates for Feb/2017 Shell, that includes interrupts, wider and more buses, DMA, Virtual LED and other. (Please refer to Release Notes for details)

2017/07/29 - Updates for Jul/2017 Shell

2017/11/16 - Updates for v1.3.4

2018/05/10 - Updates for 1.4

Table of Contents:

  1. Overview

    1a. Architecture and Version

    1b. Conventions

  2. Shell Interfaces

  3. External Interfaces

    3a. EC2 Instance view of FPGA PCIe

    3b. DDR4 DRAM Interfaces

    3c. DMA

  4. Interfaces between Shell and CL

    4a. CL/Shell AXI Interfaces

    4b. Clocks and Resets

    4c. DDR4 AXI

    4d. PCIS Interface

    4e. PCIM Interface

    4f. AXI-Lite interfaces for register access

    4g. Interrupts

    4h. Miscellanous Interfaces(vLED, vDIP..)

  5. Implementation Tips

    5a. Multi-SLR FPGA

    5b. Logic Levels

    5c. Reset

    5d. Pipeline Registers

    5e. Vivado Analysis


With Amazon EC2 FPGA instances, each FPGA is divided into two partitions:

  • Shell (SH) – AWS platform logic implementing the FPGA external peripherals, PCIe, DRAM, DMA, and Interrupts.

  • Custom Logic (CL) – Custom acceleration logic created by an FPGA Developer.

At the end of the development process, combining the Shell and CL creates an Amazon FPGA Image (AFI) that can be loaded onto the Amazon EC2 FPGA Instances.

This document specifies the hardware interface and functional behavior between the SH and the CL.

Architecture and Version

This specification applies to Xilinx Virtex Ultrascale Plus platform available on EC2 F1, each update of the Shell is tagged with a revision number. Note while AWS tries to keep the revision constant, sometimes it is necessary to update the revision due to discovered issues or added functionality. The HDK release includes the latest Shell version under /hdk/common/shell_latest

Starting from 1.4, The shell is reconfigurable, allowing, in most cases, developers to select which shell version to create the AFI with. Going forward, new shell versions will NOT require updated CL implementation and regenerating the AFI (still a requirement with 1.4 shell.)


CL – Custom Logic: the Logic to be provided by the developer and integrated with AWS Shell.

DW – Doubleword: referring to 4-byte (32-bit) data size.

AXI-4 ARM Advanced eXtensible Interface.

AXI-4 Stream – ARM Advanced eXtensible Stream Interface.

M – Typically refers to the Master side of an AXI bus.

S – Typical refers to the Slave side of AXI bus.

Shell Interfaces

The following diagram and table summarize the various interfaces between the Shell and CL as defined in cl_ports.vh.

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Interface Description
Clocks and Resets There are multiple clocks and resets provided by the Shell to the CL. Refer to the Clocks and Resets section for more information.
DDR4 The master interface to the DDR4 Controllers utilizes an AXI-4 interface. Refer to the DDR4 AXI Interface section for more information.
DMA_PCIS The PCIe Slave (PCIS) Interface is an AXI-4 interface used for Inbound PCIe transactions. Refer to the PCIS Interface section for more information.
PCIM The PCIe Master (PCIM) Interface is an AXI-4 interface used for Outbound PCIe transactions. Refer to the PCIM Interface section for more information.
SDA The SDA Interface is an AXI-Lite interface associated with MgmtPF and BAR4. Please refer to the AXI-Lite Interfaces for more information.
OCL The OCL Interface is an AXI-Lite interface associated with AppPF and BAR0. Please refer to the AXI-Lite Interfaces for more information.
BAR1 The BAR1 Interface is an AXI-Lite interface associated with AppPF and BAR1. Please refer to the AXI-Lite Interfaces for more information.
Interrupts There are 16 user interrupts available. Refer to the Interrupts section for more information.
Miscellaneous There are various generic signals, such as ID's, status, counters, etc., between the Shell and CL that are described in the Miscellaneous Signals section.

External F1 FPGA Interfaces

The F1 FPGA platform includes the following external interfaces:

  • One x16 PCI Express 3.0 Interface.

  • Four DDR4 RDIMM interfaces, each interface is 72-bit wide (including ECC).

