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

[hmolesworth]

Ok - and good job in getting your code running!

[kevy]

Hi Dmitry. This technique is working pretty well (thanks again!) but I seem to have an issue when the SoftDevice is transmitting data. It is almost as if the SoftDevice is dropping back to the internal LC oscillator then switching back to the external CLK when it returns control to me.

When I transmit data via BT as a Characteristic, the readings vary +-150Hz (like it was before I knew about the internal vs. external oscillator selection above). This does NOT happen when I break at the point where the data is transmitted and look at the reading - the breakpoint seems to shut down the BT link and all subsequent readings are stable as a rock each time I continue with the breakpoint and break again the next time around (until the SoftDevice panics and throws an exception like it always does). 

So basically when I break at my transmission point to see what data it is going to transmit, the link drops and the frequencies are stable. But when I let it run on it's own and transmit the frequency reading every second (from a 1 second timer interrupt), the data transmitted varies about +-150Hz or so. This variance also happens when I turn off BT and the SoftDevice all together which defaults the application to use the internal LC oscillator - that is expected.

Thoughts about the SoftDevice toggling oscillator sources?

Also, is there a way to tell the SoftDevice NOT to panic when I am single stepping through code?

Thanks!

[Dmitry]

Hi Kevin,

Happy to know that your code works :)  With softdevice, sd_clock_hfclk_request() call should help.

kevy said:

Also, is there a way to tell the SoftDevice NOT to panic when I am single stepping through code?

It's annoying, but it seems there's no way to single-step while connection is active.

[kevy]

Thanks Dmitry. Any thoughts about my first question about the SoftDevice seemingly changing internal oscillators or something like that...?

[kevy]

Please ignore the above. I missed your first reply... Investigating now...