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Explain seed argument to foboot-bitstream.py

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Update README to say what to do if the place-and-route process fails
to find an appropriate solution.
This commit is contained in:
Peter Chubb 2020-01-23 10:26:34 +11:00
parent be3dc254c5
commit 485d232563
1 changed files with 23 additions and 2 deletions
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23
README.md
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@ -23,11 +23,32 @@ To build the software, you need:
The hardware half will take care of building the software half, if it is run with `--boot-source bios` (which is the default). Therefore, to build Foboot, enter the `hw/` directory and run:
```
$ python3 foboot-bitstream.py --revision hacker
$ python3 foboot-bitstream.py --revision hacker --seed 19
```
This will verify you have the correct dependencies installed, compile the Foboot software, then synthesize the Foboot bitstream. The resulting output will be in `build/gateware/`. You should write `build/gateware/top-multiboot.bin` to your Fomu device in order to get basic bootloader support.
The `seed` argument is to set initial conditions for the
place-and-route phase. `nextpnr-ice40` uses a simulated annealing
algorithm that can result in one of several locally optimal layouts.
Only some of these will meet the timing requirements for Fomu.
If you see something like
```
ERROR: Max frequency for clock 'clk48_$glb_clk': 45.41 MHz (FAIL at 48.00 MHz)
```
try a different seed. You can search for an appropriate seed with:
```
for seed in $(seq 0 100)
do
python3 ./foboot-bitstream.py --revision pvt --seed $seed 2>&1 |
grep 'FAIL at 48.00 MHz' && continue
echo "Working Seed is $seed"
break
done
```
This can take a considerable time.
### Usage
You can write the bitstream to your SPI flash. If you're using `fomu-flash`, you would run the following: