NRF51 sine wave with pwm and timer

RE: NRF51 sine wave with pwm and timer

gammarui — Mon, 29 Jun 2015 12:07:27 GMT

I tried to generate a sine with the same method as before but without sd and it works, sometimes. When I set NRF_TIMER1->CC[0] = X, most values for X generates the same inverted pwm problem I got with the softdevice. However, the glitches come in a predictable pattern and it seems like it is caused by some wrap around problem. I tested a few values on X and it seems 98, 107, 118, 150 works but I cant find any least common denominator that makes any sense. Any idea what could cause this?

RE: NRF51 sine wave with pwm and timer

JohnBrown — Fri, 26 Jun 2015 16:01:29 GMT

One other thing. In your code

// Configure GPIOTE channel 0 and 1 to change pin state
  nrf_gpiote_task_config(0, pin, NRF_GPIOTE_POLARITY_LOTOHI,
      NRF_GPIOTE_INITIAL_VALUE_LOW);

  nrf_gpiote_task_config(1, pin, NRF_GPIOTE_POLARITY_HITOLO,
      NRF_GPIOTE_INITIAL_VALUE_LOW);

it looks like you are trying to use two GPIOTE task that target the same pin. You can't do that. You have to use the toggle function. That's why there are so many PWM problem questions. It would be a much better system if you could SET and CLEAR the same pin via PPI/GPIOTE, but I guess the silicon designers have to make compromises. I believe the new nRF52 has proper PWM hardware.

RE: NRF51 sine wave with pwm and timer

JohnBrown — Fri, 26 Jun 2015 14:53:31 GMT

Some of the formatting has been screwed up in the copy and paste. Sorry.

RE: NRF51 sine wave with pwm and timer

JohnBrown — Fri, 26 Jun 2015 14:49:58 GMT

OK Here is my code. Bear in mind that I am generating two sine waves, and I am using a fractional accumulator method(or whatever it's called) This is completely self contained, i.e. all the setting up and clearing up is in this function. As I said, I do not have the SD running at this point, but if I were you I'd get it working without first, then see if you can make it work with the SD.

#include "dtmf.h"
#include "nrf_gpiote.h"
#include "nrf.h"
#include "nrf51_bitfields.h"
#include "nrf_soc.h"
#include "nrf_gpio.h"


#define	DIGIT_LEN	100000/20		// 100 mS, PWM max is 20uS
#define	GAP_LEN		100000/20		// 100 mS

#define GPIOTE_CHAN_FOR_PWM_TASK	3	//The GPIOTE Channel used for PWM
#define DTMF_TIMER	NRF_TIMER1		// Timer used


enum
{
DTMF_1, DTMF_2, DTMF_3, DTMF_A,
DTMF_4, DTMF_5, DTMF_6, DTMF_B,
DTMF_7, DTMF_8, DTMF_9, DTMF_C,
DTMF_STAR, DTMF_0, DTMF_HASH, DTMF_D
};

#define HF1663	(1663 * 32678 + 20000) / 40000
#define HF1477	(1477 * 32678 + 20000) / 40000
#define HF1336	(1336 * 32678 + 20000) / 40000
#define HF1209	(1209 * 32678 + 20000) / 40000

#define LF941	(941 * 32678 + 20000) / 40000
#define LF852	(852 * 32678 + 20000) / 40000
#define LF770	(770 * 32678 + 20000) / 40000
#define LF697	(697 * 32678 + 20000) / 40000

const uint16_t HighGroup [4] =
	{
	HF1209,
	HF1336,
	HF1477,
	HF1663
	};
const uint16_t LowGroup [4] =
	{
	LF697,
	LF770,
	LF852,
	LF941
	};

const uint8_t SineTable [128] = {
	0,0,0,0,1,1,2,3,
	4,6,7,9,10,12,14,16,
	18,21,23,25,28,31,33,36,
	39,42,45,48,51,54,57,60,
	64,67,70,73,76,79,82,85,
	88,91,94,96,99,102,104,106,
	109,111,113,115,117,118,120,121,
	123,124,125,126,126,127,127,127,
	127,127,127,127,126,126,125,124,
	123,121,120,118,117,115,113,111,
	109,106,104,102,99,96,94,91,
	88,85,82,79,76,73,70,67,
	64,60,57,54,51,48,45,42,
	39,36,33,31,28,25,23,21,
	18,16,14,12,10,9,7,6,
	4,3,2,1,1,0,0,0
	};

void SendDTMF(uint8_t code)
{
uint16_t StepLo = 0;
uint16_t StepHi = 0;
uint16_t SineIndexLo = 0;
uint16_t SineIndexHi = 0;
uint16_t pwm_value = 1;
uint8_t temph;
uint8_t templ;
uint16_t duration;
nrf_gpio_cfg_output(PWM_OUT);
nrf_gpio_cfg_output(DTMF_LED);

