Setup Circuits.SPI GPIO?

Nerves Forum Nerves Questions / Help
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i’m trying to use Circuits.SPI from the basic mix nerves.new template. I can ssh in and then from the iex terminal I can open a SPI bus ({:ok, spi0} = Circuits.SPI.open("spidev0.0")) . Then I can run a transfer: {:ok, resp} = Circuits.SPI.transfer(spi0, <<0xAA, 0xFF>>) which returns {:ok, <<0, 0>>}. So from a software perspective, everything seems like it’s working. But I can only get 0’s back, and when I look at the SPI pins with an oscilloscope I see no activity on any of them during when the transfers are supposed to happen.

I’ve noticed that if I check the status of the relevant GPIO, that pins that ought to be outputs are reported as inputs:

iex(15)> Circuits.GPIO.status("P9_22 [spi0_sclk]")
{:ok, %{pull_mode: :none, direction: :input, consumer: "P9_22"}}

Does that indicate something significant, and do I need to do something more to setup SPI? The last time I used nerves was several years ago and I remember stuff like this being very fluid - no extra setup required, so I’m suspicious that I’ve done something wrong…

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I don’t know why I do this so often, but it seems like every time I write up a question about something on the internet I figure the answer out for myself 10 minutes later.

For posterity, what I was missing was that I need to use config-pin on the bbb to setup the pins in SPI mode. There is a line about this in the Circuits.SPI readme, but because the SPI peripheral is enabled in the device tree and shows up I incorrectly thought this was not the issue.

Sorry for the noise!

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