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]

Thank you again Dmitry! Your idea worked. Again.   This code is needed to maintain accurate frequency results when using a SoftDevice. If not using a SoftDevice, use other means to enable the external 32MHz oscillator.

static void ensureExternalClkRunning(void)
{
	uint32_t isRunning = 0;
	int cnt = 0;
	sd_clock_hfclk_request();
	do {
		sd_clock_hfclk_is_running(&isRunning);
		cnt++;
		if (cnt > 1000000) {} // Handle non transition. Cnt is usually on the order of 80-ish
	} while (isRunning == 0);
}

[kevy]

Thanks Hugh. I'll try that out! MUCH appreciated.  

[kevy]

Hi Dmitry,

So this freq detector has been working rather well. I can get down to about 1Hz resolution for a few hundred KHz signal. However I am seeing a strange anomaly that I'd like some help with. Every so often - maybe every 2 to 10 seconds, the frequency detector reports a slightly higher frequency (when sampling at about 10x/sec) of around 100hz. The raw count coming out of NRF_TIMER3->CC[0] just reports a slightly lower count making me think there is some kind of phase sampling error or something of that nature. Most of the times it is dead accurate, but sometimes like I said, it counts about 110 fewer pulses then immediately recovers on the next reading.

I tried a bunch of things but can't figure out why, perhaps once every 100 or 1000 samples it misses about 100 pulses. My signal generator is dead on and verified with a scope so it is not the signal coming in. I am driving it with about a 3 foot coax so there is a little ringing due to inductance/capacitance but I don't think that is the cause. 

Do you have any thoughts of what might be going on given the code sample above? FWIW, I'm detecting over 1500 pulses, counting the number of 16MHz pulses. I am also using the above ensureExternalClkRunning function to ensure an accurate CLK.

Thoughts? Thanks!

[Dmitry]

Hi Kevin,

really have no idea.. if the difference is one input pulse, I would guess some race condition because timers are not synchronized internally, but 100 pulses is too much to be a hardware issue.

[hmolesworth]

One thought: If the sig gen is on high impedance output try switching to 50 Ohm output mode and double the source amplitude to compensate for the halved signal at the input - use a 'scope to be sure. Maybe even add a terminating 50R. 3 foot coax is very long ..

[kevy]

Thanks Hugh. I'll try that later today. But even on high impedance output, wouldn't the sig gen still be driving the signal hard into the input (which is also high impedance - an I/O pin on the µC?)

[hmolesworth]

Simple answer - no; the high impedance output cannot drive "hard" and the coax cable capacitance will therefore produce a very soggy rise and fall at the I/O pin on the uC - which is very susceptible to noise and bounce and hence incorrect counts.