FPGA PCIe Representation to EC2 Instance

There are two PCIe Physical Functions (PFs) presented to the F1 instance:

  • Management PF – This PF is used for management of the FPGA using the FPGA Management Tools and FPGA Management Libraries. The Management PF provides access to various control functions like Virtual-LED, Virtual-DIPSwitch, Virtual JTAG, FPGA metrics, and AFI management (load, clear, etc...).

  • Application PF (AppPF)– The AppPF is used for CL specific functionality.

The Software Programmers' View provides the intended software programmer's view and associated software modules and libraries around the two before mentioned PFs.

Please refer to PCI Address map for a more detailed view of the address map.

Management PF (MgmtPF)

The Management PF details are provided for reference to help understanding the PCIe mapping from an F1 instance. This interface is strictly used by the AWS FPGA Management Tools linux shell commands, and FPGA Management Library for integration with C/C++ applications, as well as AWS OpenCL Runtime ICD/HDL, and does not support any interface with the CL code.

The Management PF exposes:

a) Amazon’s specific and fixed PCIe VendorID (0x1D0F) and DeviceID (0x1041).

b) Three BARs:

  - BAR0 - 16KiB
  - BAR2 - 16KiB
  - BAR4 - 4MiB  

c) No BusMaster support.

d) A range of 32-bit addressable registers.

The Management PF is persistent throughout the lifetime of the instance, and it will not be reset or cleared (even during the AFI Load/Clear process).

Application PF (AppPF)

The Application PF exposes:

a) PCIe BAR0 as a 32-bit non-prefetchable BAR sized as 32MiB. This BAR maps to the the OCL AXI-Lite interface.

b) PCIe BAR1 as a 32-bit non-prefetchable BAR sized as 2MiB. This BAR maps to the the BAR1 AXI-Lite interface.

c) PCIe BAR2 as a 64-bit prefetchable BAR sized as 64KiB. This BAR is not CL visible. This BAR maps to the MSI-X tables and XDMA (if enabled).

d) PCIe BAR4 as a 64-bit prefetchable BAR sized as 128GiB. This BAR may be used to map the entire External/Internal memory space to the instance address space if desired, through mmap() type calls or use fpga_pci_lib APIs.

e) BusMaster capability to allow the CL to master transactions towards the instance memory.

f) CL’s specific PCIe VendorID, DeviceID, VendorSystemID and SubsystemID as registered through aws ec2 fpgaImageCreate

The Developer can write drivers for the AppPF or leverage the reference driver provided in the SDK.

The PCIe interface connecting the FPGA to the instance is in the Shell, and the CL can access it through two AXI-4 interfaces:


One of the four DRAM interface controllers is implemented in the Shell, and three are implemented in the CL. This allows for optimized resource utilization of the FPGA (allowing higher utilization for the CL place and route region to maximize usable FPGA resources). For those interfaces, the designs and the constraints are provided by AWS and must be instantiated in the CL (by instantiating in the CL design).

There are four DRAM interfaces labeled A, B, C, and D. Interfaces A, B, and D are in the CL while interface C is implemented in the Shell. The sh_ddr design block,, instantiates the three DRAM interfaces in the CL (A, B, D).

For DRAM interface controllers that are implemented in the CL, the AXI-4 interfaces do not connect into the Shell, but connect locally inside the CL to the AWS provided blocks. Refer to the DDR4 AXI section for more information about these interfaces.

NOTE: Even if no DDR4 controllers are desired in the CL, the block must be instantiated in the CL (parameters are used to remove DDR controllers). WARNING If the CL does not instantiate the block, it will result in implementation errors. Additionally, if the DDR C controller that's instantiated in the Shell is not desired, then the cl_sh_ddr_rready can be tied-off to 1'b0.

There are three parameters (all default to '1') that define which DDR controllers are implemented:


These parameters are used to control which DDR controllers are impemented in the CL design. An example instantiation (includes DDR_A and DDR_B, excludes DDR_D):

   sh_ddr #(.DDR_A_PRESENT(1),
          SH_DDR (

DRAM Content Preservation between AFI Loads (Future)

In future Shell versions a DRAM content preservation feature will be implemented. This feature allows the DDR state to be preserved when dynamically changing CL logic. The current Shell version will not guarantee preservation of DRAM contents if the CL logic is re-loaded.


There is an integrated DMA controller inside the Shell (Xilinx DMA), which writes/reads data to/from the CL via the sh_cl_pcis_dma bus. Because of the shared DMA/PCIS interface, this maps to the same address space exposed by the AppPF BAR4 address. Drivers and example software are provided, please refer to the example design CL_DRAM_DMA Example.