// Set up the PPI
// PPI channel 0 event is Timer1 compare 2
NRF_PPI->CH[0].EEP = (uint32_t) &DTMF_TIMER->EVENTS_COMPARE[2];

// PPI channel 0 task is GPIOTE task 3(toggle output)
NRF_PPI->CH[0].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[3];

// PPI chan 1 event is Timer n compare 0
NRF_PPI->CH[1].EEP = (uint32_t) &DTMF_TIMER->EVENTS_COMPARE[0];

// PPI chan 1 task is GPIOTE task 3(toggle output)
NRF_PPI->CH[1].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[3];

// Enable PPI chan 0
NRF_PPI->CHENSET = (1 << 0);

// Enable PPI chan 1
NRF_PPI->CHENSET = (1 << 1);

DTMF_TIMER->MODE = TIMER_MODE_MODE_Timer;    // Set the timer in Timer Mode.
DTMF_TIMER->PRESCALER = 0; // Prescaler 0 produces 16mHz Hz timer frequency => 1 tick = 62.5nS.
DTMF_TIMER->BITMODE = TIMER_BITMODE_BITMODE_16Bit;  // 16 bit mode.
DTMF_TIMER->TASKS_CLEAR = 1; // clear the task first to be usable for later.

DTMF_TIMER->CC[0] = 1;		// cc 0 is the end of the PWM ON period, avoid zero
DTMF_TIMER->CC[1] = 255;	// cc 1 is the point where we update the period (cc0)
DTMF_TIMER->CC[2] = 399;	//319;	// cc 2 is the point where we clear the counter and set the PWM out high
// cc2 clears the counter
DTMF_TIMER->SHORTS = TIMER_SHORTS_COMPARE2_CLEAR_Msk;

// Configure the GPIOTE Task to toggle the PWM output.
nrf_gpiote_task_config(GPIOTE_CHAN_FOR_PWM_TASK,
                           PWM_OUT,
                           NRF_GPIOTE_POLARITY_TOGGLE,
                           NRF_GPIOTE_INITIAL_VALUE_HIGH);

StepLo = LowGroup[code >> 2 & 3];
StepHi = HighGroup[code & 3];

duration = DIGIT_LEN + GAP_LEN;
DTMF_TIMER->TASKS_START = 1;  // Start timer.

nrf_gpio_pin_set(DTMF_LED);

// Because my app was doing nothing else I polled the timer compare
// but it should work as an IRQ
while(duration)
    {
    if(DTMF_TIMER->EVENTS_COMPARE[1])
	{
	DTMF_TIMER->CC[0] = pwm_value;
	DTMF_TIMER->EVENTS_COMPARE[1] = 0;
	// Now calculate value for next time time.
	// Move pointers a step width ahead
	SineIndexHi += StepHi;
	SineIndexLo += StepLo;
	temph = SineTable[SineIndexHi >> 8 & 127];
	templ = SineTable[SineIndexLo >> 8 & 127];
	templ -= (templ >> 2);		// Low * 0.75
	temph += templ;
	// Avoid a zero value. I can do this safely as the sine table only goes up to 127
	// and the lower of the two tones has been multiplied by 0.75 abave
	temph += 1;
	pwm_value = (uint16_t)temph;

	if(duration == GAP_LEN)
	    {
	    nrf_gpio_pin_clear(DTMF_LED);	// This turns off the LED after 50mS or so
	    }
	if(duration < GAP_LEN)
		{
		if(temph == 1)			// This is because I want to turn the tones off when the output is close to 0V
			{
			StepHi = 0;
			StepLo = 0;
			nrf_gpiote_unconfig(GPIOTE_CHAN_FOR_PWM_TASK);
		        nrf_gpio_pin_write(PWM_OUT, 0);
			}
		}
	if(duration)
		{
		duration--;
		}
	}
    }// ends while(duration)

// Dismantle everything
DTMF_TIMER->TASKS_STOP = 1;  // Stop timer.
DTMF_TIMER->TASKS_CLEAR = 1;  // Clear timer.
DTMF_TIMER->SHORTS = 0x00;
DTMF_TIMER->EVENTS_COMPARE[0] = 0;
DTMF_TIMER->EVENTS_COMPARE[1] = 0;
DTMF_TIMER->EVENTS_COMPARE[2] = 0;

// Disable PPI chan 0
NRF_PPI->CHENCLR = (1 << 0);
// Disable PPI chan 1
NRF_PPI->CHENCLR = (1 << 1);
}

RE: NRF51 sine wave with pwm and timer

gammarui — Fri, 26 Jun 2015 14:00:00 GMT

Hi John, Thanks for your workaround. I tried to implement it but I am not sure if I understand you correctly, is it something like this you mean:

namespace {

uint8_t phase = 1;

bool active = false;

const uint8_t sine26[] = {0, 4, 16, 35, 59, 88, 120, 152, 182, 210, 232, 247,
    255, 255, 247, 232, 210, 182, 152, 120, 88, 59, 35, 16, 4, 0};

