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pcb/main board: add errata
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# Main Board
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50x50mm microcontroller module, driving LedArrayMux modules. Contains an RP2040 microcontroller, a CAN transceiver, PWM audio output, current sensing, and level shifters to work around LedArrayMux bugs.
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## Revisions
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### Rev A
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Errata:
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1. Level shifters are too slow to be useful
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- Workaround: decrease pullup values to obnoxiously low ones (100-180Ω); do not use level shifters; increase PIO delays until functional
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2. CAN receive signal is not level shifted, which exposes RP2040 to a 5V signal
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- Workaround: Do nothing; replace with a 3v3 CAN transceiver
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3. The 431 shunt voltage reference does not work correctly
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- Workaround: use main 3v3 regulator for vref
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4. PWM audio signal filter is suspected to be responsible for poor quality audio
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