Toggle gpio certain number of times with PPI

RE: Toggle gpio certain number of times with PPI

grantka — Thu, 28 Mar 2019 14:08:31 GMT

This is what I ended up doing, except I took out PPI entirely. My /DRDY event interrupt disables the event and kicks off the SPI transfer, then the SPI completion interrupt re-enables the GPIOTE event. Very simple and works well.

RE: Toggle gpio certain number of times with PPI

hmolesworth — Wed, 27 Mar 2019 17:24:29 GMT

I see the issue; that would imply using the /DRDY event to start a hardware timer which would provide a delay long enough to ensure the interrupt would execute before that timer would trigger the SPI transfer via a PPI compare event. Alternatively and simpler since the update rate is so slow - 10 or 80 times per second - the event interrupt could simply set a flag which the wakeup in main() following this interrupt would check and use to first disable the PPI and restore the /DRDY and then manually start the SPI transfer. The SPI completion interrupt would then revert the /DRDY back to PPI wakeup.

RE: Toggle gpio certain number of times with PPI

grantka — Wed, 27 Mar 2019 16:32:27 GMT

I had to change this to not be the answer. I cannot seem to get this solution working reliably. As far as I know, there is no way to disable the ppi channel after the /DRDY event is triggered to prevent it from re-firing besides using the gpiote event callback. Unfortunately, the timing is such that every once in a while the PPI event fires twice before the irq can disable it, resulting in erroneous reads.

I proved to myself that this is the cause by injecting a delay in the irq before calling nrfx_gpiote_in_event_disable and noted that the probability of misreads went up as more delay was injected, and it only took 10us to misread every time.

RE: Toggle gpio certain number of times with PPI

grantka — Wed, 20 Mar 2019 15:57:44 GMT

thanks for the response! I'm trying to implement this now but not sure how to appropriately change /DRDY back to SPI MISO function.

I have a GPIOTE event when /DRDY transitions HITOLO, and I have that hooked up to PPI to start the SPI transfer.

Can I use the GPIOTE interrupt to disable GPIOTE on /DRDY while the SPI transfer takes place? Or will that interfere with how PPI receives the event?

RE: Toggle gpio certain number of times with PPI

hmolesworth — Fri, 15 Mar 2019 16:50:02 GMT

You might try SPI; you would still have to use /DRDY as the trigger by changing the pin to the input sense and then changing it back to the SPI MISO function. Unless you require to poll /DRDY at other places in the code then the 25th clock is superfluous; just use 3-byte 24-bit transfer then revert /DRDY to wait for the next transition. The change from the L or H state - depending on last data bit - would be handled by the AFE when it generates the next falling edge, as it automatically pusghes /DRDY high before issuing falling edge.