# ocularium test firmware

This is Arduino-based test firmware written to derisk systems quickly before moving to a Rust
implementation.

## SD bug (?)

This firmware _doesn't_ work with the stock `SD.h` that ships with the RP2040 Arduino core. It needs
the following change at the end of `Sd2Card.cpp:cardCommand`:

```c++
  // XXX: Assume CMD0 succeeds.
  if (cmd == CMD0)
  {
      spiRec();
      status_ = 0x1;
  }
```

With this, it works perfectly, but without, it gets stuck unable to ever initialize the card.

Logic analyzer says the card (I've tried multiple brands) sends an `0x7f` (general error) in the
byte after the CMD0 sequence completes, and _then_ sends the expected `0x01` R1 in the following
byte. The SD library is looking for any byte starting with a 0 bit (indicating message presence), so
treats this as a genuine error and always fails. If we change the implementation to ignore this
byte, the rest of the initialization succeeds without error.

Currently, I'm just applying a transient patch to my local PlatformIO download of the RP2040 core to
resolve this temporarily.