Interfaces between Shell and CL

CL/Shell AXI Interfaces (AXI-4 and AXI-Lite)

All interfaces except the inter-FPGA links use the AXI-4 or AXI-Lite protocol. The AXI-L buses are for register access use cases, and can access lower speed control interfaces that use the AXI-Lite protocol.

For bulk data transfer, wide AXI-4 buses are used. AXI-4 on the CL/Shell interfaces have the following restrictions:

  • AxBURST – Only INCR burst is supported.

  • AxLOCK – Lock is not supported.

  • AxCACHE – Memory type is not supported.

  • AxPROT – Protection type is not supported.

  • AxQOS – Quality of Service is not supported.

  • AxREGION – Region identifier is not supported.

These signals are not included on the AXI-4 interfaces of the shell. If connecting to a fabric or component that supports these signals, these constant vaules should be used:

Signal Value
AxBURST[1:0] 0b01
AxLOCK[1:0] 0b00
AxCACHE[3:0] 0b000x (bit 0 is Bufferable bit and may be 0 or 1)
AxPROT[2:0] 0b000
AxQOS[3:0] 0b0000
AxREGION[3:0] 0b0000

Clocks and Reset


There are multiple clocks provided by the Shell to the CL, grouped into 3 groups marked _a, _b and _c:

  • clk_main_a0

  • clk_extra_a1

  • clk_extra_a2

  • clk_extra_a3

  • clk_extra_b0

  • clk_extra_b1

  • clk_extra_c0

  • clk_extra_c1

clk_main_a0 is the main clock. All interfaces between the CL and SH are synchronous to clk_main_a0, and must be used by the CL.

The clocks within each group are generated from a common VCO/PLL, which restrict what combinations of frequencies are allowed within a group.

The maximum frequency on clk_main_a0 is 250MHz.

Clocks within a group are phase aligned.

NOTE: Even though clocks within a group are phase aligned, treating them asynchronous generally allows for higher individual frequencies and easier timing closure.

NOTE: The Developer must NOT assume frequency lock or alignment between clocks from different groups, even if they are set for same frequencies.

Defining Clock frequencies by Developer

There Developer can select among a set of available frequencies, provided in the clock recipe table. The recipe names are called Ax, By, Cz for group A recipe x, group B recipe y and group C recipe z respectively.

Group A recipe must be defined in the AFI Manifest, which is included in the tar file passed to aws ec2 create-fpga-image AFI registration API. Group B and C recipes are optional in the manifest file, and if they are missing the recipe B0 and/or C0 are used as default.


The shell provides an active_low reset signal synchronous to clk_main_a0: rst_main_n. This is an active low reset signal, and combines the board reset and PCIe link-level reset conditions.


Each of the four DDR4 Controllers has an AXI-4 interface with a 512 bit data bus. The AXI-4 interfaces for the three DDR4 Controllers instantiated in the CL ( are connected inside of the CL. However, for the DDR-4 controller that is instantiated in the Shell, DDRC, there is an AXI-4 interface between the Shell and CL.

Each DRAM interface is accessed via an AXI-4 interface:

  • AXI-4 (CL Master and DRAM controller is slave) – 512-bit AXI-4 interface to read/write DDR.

There is a single status signal that the DRAM interface is trained and ready for access. DDR access should be gated when the DRAM interface is not ready. The addressing uses ROW/COLUMN/BANK (Interleaved) mapping of AXI address to DRAM Row/Col/BankGroup. The Read and Write channels are serviced with round-robin arbitration (i.e. equal priority).

The DRAM interface uses the Xilinx DDR-4 Interface controller. The AXI-4 interface adheres to the Xilinx specification. Uncorrectable ECC errors are signaled with RRESP. ECC error status can be read using AWS Management Software APIs.

Additionally, there are three statistics interfaces between the Shell and CL (one for each CL DDR controller). If the DDR controllers are being used by the CL, then the interfaces must be connected between the Shell and the DRAM interface controller modules.

WARNING: If the stats interfaces are not connected, the DDR controllers will not function. However, the CL developer should not otherwise use them since they are specific to Shell management functions. If the DDR controllers are not used by the CL, then the interfaces should be left unconnected.

