This repository contains Verilog modules that are common to multiple NewAE projects.

Notes regarding FIFOs

These FIFOs were developed to overcome issues with simulating the Xilinx FIFOs with the Icarus Verilog simulator. They are not (yet) intended to be a full replacement for Xilinx FIFOs in implementation. Tests show that the Xilinx FIFOs result in better resource utilization and timing closure. However, these FIFOs simulate much faster than the Xilinx ones.

The fifo_sync module aims to have the exact same behaviour as the Xilinx synchronous FIFO, and while there is some limited testing for this, this is not exhaustively tested or proven (yet).

The fifo_async currently does not have the same behaviour as the Xilinx asynchronous FIFO (i.e. in terms of timing of its empty/full status signals).

Notes regarding SS2 wrapper

ss2.v implements the SimpleSerial v2.1 protocol (SS2) and maps SS2 commands to CW305 register read/writes. This allows existing CW305 design which use the SAM3U USB parallel interface to be very easily ported to the CW312T-A35 and CW312T-iCE40 target boards.

Examples exist for our existing CW305 AES, CW305 AES pipeline and CW305 ECC projects, ported to the CW312T-A35.

For the iCE40, there is the CW305 AES project.

SS2 Wrapper Usage

The SS2 wrapper was designed to minimize the work required to port CW305 target designs to target platforms which lack the CW305's SAM3U USB parallel interface, on both the FPGA development side and the host Python side (for CW305 designs that follow our example AES/ECC designs).

On the Verilog side, refer to the AES and ECC examples linked above to see what's needed. As per these examples, the wrapped CW305 top level module's pBYTECNT_SIZE parameter should be set to 8, and its pADDR_WIDTH parameter should be set to 32. The latter is required by ss2.v; the former could in theory be changed, but CW305.py expects it to be 8. Stray from these recommended values at your own risk.

On the Python side, connect to an SS2 target design by adding the platform argument:

target = cw.target(scope, cw.targets.CW305, bsfile=<bitstream file>, platform='ss2')

and everything should "just work". Calls to methods tied to CW305-specific hardware which does not exist on the CW312T-A35 or CW312T-iCE40 (external PLL, programmable supply voltage, USB clocking) will result in warnings.

Look at the CW305 AES and ECC notebooks to see how the same Python code can interact with a CW305 target running on either the CW305 natively, or wrapped by SS2 on the CW312T-A35 / CW312T-iCE40.

SS2 Wrapper Resource Overhead

The SS2 wrapper uses relatively little FPGA resources: 2% of the xc7a35's slice LUTs and less than 1% of its slice registers.

On the much smaller iCE40, the overhead is much more significant: 20% of logic cells. However, the basic CW305 AES target does fit (just barely!).

SS2 Wrapper Clocking

The SS2 wrapper easily meets timing at 100 MHz on Xilinx 7-series. However it is intended to be clocked at 7.37 MHz since its baud rate (230400 bps) is hard-coded in the Verilog source. This can be easily modified via the pBIT_RATE parameter in ss2.v, which indicates the number of clock cycles per UART bit. Of course it's also possible to scale the clock rate and baud rate together (e.g. double the baud rate to 230400 * 2 = 460800 bps and double the clock rate to 7.37e6 * 2 = 14.74e6 MHz).

SS2 Wrapper Performance

While SimpleSerial v2.1 runs at a speedy 230400 baud, it is still considerably slower than the CW305's SAM3U-based parallel register interface. You can expect captures to run at about half the rate they would on the CW305. Performance will depend on the length of the target operation and the number and size of the target reads and writes that are required.

SS2 Wrapper Commands

Most SS2 wrapper commands use 0x23 as a command byte. The SS2 wrapper supports 3 subcommands:

• write: cmd=0x23, scmd=0x57, dlen=[size of data], data=[4-byte address, write data]
• read: cmd=0x23, scmd=0x52, dlen=5, data=[4-byte address, number of bytes to read]
• echo: cmd=0x23, scmd=0x45, dlen=4, data=[4-byte payload]

The purpose of the echo command is to test that UART communication is up: it is handled automatically by the SS2 wrapper without involving the underlying target.

These commands are exhaustively tested in simulation by the SS2 testbench.

There is also a command to reset the SS2 wrapper (and only the SS2 wrapper):

• ss2_reset: cmd=0x24, scmd=0x72, dlen=1, data=[single byte (not used)]

(This is not yet properly validated in simulation, but on-target testing shows that it works as intended.)

If you use CW305.py then you don't need to know about these details; everything is handled under-the-hood by mapping fpga_write() and fpga_read() to the corresponding SS2 commands.

SS2 Next Steps

A future goal is to allow for CW305 targets which use both the parallel SAM3U USB interface and UART (such as our DesignStartTrace) by expanding the SS2 wrapper to allow encapsulating UART commands inside SS2 commands.