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

PaulELong

@giuliomoro how do you disable ADC? I have already have an ESP32 connected via SPI that I was experimenting on my bela mini. But now I want to try the same thing on bela gem. Are there steps you can point me to?

giuliomoro

You don't need to disable the ADC to use SPI: SPI0 is available on the pins without needing to disable SPI2 which is used by the Bela ADC. If you want to disable the ADC and take over SPI2, simply turn off "Use analog" in the analog inputs in the project settings.

PaulELong

Thanks @giuliomoro , I'm using d0-d11, so I think that means I can't use SPI1? I'm also using supercollider, so do I need to change a setting in the configuration?

giuliomoro

SPI0_D1 -> P2.25 (D10)
SPI0_D0 -> P2.27 (D11)
SPI0_CLK -> P2.29 (D12)

If you do unidirectional SPI, you only need CLK and either of D1 or D0; if you do bidirectional, you need all three. If you are using only D0 to D11, then you can move the signals from D10 and D11 to other pins (e.g.: D13, D14, D15).

To enable SPI0 you need to have a dedicated device-tree overlay that sets the pinmuxing for those pins to the desired one. At that point, D10, D11 and D12 will just be disconnected from the pin and so there's no need to change any settings in supercollider if you use SPI0.

If you use SPI2, then you need to disable the Bela ADC from Supercollider. That's done with

s.options.numAnalogOutChannels = 0;
s.options.numAnalogOutChannels = 0;

I believe.

PaulELong

Does that [s.options.numAnalogOutChannels=0] disable line out as well? or just A0-A7?

PaulELong

I see this when setting to 0:
Invalid number of analog channels: 0. Valid values are 2, 4, 8.
Error in SC_BelaDriver:😃riverSetup(): unable to initialise audio
could not initialize audio.
Server 'localhost' exited with exit code 1.

PaulELong

To be clear @giuliomoro I need to get analog in and out, does that mean SPI2 is not an option?

giuliomoro

PaulELong I see this when setting to 0:
Invalid number of analog channels: 0. Valid values are 2, 4, 8.
Error in SC_BelaDriver:😃riverSetup(): unable to initialise audio
could not initialize audio.
Server 'localhost' exited with exit code 1.

then maybe analog inputs cannot be disabled from within Supercollider.

PaulELong I need to get analog in and out, does that mean SPI2 is not an option?

There's no analog out as such on Gem: audio outputs 2 to 9 work similarly to legacy Bela analog outputs (DC-coupled, can reach +5V), but are addressed as audio outputs (because that's what they are). If using analog inputs, then SPI2 is not an option and you should use SPI0 instead, which conflicts with some of the D pins as mentioned above. If you don't need a lot of bandwidth from the SPI, the spi-gpio driver is available to bitbang SPI on any set of spare GPIO pins; you'll probably get around 100kHz bit clock that way.