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Explain seed argument to foboot-bitstream.py
Update README to say what to do if the place-and-route process fails to find an appropriate solution.
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README.md
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README.md
@ -23,11 +23,32 @@ To build the software, you need:
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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:
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```
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$ python3 foboot-bitstream.py --revision hacker
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$ python3 foboot-bitstream.py --revision hacker --seed 19
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```
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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.
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The `seed` argument is to set initial conditions for the
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place-and-route phase. `nextpnr-ice40` uses a simulated annealing
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algorithm that can result in one of several locally optimal layouts.
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Only some of these will meet the timing requirements for Fomu.
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If you see something like
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```
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ERROR: Max frequency for clock 'clk48_$glb_clk': 45.41 MHz (FAIL at 48.00 MHz)
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```
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try a different seed. You can search for an appropriate seed with:
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```
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for seed in $(seq 0 100)
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do
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python3 ./foboot-bitstream.py --revision pvt --seed $seed 2>&1 |
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grep 'FAIL at 48.00 MHz' && continue
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echo "Working Seed is $seed"
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break
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done
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```
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This can take a considerable time.
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### Usage
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You can write the bitstream to your SPI flash. If you're using `fomu-flash`, you would run the following:
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@ -69,4 +90,4 @@ $ dfu-util -e
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## Building the Software
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Software is contained in the `sw/` directory.
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Software is contained in the `sw/` directory.
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