NOTE: There is no performance or frequency difference between the four DRAM controllers regardless whether they resides in the CL or the Shell logic

DMA_PCIS Interface -- AXI-4 for Inbound PCIe Transactions (Shell is Master, CL is Slave, 512-bit)

This AXI-4 bus is used for:

  • PCIe transactions mastered by the instance and targeting AppPF BAR4 (PCIS)
  • DMA transactions (if enabled) (XDMA)

It is a 512-bit wide AXI-4 interface.

A read or write request on this AXI-4 bus that is not acknowledged by the CL within a certain time window, will be internally terminated by the Shell. If the time-out error happens on a read, the Shell will return 0xFFFFFFFF data back to the instance. This error is reported through the Management PF and can be retrieved by the AFI Management Tools metric reporting APIs. Refer to DMA_PCIS Interface Timeout Details and for more details.

The AXI ID can be used to determine the source of the transaction:

  • 0x20 : PCI Interface
  • 0x00 : XDMA Channel 0
  • 0x01 : XDMA Channel 1
  • 0x02 : XDMA Channel 2
  • 0x03 : XDMA Channel 3

The XDMA interface has backpressure signals to stop read/write transactions. There is an independent signal per transaction type:

  • cl_sh_dma_wr_full : Stop additional write transaction requests from the XDMA
  • cl_sh_dma_rd_full : Stop additional read transaction requests from the XDMA

When asserted, the XDMA will stop sending further requests for the transaction type (AW or AR channel). Note some number of requests may be asserted after the assertion of backpressure due to FIFO'ing and register slices. This backpressure may be used to throttle XDMA requests if the XDMA requests may delay servicing PCIS requests such that the PCIS request responses would exceed the interface timeout time.

DMA_PCIS Interface Timeout Details

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The DMA_PCIS interface multiplexes the XDMA requests and PCIS requests. Each type of request has a different timeout time:

  • XDMA (DMA transactions) : 5 seconds
  • PCIS (PCIe transactions mastered from the instance) : 8 us

Transactions on the DMA_PCIS interface must complete before the associated timeout time or the SH will timeout the transactions and complete the transactions on behalf of the CL (BVALID/RVALID). Each "issued" transaction has an independent timeout counter. For example if 4 transactions are issued from the PCIS interface "simultaneously" (i.e. back-to-back cycles), then all 4 must complete within 8us. A transaction is considered "issued" when the AxVALID is asserted for the transaction by the Timeout Detection block. AxREADY does not have to be asserted for the transaction to be considered "issued". Note there is a 16 deep clock crossing FIFO between the Timeout Detection block and the CL logic. So if the CL is asserting backpressure (de-asserting AxVALID) there can still be 16 transactions issued by the Timeout Detection block. The SH supports a maximum of 32 transactions outstanding for each type (read/write). It is advisable for the CL to implement enough buffering for 32 transactions per type so that it is aware of all issued transactions.

Once a transaction is issued, it must fully completed within the timeout time (Address, Data, Ready). Any transaction that does not completed in time will be terminated by the shell. This means write data will be accepted and thrown away, and default data (0xffffffff) will be returned for reads.

If a timeout occurs, the Shell will timeout all further transactions in 16ns for a moderation time (4ms).

WARNING: If a timeout happens the DMA/PCIS interface may no longer be functional and the AFI/Shell must be re-loaded. This can be done by adding the "-F" option to fpga-load-local-image.

PCIM interface -- AXI-4 for Outbound PCIe Transactions (CL is Master, Shell is Slave, 512-bit)

This is a 512-bit wide AXI-4 interface for the CL to master cycles to the PCIe bus. This can be used, for example, to push data from the CL to instance memory, or read from the instance memory.

WARNING: The CL must use Physical Addresses, and developers must be careful not to use userspace/virtual addresses.

The following PCIe interface configuration parameters are provided from the Shell to the CL as informational:

  • sh_cl_cfg_max_payload[1:0] – PCIe maximum payload size:
Value Max Payload Size
0b00 128 Bytes
0b01 256 Bytes (Most probable value)
0b10 512 Bytes
0b11 Reserved
  • sh_cl_cfg_max_read_req[2:0] - PCIe maximum read request size:
Value Max Read Request Size
0b000 128 Bytes
0b001 256 Bytes
0b010 512 Bytes (Most probable value)
0b011 1024 Bytes
0b100 2048 Bytes
0b101 4096 Bytes
Others Reserved

Outbound PCIe AXI-4 Interface Restrictions:

  • Transfers must not violate PCIe byte enable rules (see byte enable rules below).
  • Transfers must adhere to all AXI-4 protocol rules

Byte Enable Rules

All AXI-4 transactions to the PCIe interface must adhere to the PCIe Byte Enable rules (see PCI Express Base specification). Rules are summarized below:

  • All transactions larger than two DW must have contiguous byte enables.
  • Transactions that are less than two DW may have non-contiguous byte enables.

