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]

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

[kevy]

Thanks again for the input Hugh. If I understand you correctly, your idea is to try to accurately resolve the very last pulse in the 1000 pulse train - basically to compare the phase difference between when the last pulse happens and when the last sampling of that pulse happens, right? 

If that is a correct understanding, I don't know how much that solution might help given that I am sampling 1000 pulses in order to determine frequency and even if the last pulse was mis-measured the worst it could be (1/2 the pulse width), wouldn't the averaging of the other 999 pulses wash that mistaken measurement out for the most part? I could see where your idea would totally help if I were measuring only a single pulse, but given the large sampling and averaging that is happening, I think that might not result in such a huge accuracy improvement.

Or maybe I don't fully understand what you are resolving with this refinement...

In any case, thanks for the input none-the-less!

[Dmitry]

Did you start HFCLK? Don't expect an accurate result from an internal oscillator.

[hmolesworth]

Not quite what I was suggesting. You might explain the requirement in more detail before moving forward; not sure why you mention averaging 999 pulses. The stability of the measurement window is probably 20ppm (part per million) given you have HFCLK enabled as suggested, and over a short measurement that would be even better.

Is the signal encoding an analogue (analog) waveform modulated about the center frequency? If so you don't care about the actual frequency, just the delta modulation which becomes a ratiometric measurement much easier to make (and more accurate) as there are some other techniques available.

Is the signal instead a changing frequency which has to be measured absolutely, say for a hand-held frequency meter?

Is the signal a series of digital step increments? Where does the 1Hz resolution requirement come from?

[kevy]

I incorrectly assumed the NRF52-DK automatically had that designed into it since it has an external xtal. Now I understand the external xtal has to manually be enabled (right?). When I do so (through enabling a BTLE SoftDevice) the frequency measurements are stable as a rock. It is off by about 2Hz in absolute accuracy, but I can live with that. It oscillates by about 1/2Hz to 1Hz. Very nice performance (on an NRF52-DK).

So... Your idea worked well! Thank You. I'll post the final code here in case anyone else needs a frequency detector with a few Hz resolution that can measure up to the hundreds of KHz. Just make sure you use a very stable external CLK.

#define FREQ_MEASURE_PIN  11												// P0.11
#define kPulseDetectionCount 1000											// Detect this many +edges

static volatile uint32_t pulsesDetected = 0;
static volatile uint32_t freqDetected = 0;


extern "C" void TIMER4_IRQHandler(void)
{
	if (NRF_TIMER4->EVENTS_COMPARE[0]) {
		NRF_PPI->CHENCLR = 1 << 1;				// Disable PPI channel CH1 that initiated counting

		pulsesDetected = NRF_TIMER3->CC[0];		// Get detected pulses (count was captured during PPI event): total count for 1000 +edge events (in 0.0625us units)
		freqDetected = (pulsesDetected == 0) ? 0 : (16000000.0 * (float)kPulseDetectionCount) / pulsesDetected;

		NRF_TIMER3->TASKS_CLEAR = 1;			// Reset timers
		NRF_TIMER4->TASKS_CLEAR = 1;
		NRF_TIMER4->EVENTS_COMPARE[0] = 0;		// Reset for next
		NRF_PPI->CHENSET = 1 << 1;				// Re-enable PPI - go again
	}
}


// Must use a stable external CLK for accurate measurements
static void freqDetectorInit(void)
{
		// Pin & GPIOTE init
    IOPinConfig(0, FREQ_MEASURE_PIN, 0, IOPINDIR_INPUT, IOPINRES_NONE, IOPINTYPE_NORMAL);
	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

		// Calls TIMER4_IRQHandler
	NVIC_SetPriority(TIMER4_IRQn, APP_IRQ_PRIORITY_LOW);
	NVIC_EnableIRQ(TIMER4_IRQn);

		// Timer 4: Rising edge event counter - detect 1000 +edges then generate an interrupt
	NRF_TIMER4->TASKS_STOP = 1;
	NRF_TIMER4->MODE = TIMER_MODE_MODE_Counter;												// Counting external pulses
	NRF_TIMER4->BITMODE = TIMER_BITMODE_BITMODE_16Bit << TIMER_BITMODE_BITMODE_Pos;			// Only need 16 bits to count 1000 pulses
	NRF_TIMER4->CC[0] = kPulseDetectionCount + 1;											// # pulses to detect (+1)
	NRF_TIMER4->TASKS_CLEAR = 1;
	NRF_TIMER4->INTENSET = TIMER_INTENSET_COMPARE0_Enabled << TIMER_INTENSET_COMPARE0_Pos;	// Generate int when done to get results
	NRF_TIMER4->TASKS_START = 1;

		// Timer 3: 16MHz timer during 1000 events
	NRF_TIMER3->TASKS_STOP = 1;
	NRF_TIMER3->MODE = TIMER_MODE_MODE_Timer;
	NRF_TIMER3->BITMODE = TIMER_BITMODE_BITMODE_32Bit << TIMER_BITMODE_BITMODE_Pos;
	NRF_TIMER3->PRESCALER = 0;
	NRF_TIMER3->CC[0] = 0;
	NRF_TIMER3->TASKS_CLEAR = 1;

		// Using PPI CH0, when Timer 4 compares to kPulseDetectionCount +transitions events, capture the count of 0.0625us periods
		// Add a 2nd task to stop 16MHz timer
	NRF_PPI->CH[0].EEP = (uint32_t)&NRF_TIMER4->EVENTS_COMPARE[0];
	NRF_PPI->CH[0].TEP = (uint32_t)&NRF_TIMER3->TASKS_CAPTURE[0];
	NRF_PPI->FORK[0].TEP = (uint32_t)&NRF_TIMER3->TASKS_STOP;
	NRF_PPI->CHENSET = 1 << 0;

		 // On PPI CH1, when a GPIOTE rise event happens start TIMER3 (the first time) and increment TIMER4 every time
	NRF_PPI->CH[1].EEP = (uint32_t)&NRF_GPIOTE->EVENTS_IN[0];
	NRF_PPI->CH[1].TEP = (uint32_t)&NRF_TIMER3->TASKS_START;
	NRF_PPI->FORK[1].TEP = (uint32_t)&NRF_TIMER4->TASKS_COUNT;
	NRF_PPI->CHENSET = 1 << 1;										// Go
}

[kevy]

Hugh, I'll reply to your questions offline, OK?

[hmolesworth]

Ok - and good job in getting your code running!