Frequency detector possible?

[kevy]

Hi. Can anyone think of a way to use the nRF52832 (probably Timer) to detect approximately 125KHz-150KHz variable square wave down to about a 1Hz resolution? I'd like to detect frequency changes at about 100 times a second though. Ideally I'd like to do this without any external hardware (like a heterodyne mixer). Thoughts? Thanks!

(FINAL answer at very bottom. The answer is YES, the nRF52832 can be used even with a SoftDevice to accurately detect a few hundred KHz signal down to a few Hz resolution)

[Dmitry]

Hi,

Yes, you can do this with GPIOTE, PPI, and two timers - for example, TIMER1 is a pulse counter, TIMER2 is a 16-MHz timer.
- configure TIMER1->CC[0] for a number of pulses to measure plus 1 (in your case, 1000 pulses is about 0.08 sec that meets your requirements)
- configure first PPI channel to start TIMER2 and increment TIMER1 by GPIOTE rise event
- configure second PPI channel to capture TIMER2 value into CC[0] by counter's TIMER1->COMPARE[0] event (after 1000 pulses)
- to start measurement, clear TIMER1 and TIMER2, then enable both PPI channels
- after TIMER1->COMPARE[0] event, TIMER2->CC[0] will contain total time for 1000 pulses in 1/16 usec units.

1Hz resoultion is a challenge. A difference between 149999 and 150000 Hz is about 0.04 usec at 1000 periods, resolution of nRF52 timer is 1/16 usec - I believe you can get about 2-3 Hz resolution if everything is done carefully.

[kevy]

Thanks for the input Dimitry. That's the issue: The different between say 150000Hz and 150001Hz is not 0.04 usec. It's 0.04 nsec - that's about 40 picoseconds (0.00000666662 vs 0.00000666666). I think the fundamental issue is detecting changes accurately down to the 10's of ps level - probably unlikely with a 52832. Thoughts?

Not only that but I want to use an S132 SoftDevice and as I understand it, the SoftDevice uses Timers 0 and 1. So I think I would need Timers 2 and 3, right? Aren't there limitations to timers above 2?

[kevy]

Actually I stand corrected! You are speaking of 1000 periods of detection so you are correct - it is 0.04 usec. Detecting a single pulse would be 40ps.

So the remaining question about timers stands. Any issues using Timers 2 & 3?

[Dmitry]

AFAIK SoftDevice needs only TIMER0, so you can use any others.

[kevy]

Hi Dmitry,

I got it basically working largely from code from https://devzone.nordicsemi.com/f/nordic-q-a/9036/measuring-input-gpio-pin-frequency-with-soft-device-running. My code looks like this:

static void freqDetectorInit(void)
{
    IOPinConfig(0, FREQ_MEASURE_PIN, 0, IOPINDIR_INPUT, IOPINRES_NONE, IOPINTYPE_NORMAL);

	NVIC_SetPriority(TIMER3_IRQn, APP_IRQ_PRIORITY_LOW);
	NVIC_EnableIRQ(TIMER3_IRQn);									// Calls TIMER3_IRQHandler

    	// Timer 4: Freq counter
	NRF_TIMER4->TASKS_STOP = 1;
	NRF_TIMER4->MODE = TIMER_MODE_MODE_Counter;
	NRF_TIMER4->BITMODE = (TIMER_BITMODE_BITMODE_32Bit << TIMER_BITMODE_BITMODE_Pos);
	NRF_TIMER4->TASKS_CLEAR = 1;
	NRF_TIMER4->EVENTS_COMPARE[0] = 0;

		// Timer 3: Timed gate
	NRF_TIMER3->TASKS_STOP = 1;
	NRF_TIMER3->MODE = TIMER_MODE_MODE_Timer;
	NRF_TIMER3->PRESCALER = 0;										// Fhck / 2^0
	NRF_TIMER3->CC[0] = 16000000ULL / 1000;							// Detect 1000 events - careful changing this!
	NRF_TIMER3->BITMODE = (TIMER_BITMODE_BITMODE_32Bit << TIMER_BITMODE_BITMODE_Pos);
	NRF_TIMER3->TASKS_CLEAR = 1;
	NRF_TIMER3->INTENSET = (TIMER_INTENSET_COMPARE0_Enabled << TIMER_INTENSET_COMPARE0_Pos);
	NRF_TIMER3->EVENTS_COMPARE[0] = 0;

		// GPIOTE init
	NRF_GPIOTE->CONFIG[0] = 0x01 << 0; 								// Event mode
	NRF_GPIOTE->CONFIG[0] |= FREQ_MEASURE_PIN << 8;					// Pin number
	NRF_GPIOTE->CONFIG[0] |= GPIOTE_CONFIG_POLARITY_LoToHi << 16;	// Event rising edge

		// PPI GPIOTE counter init on PPI CH1 set up to start the count
	NRF_PPI->CHEN |= 1 << 1;										// Enable the channel - CH1
	*(&(NRF_PPI->CH1_EEP)) = (uint32_t)&NRF_GPIOTE->EVENTS_IN[0];	// Event end point
	*(&(NRF_PPI->CH1_TEP)) = (uint32_t)&NRF_TIMER4->TASKS_COUNT;	// Task end point
	NRF_PPI->CHENSET |= 1 << 1;										// Enable the SET function

		// PPI timer stop counter init on PPI CH0 set up to end the count
	NRF_PPI->CHEN |= 1 << 0;
	*(&(NRF_PPI->CH0_EEP)) = (uint32_t)&NRF_TIMER3->EVENTS_COMPARE[0];
	*(&(NRF_PPI->CH0_TEP)) = (uint32_t)&NRF_TIMER4->TASKS_STOP;
	NRF_PPI->CHENSET |= 1 << 0;

	NRF_TIMER3->TASKS_START = 1;
	NRF_TIMER4->TASKS_START = 1;
}



static volatile uint32_t freqDetected = 0;

extern "C" void TIMER3_IRQHandler(void)
{
	if (NRF_TIMER3->EVENTS_COMPARE[0] != 0)
	{
		NRF_TIMER3->EVENTS_COMPARE[0] = 0;
		NRF_TIMER4->TASKS_CAPTURE[0] = 1;

		freqDetected = NRF_TIMER4->CC[0];		// Total count for 1000 events (in 0.0625us units)

		NRF_TIMER3->TASKS_CLEAR = 1;
		NRF_TIMER4->TASKS_CLEAR = 1;

		NRF_TIMER4->TASKS_START = 1;
	} else
		hang(1);
}

I'm not sure I've fully wrapped my head around it though because the values I get for 

freqDetected

only report KHz and not down to the Hz - so a signal of 123456Hz returns 123 and I miss the 456 which is the important part. When I change to 

NRF_TIMER3->CC[0] = 16000000ULL;

then I get freqDetected values down to the hertz: 123456 but then the sampling takes a full 1000ms where I need it to take about 10ms.

What, if anything, might I be doing wrong, if you can see it?

Thanks!

Kevin