0152-2023-10-04.md

4 Oct 2023

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SPI flash ROM next steps

• Implement test bench:
  • Want to get a VCD to check SPI timing.
  • Use iverilog, verilator, or cocotb?
  • Try running cocotb on Windows? It needs iverilog too.
• When was the last time I wrote a TB and cocotb tests? Was it solo_squash? Examples:
  • solo_squash:test/test_solo_squash.py
  • tt03p5-solo-squash:src/test.py
  • wrapped_rgb_mixer:caravel_rgb_mixer/test_rgb_mixer.py
  • robojan's TT04 Tiny Breakout includes some nice tests in the src dir of the repo: tb.v; test.py; Makefile
• In Z2A, cocotb is introduced in part 2.1, and used again in 5.1

Set up Python on Windows, to be used by cocotb

I've done this, and created Tip 2310A to describe the process.

Set up cocotb and iverilog on Windows

I originally referred to the official iverilog on Windows setup guide but then found this Fandom page which shows how to compile on Windows (linking to a page with a lot more detail about doing this with MSYS2) but in the end it was just easier to use pacman in MSYS2.

Just see my guide in 2310B for how to do this.

Creating the tests

NOTE: Consider this superseded compared to steps I've now got in 0156.

1. I'll use cocotb for now.
2. In the project dir, create test/ -- NO: TT convention is to place tests directly in src/
3. In there, create the testbench, tb.v. Main points:
   • Define inputs and outputs that we want the tests to have control over, giving them meaningful names.
   • Have an initial block that specifies the $dumpfile (in this case tb.vcd).
   • Instantiate and wire up the design, calling it uut. In this case, it's the vga_spi_rom module.
4. Create a .venv for cocotb and these tests.
5. Create test.py and fill it in -- more to come.
6. Create Makefile that uses cocotb -- more to come.
7. Write README describing how to set up everything and run the tests.

Notes

• Simulated spiflash inside a Caravel testbench
• In Verilog, given an assign is like a direct wire/net connection, does it matter whether something is on LHS or RHS, if using simple direct signals rather than expressions?