SPI transfer in timer handler retrieves wrong values, hangs and crashes

RE: SPI transfer in timer handler retrieves wrong values, hangs and crashes

user112696 — Mon, 13 Jun 2022 15:12:18 GMT

OMG, I had actually read that exact paragraph (in the Zephyr docs) but I hadn't looked properly at the code and I assumed that when I use an expiry function it will always run it in the system workqueue thread.

But no, the timer handler is part of the ISR and if I want to defer the work to a "userspace" thread I have to do that myself.

I'm not really surprised that doing SPI I/O is not possible in an ISR, I just didn't know the timer handler is executed while still handling the interrupt.

RE: SPI transfer in timer handler retrieves wrong values, hangs and crashes

user111520 — Mon, 13 Jun 2022 13:42:03 GMT

So what you're doing in your code is similar to this example?

Yes, the timer expiry function is executed at interrupt level.

-Einar

RE: SPI transfer in timer handler retrieves wrong values, hangs and crashes

user112696 — Mon, 13 Jun 2022 12:42:55 GMT

Well, in the demo program above that is intentional, yes, to demonstrate that it works from the main function but not from the timer expiry function.

For now I used a workaround similar to the semaphore, I used a message queue with the main loop blocking on k_msgq_get() and the timer expiry function publishing to the message queue. But since I couldn't find anything about (not) doing SPI transfers in a timer handler I assumed I was doing something wrong.

Does the timer expiry function actually run as part of an ISR? I assumed Zephyr internally handles the actual timer interrupt and then runs the expiry function on a normal thread.

RE: SPI transfer in timer handler retrieves wrong values, hangs and crashes

user111520 — Mon, 13 Jun 2022 12:00:43 GMT

Hello!

It looks like you're calling read_byte_from_spi() both in your main loop and in your timer handler, is this intentional?

Also, I would advise against reading spi inside your timer interrupt, this could be why you're seeing unexpected behavior. Instead I would use a semaphore to signal from the interrupt that the timer has expired, and do the reading in your main function or another thread.

Best regards,

Einar