µSD-card logging with flow and loco deck finally solved

For a long time issue #270 has been bugging us. It caused the µSD-card logging to fail in combination when using either the flow or loco deck, or actually any deck that uses the deck SPI bus. Several attempts has been made to fix this issue over time and recently we decided to really dig in to it. There has been some workaround to move the µSD-card to a different SPI bus but that was tedious and required patching the deck. So it was time to fix this for good, or at least know why it doesn’t work. A SPI bus is designed to be a multi-bus so it should be possible… Timing problems is still tricky but that is another story.

The problem

The SPI driver is protecting the bus with a mutex to prevent several clients to access it at the same time. After some digging we found that the FatFs integration layer was bugged and that SPI bus handling wasn’t well done. After comparing this to some other open implementations we found that this needed to be rewritten.

The solution

After rewriting part of the integration layer to have clear path of when the SPI bus was taken, and when it was released, we immediately got some good results. µSD-card logging with flow and loco deck worked, hooray! There is of course a limit to this and as we mentioned earlier the bus is a shared resource and if it is to congested, things will slow down, or stop working. This is currently the case when LPS is put in TWR mode. The TWR is very chatty and causes around 15k transactions per seconds on the SPI bus, and since it has higher priority than the µSD-card logging, the µSD-card write task starves, causing the logging to fail.

µSD and LPS SPI bus captured with a logic analyzer, over 50ms
µSD and LPS SPI bus captured with a logic analyzer, over 6ms

So if you stay away from LPS in TWR mode µSD-card logging should now work fine. I’m pretty sure there is a workaround for the TWR mode as well. First guess is that you would need to slow down the TWR update rate which is now at its maximum.

Happy logging!

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