Note on AXI-4 byte enables are signaled using WSTRB.

AXI4 Error Handling for CL outbound transactions

Transactions on AXI4 interface will be terminated and reported as SLVERR on the RRESP/BRESP signals and will not be passed to the instance in the following cases:

  • PCIe BusMaster Enable (BME) is not set in the PCIe configuration space.

  • Illegal transaction address; i.e. addressing memory space that isn't supported by the instance.

  • Transaction crossing 4KB boundaries violating AXI-4/PCIe specifications.

  • Illegal byte-enable.

  • Illegal length (AXI-4 write doesn't match length).

  • Illegal AXI-Size (only full width 512-bit transfers, size=0b110 are supported)

  • Timeout. Each channel must complete in 8 us or it will timeout:

    1. Once AW is asserted, the write data must be supplied in 8us
    2. Once RVALID is asserted, RREADY must be asserted, and all data transferred within 8us
    3. Once BVALID is asserted, BREADY must be asserted within 8us

NOTE: If a timeout occurs, the PCIM bus will no longer be functional. This can be cleared by clearing/re-loading the AFI. Refer to

AXI-Lite interfaces for register access -- (SDA, OCL, BAR1)

There are three AXI-L master interfaces (Shell is master) that can be used for register access interfaces. Each interface is sourced from a different PCIe PF/BAR. Breaking this into multiple interfaces allows for different software entities to have a control interface into the CL:

  • SDA AXI-L: Associated with MgmtPF, BAR4. If the developer is using AWS OpenCL runtime Lib (as in SDAccel case), this interface will be used for performance monitors etc.
  • OCL AXI-L: Associated with AppPF, BAR0. If the developer is using AWS OpenCL runtime lib(as in SDAccel case), this interface will be used for openCL Kernel access
  • BAR1 AXI-L: Associated with AppPF, BAR1.

Please refer to PCI Address map for a more detailed view of the address map.

AXI Slave (AXI-Lite/PCIS) CL Error reporting

Each AXI (AXI-4/AXI-L) transaction is terminated with a response (BRESP/RRESP). The AXI responses may signal an error such as Slave Error, or Decode Error. PCIe also has error reporting for non-posted requests (Unsupported Requests/Completer Abort). The shell does not propagate the AXI-4 error responses to the PCIe bus. All PCIe cycles are terminated with non-errored responses. The AXI-4 errors are reported through the Management PF and can be retrieved by the AFI Management Tools metric reporting APIs.

Accessing Aligned/Unaligned addresses from PCIe (Shell is Master, CL is Slave):

The Shell supports DW aligned and unaligned transfers from PCIe (address is aligned/not aligned to DW-4byte boundary)

Following are a few examples of how aligned and Unaligned access from PCIe to CL on DMA_PCIS interface work:

  1. Writing 8 bytes to DW aligned address through PCIe on AXI4 Interface(DMA_PCIS- 512 bit interface):

    If the transaction on the pcie is as follows:
    Addr : 0x0000002000000000 dw_cnt : 2 first_be : 4’b1111 last_be : 4’b1111

    Then the transaction on the AXI4 interface will have the following axi attributes: awaddr 64’h0000_0000_0000_0000 awlen = 0 wstrb = 64’h0000_0000_0000_00ff

  2. Writing 8 bytes to DW un-aligned address on AXI4 Interface(DMA_PCIS- 512 bit interface):

    If the transaction on the pcie is as follows:
    Addr : 0x0000002000000001 dw_cnt : 3 first_be : 4’b1110 last_be : 4’b0001

    Then the transaction on the AXI4 interface will have the following axi attributes: awddr = 64’h0000_0000_0000_0001 awlen = 0 wstrb = 64’h0000_0000_0000_01fe

The addresses for the Read transactions will work similar to writes.

Note: If a transaction from PCIe is initiated on AXI-Lite (SDA/OCL/BAR1) interfaces with dw_cnt greater than 1, i.e. >32bits, the transaction is split into multipe 32 bit transactions by the Shell.