}
void initSimplePWM(uint8_t pin);

extern "C" {

void TIMER2_IRQHandler(void) {
  NRF_TIMER2->CC[0] = sine26[phase];
  if (phase < 26) {
    phase++;
  } else {
    phase = 1;
  }
}

} // end of extern C

int main() {
  setupLEDPins();

  pinOutput(PIEZO_PWM_PIN);

// Init softdevice
  BluetoothPeripheral::instance().initialize();

  initSimplePWM(PIEZO_PWM_PIN);
  NRF_TIMER2->TASKS_START = 1;               // Start timer.

  while (1) {
    delayMs(100);
    pinToggle(LED_RED);
  }
  
  return 0;
}

void initSimplePWM(uint8_t pin) {
  // Configure GPIOTE channel 0 and 1 to change pin state
  nrf_gpiote_task_config(0, pin, NRF_GPIOTE_POLARITY_LOTOHI,
      NRF_GPIOTE_INITIAL_VALUE_LOW);

  nrf_gpiote_task_config(1, pin, NRF_GPIOTE_POLARITY_HITOLO,
      NRF_GPIOTE_INITIAL_VALUE_LOW);

  // Configure PPI channel 0 to toggle PWM_OUTPUT_PIN on every TIMER2 COMPARE[0] match.
  NRF_PPI->CH[0].EEP = (uint32_t)&NRF_TIMER2->EVENTS_COMPARE[0];
  NRF_PPI->CH[0].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[0];
  NRF_PPI->CH[1].EEP = (uint32_t)&NRF_TIMER2->EVENTS_COMPARE[1];
  NRF_PPI->CH[1].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[1];

  // Enable PPI channel 0.
  NRF_PPI->CHEN = (PPI_CHEN_CH0_Enabled << PPI_CHEN_CH0_Pos |
  PPI_CHEN_CH1_Enabled << PPI_CHEN_CH1_Pos);
  //sd_ppi_channel_enable_set(1 << 0);

  // Init TIMER2 for 10kHz PWM
  NRF_TIMER2->MODE = TIMER_MODE_MODE_Timer;     // Set the timer in Counter Mode
  NRF_TIMER2->TASKS_CLEAR = 1;   // Clear the tasks first to be usable for later
  NRF_TIMER2->PRESCALER = 4; // 2^PRESCALER (values authorized 1 to 9) 4 : 16MHz / 16 = 1MHz
  NRF_TIMER2->BITMODE = TIMER_BITMODE_BITMODE_16Bit; //Set counter to 16 bit resolution
  NRF_TIMER2->CC[0] = phase; //1Mhz timer with prescaler 4
  NRF_TIMER2->CC[1] = 400; //1Mhz timer with prescaler 4
  NRF_TIMER2->CC[2] = 256; //1Mhz timer with prescaler 4
  NRF_TIMER2->SHORTS = (TIMER_SHORTS_COMPARE1_CLEAR_Enabled
      << TIMER_SHORTS_COMPARE1_CLEAR_Pos);

  NRF_TIMER2->INTENSET = (TIMER_INTENSET_COMPARE2_Enabled
      << TIMER_INTENSET_COMPARE2_Pos);
  sd_nvic_SetPriority(TIMER2_IRQn, 3);
  sd_nvic_EnableIRQ(TIMER2_IRQn);
}

This code does not work but if this is what you meant and you got it to work I will continue on this path as there does not seem to be any other way to generate a sine wave output with the nrf51 device.

RE: NRF51 sine wave with pwm and timer

JohnBrown — Fri, 26 Jun 2015 11:48:40 GMT

The way I got round this in order to generate DTMF was to set the maximum count to 400, but limit the on-time to 256. This way I was able to set the next duty cycle in the dead time between 256 and 399. The only IRQ was the one at the 256 count, when I set the compare for the off-time and then looked up the value for next time The downside:

a) I had to avoid the value of zero for the on-time(I used 1 to 256 rather than 0 to 255)

b) The resulting voltage levels were not 0 to Vdd(didn't matter in my app.)

So one compare register is set at 400. This clears the timer(via SHORTS) and toggles the output to ON.

Another compare register is set to the duty, this toggles the output to OFF.

A third compare is set to 256, this generates an IRQ in which I update the duty compare with the value I calculated last time, then calc the value for next time.

Admittedly, I was not running the SD during this, but I reckon that if you can cope with a lower amplitude sine wave, this could still work, you might need to increase the 400 to something higher...

RE: NRF51 sine wave with pwm and timer

gammarui — Fri, 26 Jun 2015 11:47:40 GMT

Thanks for your quick reply. Yes, I am using S110.

Looking forward to a fix in future releases!

RE: NRF51 sine wave with pwm and timer

Susheel Nuguru — Fri, 26 Jun 2015 11:34:20 GMT

If you are using softdevice, then it is a known issue devzone.nordicsemi.com/.../ This issue is currently under investigation and we unfortunately have to wait until further information is available.