SPI not transferring when sck pin not used

[JVKran]

Hi there!

I'm in the process of migrating a project from a nRF52832 to nRF52840 due to insufficient memory and of course some problems arise during the process. Most of the problems are configuration related, but I believe this one to be a bug.

When the sck_pin is set to NRF_DRV_SPI_PIN_NOT_USED, the transfer isn't completed. In fact, the pin is made high and becomes low after 1.6ms; then all nrf_drv_spi_transfer calls return NRF_ERROR_BUSY. This is the case in both, the project and the spi example in the most recent version of the SDK. What I find interesting, is that setting the sck_pin to NRF_DRV_SPI_PIN_NOT_USED works with the nRF52832 DK, but not with the nRF52840 DK.

The problem is temporarily solved by just initializing the sck_pin with a random (unused) pin. Anyway, I thought I'd raise this question to save a couple days of debugging for people who are also coping with this problem and to get to know why this is happening.

Thanks in advance,

Jochem

[awneil]

Why would you set the SCK pin as 'not used' - surely that makes no sense?!

SCK is essential to SPI.

[JVKran]

Sorry, forgot to add that I wrote a library for the WS2812 leds. Using SPI at 4MHz with DMA is a very efficient way to control these leds. Then, 4 bits represent 1 bit in the WS2812 protocol. Hence MOSI is connected straight to the DIN pin of the leds and other pins are not used nor necessary.

[awneil]

Aha - that's a key bit of information! Yes, I have done this on nRF52832; haven't tried nRF52840.

Strictly, it is using the SPI peripheral in a way it's not intended - I guess we'll have to wait for the Nordic guys to say if there's anything that can be done ...

[JVKran]

Yep, haha. I know it's kinda mis-using SPI, but still; I wouldn't expect the macro to completely prevent transactions from completing. 

At least there's a temporary fix; wouldn't be the end of the world if there isn't a proper solution.

[awneil]

Have you tried looking at (or stepping through) the code to see if there's anything obvious that's messed up by having the SCK pin as 'Not Used' ... ?