Following are a few examples of how aligned and Unaligned access from PCIe to CL on SDA/OCL/BAR1 AXI-Lite interfaces work:

  1. Writing 8 bytes to DW aligned address on AXI Lite Interface(SDA/OCL/BAR1- 32 bit interface):

    If the transaction on the pcie is as follows:
    Addr : 0x0000000002000000 dw_cnt : 2 first_be : 4’b1111 last_be : 4’b1111

    Then the transaction on the AXI-Lite interface will be split and will have the following axi attributes: Transaction is split into 2 transfers.

    1st transfer awaddr = 32’h0000_0000 wstrb = 4’hf

    2nd transfer awaddr = 32’h0000_0004 wstrb = 4’hf

  2. Writing 64 bits to DW un aligned address on AXI Lite Interface(SDA/OCL/BAR1- 32 bit interface):

    If the transaction on the pcie is as follows:
    Addr : 0x0000000002000001 dw_cnt : 3 first_be : 4’b1110 last_be : 4’b0001

    Transaction on AXI-Lite interface will be split and will have the following axi attributes: Transaction is split into 3 transfers.

    1st transfer awaddr = 32’h0000_0001 wstrb = 4’he

    2nd transfer awaddr = 32’h0000_0004 wstrb = 4’hf

    3rd transfer awaddr = 32’h0000_0008 wstrb = 4’h1

The transaction splitting and addresses for the Read transactions will work similar to writes.


16 user interrupt source are supported. There is mapping logic that maps the user interrupts to MSI-X vectors. Mapping registers in the DMA controller map the 16 user interrupt sources to MSI-X vectors.

There are two sets of signals to generate interrupts:

  • cl_sh_apppf_irq_req[15:0] (from CL to SH)
  • sh_cl_apppf_irq_ack[15:0] (from SH to CL)

This interface uses single clock pulses for the req/ack. The CL asserts (active high) cl_sh_apppf_irq_req[x] for a single clock to assert the interrupt request to the SH. The SH will respond with a single clock pulse on sh_cl_apppf_irq_ack[x] to acknowledge the interrupt. Once the CL asserts a request on a particular bit[x], it should not assert a request for the same bit[x] until it has recieved the ack for bit[x] from the SH. The CL may assert requests on other bits[y] (y!=x).

Miscellaneous signals

There are some miscellaneous generic signals between the Shell and CL.

Integrity Check

The 64-bit ch_sh_id0/id1 are used by AWS to validate the signature of the DCP while being loaded into an FPGA in AWS.

Initial versions of the HDK and Shell used the 4-tuple: PCIe VendorID, DeviceID, SubsystemVendorID and SubsystemID (which are used during AFI registration via aws ec2 create-fpga-image API) as the Integrity check mechanism, following the next mapping

  • cl_sh_id0

    • [15:0] – Vendor ID

    • [31:16] – Device ID

  • cl_sh_id1

    • [15:0] – Subsystem Vendor ID

    • [31:16] – Subsystem ID

In future revisions of the HDK, AWS scripts may override the cl_sh_id0/id1 to include an integrity hash function.

General Control/Status

The functionality of these signals is TBD.

  • cl_sh_status0[31:0] – Placeholder for generic CL to Shell status.

  • cl_sh_status1[31:0] – Placeholder for generic CL to Shell status.

  • sh_cl_ctl0[31:0] – Placeholder for generic Shell to CL control information.

  • sh_cl_ctl1[31:0] – Placeholder for generic Shell to CL control information.

  • sh_cl_pwr_state[1:0] – This is the power state of the FPGA.

    • 0x0 – Power is normal

    • 0x1 – Power level 1

    • 0x2 – Power level 2

    • 0x3 – Power is critical and FPGA may be shutting off clocks or powering down

Virtual LED/DIP

There are virtual LED/DIP switches that can be used to control/monitor CL logic. There are 16 LEDs and 16 DIP Switches. Registers exposed to the Management PF are used to control/monitor the LED/DIP Switches.

vLED - There are 16 virtual LEDs that can be driven from the CL logic to the SH (cl_sh_status_vled[15:0]). The value of these signals can be read by S/W in the Instance. An API is also provided through AWS Management Software.

vDIP - There are 16 virtual DIP switches that drive from the SH to the CL logic (sh_cl_status_vdip[15:0]). These can be used to control logic in the CL. The value of these signals can be written/read by S/W in the instance. An API is also provided through AWS Management Software.

