nrf51822 UART tx interrupt not firing, arbitrarily when soft device is enabled.

RE: nrf51822 UART tx interrupt not firing, arbitrarily when soft device is enabled.

Ulrich Myhre — Sun, 17 Apr 2016 19:15:09 GMT

Sorry, I can't really explain that behavior. The UART should not be touched at all by the SoftDevice. Are you checking for UART errors in the IRQ handler as well? Is the chip otherwise operative in the interrupt-less state, or is it sleeping? If it is doing e.g. directed advertisements, it will block all incoming interrupts for 1.28s. This is due to a PAN in the earlier chip revisions that requires the SD to own the CPU whenever the radio is on.

RE: nrf51822 UART tx interrupt not firing, arbitrarily when soft device is enabled.

SBatt — Sun, 17 Apr 2016 13:33:22 GMT

Thanks Ulrich for the response, though I am not using WFI, WFE or sd_app_evt_wait() anywhere. I removed it from the ble_hrs app because I have processes in the main loop.

I did solve the problem, however, on a whim. Anytime I send a packet out my serial driver I have to start tx (NRF_UART0->TASKS_STARTTX = ENABLE) and then when the entire packet is emptied in the interrupts I had to stop (NRF_UART0->TASKS_STOPTX = ENABLE). Prior to this I just enabled TX during initialisation, which as mentioned in my original question, works fine when the soft device isn't enabled.

Any ideas why this fixed my problem?

I couldn't find it mentioned in the reference manual, but I may have missed it. I'm hesitant to call it 100% fixed because I can't explain why it worked. I'm running some tests now and it's been successful so far.

Cheers, Sam

RE: nrf51822 UART tx interrupt not firing, arbitrarily when soft device is enabled.

Ulrich Myhre — Sun, 17 Apr 2016 10:42:02 GMT

Except for the peripherals that are reserved/restricted, the SoftDevice should not touch anything else. What could be happening is that you call a form of WFE (e.g. sd_app_evt_wait()) right when the interrupt triggers, also from a higher (i.e. lower numbered) priority than the UART interrupt. The call will then immediately return, but also clear the pending bit. If you then WFE again, without servicing the interrupt, you will not be woken up again from it.

RE: nrf51822 UART tx interrupt not firing, arbitrarily when soft device is enabled.

SBatt — Sun, 17 Apr 2016 10:01:12 GMT

More info: When I set the priority of the UART interrupt to 0, the problem doesn't occur. Though the stack we become un-reliable, so this is not a solution. Is it possible that the BLE SD is clearing the UART TX interrupt within a higher/equal priority interrupt?