SPI on Gem

minovski

Thanks for the Gem - really thrilled to see Bela evolve.

I’m trying to drive NeoPixels with it which require very strictly timed signal. So far I’ve tried the SPI example project but that seems to default to /dev/spidev2.1, which doesn't exist on my system (I only see /dev/spidev2.0). When I try to run the project as-is, I get:

SPI: Failed to open /dev/spidev2.1 |: No such file or directory
SPI: SPI_IOC_MESSAGE Failed |: Bad file descriptor

I suspect the Device Tree overlay for the Gem only exports the first chip select so maybe a .dtbo is missing or some tweak in uEnv but not sure how that'd mix with Bela's internal comms.

I've used the second PRU before for this as an alternative route but noticed the assembler example project isn't there anymore. I realize the AM62x architecture is different from the old AM335x (also why config-pin CLI is missing), for which there was a dedicated PRU package so that seems like a tougher one now.

Any hints would be appreciated.

giuliomoro

/dev/spidev2 is busy on Gem for the ADC.

Other SPI busses are available, but I recommend you use the O2L project for LEDs. This runs the neopixels from the second PRU and works with Gem. Start from this branch: https://github.com/giuliomoro/O2L/tree/andrew. As is, it will simply write to the neopixels these test values https://github.com/giuliomoro/O2L/blob/andrew/main.cpp#L178-L190 . For more useful applications, you'll want to uncomment #define USE_OSC at the top and send OSC to it from a running Bela program, as exemplified in the local.pd or local.scd files.

This sends the output from P2.20, though you need to run devmem2 0xf40c8 w 0x00040007 to enable that pin after each reboot. This is not a Bela project (i.e.: it doesn't process audio), but can be run as a service alongside a Bela project. For that, do:

cp projects/O2L/O2L.service /lib/systemd/system/
systemctl daemon-reload
systemctl enable --now O2L

minovski

Thanks! Forked that to support APA102 since these have much better brightness control. Leaving that here in case anyone finds it useful.

Ward

giuliomoro /dev/spidev2 is busy on Gem for the ADC.

Can I use spi2 for other stuff if I disable the Gem ADC (or if it isn't even connected), like we can with the spi used for the ADC/DAC's on the legacy Bela?

giuliomoro

yes