These signals are asynchronous to the CL clocks, and the following must be done when using these signals:

  • vLED: In implementation a false path should be set from the vLED signals. For example in the constraints for place and route add:

       set_false_path -from [get_cells CL/cl_sh_status_vled_reg*]
  • vDIP: The vDIP signals should be synchronized to a CL clock before being used.

       always @(posedge clk_main_a0)
           pre_sync_vdip <= sh_cl_status_vdip;
           sync_vdip <= pre_sync_vdip;
        my_logic = sync_vdip[0];

Global Counters

There are two global counter outputs that increment every 4ns. These can be used to measure time inside of the CL. They are synchronized to clk_main_a0. Note if clk_main_a0 is running slower than 250MHz, the counters will appear to skip values. The counters are:

  • sh_cl_glcount0[63:0]
  • sh_cl_glcount1[63:0]

Implementation Tips

Here are some implementation tips.


The VU9P FPGA is a stacked FPGA that has 3 die stacked together. Each Die is called a “Super Logic Region” (SLR). Crossing an SLR boundary is expensive from a timing perspective. It is good practice to pipeline interfaces between major blocks to allow the tool freedom to have SLR crossings between the major blocks. Even with pipelined interfaces it is possible the tool has sub-optimal logic to SLR mapping (i.e. a major block is spread out over multiple SLR's). In this case you may want to at map major blocks to specific SLRs (define the logic that should be constrained to each SLR). Any crossing of SLR’s should have flops on either side (or register slices for AXI).

It is ideal to place logic that interfaces to the shell in the same SLR as the Shell logic for that interface. If this is not possible, the first flop/register slice should be placed in the same SLR:

  • MID SLR:
    • CL_SH_DDR
    • BAR1
    • PCIM
    • PCIS
    • OCL
    • DDR STAT3
    • DDR STAT0
    • DDR STAT1
    • SDA

For the interfaces that are in both the MID/BOTTOM the recommendation is to use flops for pipelining, but don’t constrain to an SLR. Also it is recommended to not use the SDA interface because it spans two SLR's (use BAR1 or OCL instead). You can constrain logic to a particular SLR by creating PBLOCKs (one per SLR), and assigning logic to the PBLOCKs (refer to cl_dram_dma example cl_pnr_user.xdc). Dataflow should be mapped so that SLR crossing is minimized (for example a pipeline should be organized such that successive stages are mostly in the same SLR).

Here’s an example post on the Xilinx forum which points to some documentation related to solving this:

There are some good timing closure tips in this methodology doc pointed to by the Xilinx forum post:

Logic Levels

You can report all paths that are greater than a certain number of logic levels. This can be used to iterate on timing in synthesis rather than waiting for place and route. For example at 250MHz a general rule of thumb is try to keep logic levels to around 10. The following commands report on all paths that have more than 10 logic levels:

  • report_design_analysis -logic_level_distribution -of [get_timing_paths -max_paths 10000 -filter {LOGIC_LEVELS > 10}]

  • foreach gtp [get_timing_paths -max_paths 5000 ?nworst 100 -filter {LOGIC_LEVELS > 10}] {puts "[get_property STARTPOINT_PIN $gtp] [get_property ENDPOINT_PIN $gtp] [get_property SLACK $gtp] [get_propert LOGIC_LEVELS $gtp]"}


Reset fanout can be minimized in an FPGA. This helps with routing congestion. Flops can be initialized in their declaration and generally do not require resets:

logic[3:0] my_flops = 4’ha;

If logic must have a reset, use synchronous resets rather than asynchronous resets:

always @(posedge clk)
   if (reset)
      my_flop <= 4’ha;
      my_flop <= nxt_my_flop; 

If there is still significant fanout of reset, it should be replicated and pipelined. For example each major block could have its own pipelined version of reset.

Pipeline Registers

You have to be careful that pipeline registers do not infer a shift register component. The shift register is placed in a single area and does not accomplish any distance pipelining. Here is a snippet to force the tools to not infer a shift register (shreg_extract="no" directive):

  (*shreg_extract="no"*) logic [WIDTH-1:0] pipe[STAGES-1:0] = '{default:'0};

Vivado Analysis

Vivado has some nice analysis capabilities:

  • report_methodology (includes CDC report)
  • clock interaction report (see if paths between async clocks are erroneously being timed)
  • congestion heat map
  • power analysis
  • physical implementation analysis (placement, routing)
  • linked timing/schematic/physical views