LPCOMP issue with sensing multiple inputs

[lance338]

I'm using LPCOMP to sense multiple input.

I call the lpcomp_read() function below for each input pin.

If I only call this function for 1 pin only, it worked fine. 

For example:

read_value  |= lpcomp_read(4) << 1; <== this would return read_value = 2, and that is correct.

However, when I call this function multiple times for multiple pins, it only read the 1st one as if the PSEL stuck on the 1st pin. 

read_value |= lpcomp_read(2) << 0;

read_value |= lpcomp_read(4) << 1; 

I expected read_value to be 3 (as both pins should be above reference voltage). Instead, I always get read_value = 1;

Is there any clearing of the LPCOMP if I want to use it again for another pin?

uint32_t lpcomp_read (uint32_t analog_pin)
{
uint32_t lpcomp_result;

NRF_LPCOMP_Type NRT_LPCOMP;


//Set Reference Voltage to VDD *2/8
NRF_LPCOMP->REFSEL |= (LPCOMP_REFSEL_REFSEL_SupplyTwoEighthsPrescaling << LPCOMP_REFSEL_REFSEL_Pos);

NRF_LPCOMP->PSEL = (analog_pin << LPCOMP_PSEL_PSEL_Pos);

//Enable LPCOMP
NRF_LPCOMP->ENABLE = LPCOMP_ENABLE_ENABLE_Enabled;

//start LPCOMP
NRF_LPCOMP->TASKS_START = 1;

while (!NRF_LPCOMP->EVENTS_READY) {}

//Sample Comparator Value
NRF_LPCOMP->TASKS_SAMPLE = 1;

//reset result
lpcomp_result = NRF_LPCOMP->RESULT;

//stop LPCOMP
NRF_LPCOMP->TASKS_STOP = 1;

//Disable LPCOMP
NRF_LPCOMP->ENABLE = LPCOMP_ENABLE_ENABLE_Disabled;

//return result
return lpcomp_result;
}

[Kenneth]

Have you considered using the SAADC instead? Should be a better solution to read multiple pins.

In you case maybe you should also clear NRF_LPCOMP->EVENTS_READY = 0 between "reads".

[lance338]

Thanks Kenneth. Tried clearing Event_ready and it doesn't work. Do I have to wait between enables? 

Also, can you share why the ADC is better for multiple inputs?

[Kenneth]

The LPCOMP is intended to be used for a low power wakeup from sleep when an input drop a specified level, it is not very accurate either. So the way you use it is not as intended, instead the SAADC is better since it will provide a much better reading of the individual pins.

[lance338]

We don't need very accurate. Just simply a detection of voltage crossing below/above a level. The ADC has it own challenge in our application. Is there any thing you can think of to get the LPCOMP to work on multiple inputs?

[Kenneth]

I would have tried with a delay (e.g. 50us) or power cycle the LPCOMP when it's disabled like this:

*(volatile uint32_t *)0x40013FFC = 0;
*(volatile uint32_t *)0x40013FFC;
*(volatile uint32_t *)0x40013FFC = 1;