PWM Differential output

[???]

Would you please tell me how to use the nrf52832 PWM interface to implement differential output to spkr?

Here is my PWM interface configuration code:

NRF_PWM0->PSEL.OUT[0] =   (PWM_IO << PWM_PSEL_OUT_PIN_Pos) | (PWM_PSEL_OUT_CONNECT_Connected << PWM_PSEL_OUT_CONNECT_Pos);
NRF_PWM0->PSEL.OUT[1] =   (0 << PWM_PSEL_OUT_PIN_Pos) | (PWM_PSEL_OUT_CONNECT_Disconnected << PWM_PSEL_OUT_CONNECT_Pos);
NRF_PWM0->PSEL.OUT[2] =   (1 << PWM_PSEL_OUT_PIN_Pos) | (PWM_PSEL_OUT_CONNECT_Disconnected << PWM_PSEL_OUT_CONNECT_Pos);
NRF_PWM0->PSEL.OUT[3] =   (2 << PWM_PSEL_OUT_PIN_Pos) | (PWM_PSEL_OUT_CONNECT_Disconnected << PWM_PSEL_OUT_CONNECT_Pos);

NRF_PWM0->ENABLE = (PWM_ENABLE_ENABLE_Enabled << PWM_ENABLE_ENABLE_Pos);
NRF_PWM0->MODE = (PWM_MODE_UPDOWN_Up << PWM_MODE_UPDOWN_Pos);
NRF_PWM0->PRESCALER = (PWM_PRESCALER_PRESCALER_DIV_1 << PWM_PRESCALER_PRESCALER_Pos);
NRF_PWM0->COUNTERTOP = (256/*16000*/ << PWM_COUNTERTOP_COUNTERTOP_Pos);
NRF_PWM0->DECODER =   (PWM_DECODER_LOAD_Common << PWM_DECODER_LOAD_Pos) | (PWM_DECODER_MODE_RefreshCount << PWM_DECODER_MODE_Pos);

NRF_PWM0->SHORTS = (PWM_SHORTS_LOOPSDONE_SEQSTART0_Disabled << PWM_SHORTS_LOOPSDONE_SEQSTART0_Pos);

Here is my PWM audio output code:

	NRF_PWM0->SEQ[0].PTR = ((uint32_t)(pwmdata) << PWM_SEQ_PTR_PTR_Pos);
	NRF_PWM0->SEQ[0].CNT = ((sizeof(pwmdata) / sizeof(uint16_t)) << PWM_SEQ_CNT_CNT_Pos);
	NRF_PWM0->SEQ[0].REFRESH = REFRESHBD;
	NRF_PWM0->SEQ[0].ENDDELAY = REFRESHBD * ((HALFLOOPPERIOD / REFRESHBD) - ((sizeof(pwmdata) / sizeof(uint16_t))));
	
	NRF_PWM0->LOOP = (0 << PWM_LOOP_CNT_Pos);
	NRF_PWM0->TASKS_SEQSTART[0] = 1;

Is it possible to achieve PWM differential output?

Thanks!

[Jørn]

Hello

First I would recommend you use the pwm_driver module as it simplifies the use of the PWM hardware through predefined function calls.

A differential PWM can be achieved by using two outputs reading separate sequences. Page 498 of the nRF52832 product specification shows grouped mode can be used. When a sequence is played the outputs will be fed with alternating duty cycles from the sequence, output 1 will be fed duty cycles number 1, 3, 5, 7 etc. and output 2 will be fed with duty cycles number 2, 4, 6, 8 etc.

The register description on page 498 of the nRF52832 product specification shows that the MSB of the compare value read from RAM is a polarity bit. If the duty cycles are equal for both outputs, but with the MSB of the duty cycles fed to one of the outputs set, the output will be differential.

A simple example of a differential PWM signal would be.

#define PWM_CH0_DUTY 16000 //50% duty cycle
#define PWM_CH1_DUTY PWM_CH0_DUTY | (1<<15) //50% duty cycle, polarity bit set

With PWM configuration

static void PWM_config(void)
{
	uint16_t pwm_seq0[2] = {PWM_CH0_DUTY, PWM_CH1_DUTY};
	NRF_PWM0->PSEL.OUT[0] = (mypin << PWM_PSEL_OUT_PIN_Pos) | (PWM_PSEL_OUT_CONNECT_Connected << PWM_PSEL_OUT_CONNECT_Pos);
	NRF_PWM0->PSEL.OUT[2] = (mypin2 << PWM_PSEL_OUT_PIN_Pos) | (PWM_PSEL_OUT_CONNECT_Connected << PWM_PSEL_OUT_CONNECT_Pos);
	NRF_PWM0->ENABLE = (PWM_ENABLE_ENABLE_Enabled << PWM_ENABLE_ENABLE_Pos);
	NRF_PWM0->MODE = (PWM_MODE_UPDOWN_Up << PWM_MODE_UPDOWN_Pos);
	NRF_PWM0->PRESCALER = (PWM_PRESCALER_PRESCALER_DIV_128 << PWM_PRESCALER_PRESCALER_Pos);
	NRF_PWM0->COUNTERTOP = (32000 << PWM_COUNTERTOP_COUNTERTOP_Pos);
	NRF_PWM0->LOOP = (1 << PWM_LOOP_CNT_Pos);
	NRF_PWM0->DECODER = (PWM_DECODER_LOAD_Grouped << PWM_DECODER_LOAD_Pos) | (PWM_DECODER_MODE_RefreshCount << PWM_DECODER_MODE_Pos);
	NRF_PWM0->SEQ[0].PTR = ((uint32_t)(pwm_seq0) << PWM_SEQ_PTR_PTR_Pos);
	NRF_PWM0->SEQ[0].CNT = ((sizeof(pwm_seq0) / sizeof(uint16_t)) << PWM_SEQ_CNT_CNT_Pos);
	NRF_PWM0->SEQ[0].REFRESH = 0;
	NRF_PWM0->SEQ[0].ENDDELAY = 2;
}

This example simply blinks two leds with 50% duty cycle, where one led is inverted compared to the other.

For driving a speaker your sequence would be based on the audio you are going to play. For continuous play you could create two sequences, and use interrupts to determine when to update and run the two sequences in an alternating fashion.

Best regards

Jørn Frøysa

[???]

Hi Jørn,

Thank you very much for your help!

But my side of the audio output of each frame data are complete, how do I go to my audio data +-50% differential processing, and then output it?

If it is the first frame of the data selection channel A output, the second frame data selection channel B output, so that the difference does not achieve the output results.

Can you please continue to answer for me, thank you!

[Jørn]

I am sorry, but I don't quite understand what you mean. Could you please elaborate on what your problem is and your question?