LPCOMP occasionally missing interrupt

[Quantum_Quail]

Hi, 

I"m using SDK 15.3.0 with an NRF52840 dev board. I'm trying to record the timings from an incoming IR signal. In doing so I'm using LPCOMP with an analog input pin to record IR signals which will toggle at frequencies ranging from 1.2 ms to 2.4 ms. I'm using LPCOMP because the amplitude of the IR signal often doesn't reach the reference voltage (3.3V). I've attached an interrupt to LPComp that will toggle both an indicator light (LED4) and a gpio connected to the oscilloscope (OUT_PIN). LPCOMP is configured to a Prescaler of 4/8ths and meant to fire an interrupt on all Cross events. 

After hooking up the entire system on the oscilloscope, the interrupt occasionally fails to trigger. The yellow line is the OUT_PIN while the blue line is the incoming IR signal in the pictures below. Most of the time, OUT_PIN toggles properly thus emulates the incoming IR signal. However, occasionally the OUT_PIN will toggle for no reason or miss toggles (reference the oscilloscope readings pic below). In addition, I'm facing another bug where if I turn the LPCOMP Prescaler to anything other than 4/8 the indicator light (LED4) will continue toggle but the OUT_PIN will not give any readings on the oscilloscope and stay flat.

Is there perhaps a way to increase the frequency of LPCOMP? Or any other way to ensure that an interrupt is not skipped? 

Oscilloscope Readings: 

In this picture below, the LPCOMP interrupt misses a few edges and also toggles quite a bit extra following the end of a pulse.

LPComp Init:

[Jørgen Holmefjord]

Hi,

Are you testing this with a minimal example, or does the application perform other tasks as well? If other higher priority tasks are blocking you from clearing the event in the interrupt handler, the event/interrupt may not be generated on the subsequent crossings.

Could you also try to toggle the GPIO using PPI+GPIOTE, to make sure this is only done by this event? You can use this sample code, but connect the EVENTS_CROSS from LPCOMP to PPI EEP, instead of the TIMER event.

Best regards,
Jørgen

[Jørgen Holmefjord]

You can use PPI to trigger one of the CAPTURE tasks of the TIMER, which will store the counter value in the corresponding CC register. You will have to read out the CC value in the interrupt, but at least you avoid the delay from event to interrupt is triggered to capture the value. As far as I know, there is no way to store CC value directly into RAM buffer.

[Quantum_Quail]

My original setup is exactly as you have detailed, PPI connecting a TIMER Task out and LPCOMP Event, with an LPCOMP, interrupt attached that collects the CC value. However, since the LPCOMP interrupt misses edges, it throws the entire array of times off. Do you have any suggestions/alternative solutions you could recommend for timing an IR signal?

[Jørgen Holmefjord]

I see. Are you running the LPCOMP interrupt at the highest possible priority? Are you using the softdevice in your application?

[Quantum_Quail]

I am using the soft device and I've tried setting the interrupt to priority 2 but the results are still the same. Do you think amplifying the signal and using GPIOTE will result in any differences? Or perhaps there are other libraries I can use?

[Mojo]

I am using this library https://github.com/IOsonata/IOsonata/blob/master/ARM/Nordic/src/analog_comp_nrf5x.cpp

It works fine for me.

[Jørgen Holmefjord]

I think you will be facing the exact same issue if you switch to GPIOTE, as it seems that the issue is related to handling the interrupt fast enough. There is no operation in the softdevice specifications that should block you from handling the interrupt within the required time, as it seems to be more than 1 ms between the two missed events. Please make sure that you do not run any other peripherals at the same high priority, and that you do not spend excessive time in the interrupt handler.

You can also try with different prescaling/reference and with/without the hysteresis, to see if this makes any difference.