PWM with a piezo buzzer

RE: PWM with a piezo buzzer

Honey — Wed, 27 May 2015 18:18:42 GMT

hi please check this code.i have problem in buzzer while using softdevie....

ble_app_uart_pwm.zip

RE: PWM with a piezo buzzer

Eliot Stock — Mon, 31 Mar 2014 17:32:46 GMT

It seems we have our pins 1 and 2 round the wrong way on Q3! So indeed, a hardware issue. Thanks again for your help.

RE: PWM with a piezo buzzer

Michael Antwih — Mon, 31 Mar 2014 14:57:58 GMT

No worries. Hopefully that will turn out to be your issue.

RE: PWM with a piezo buzzer

Eliot Stock — Mon, 31 Mar 2014 14:41:41 GMT

Thanks awfully for your help on this. Q5 has changed since that schematic was printed and is now one that switches at a lower voltage, but I think your point remains valid. Must admit I am quickly out of my depth on the hardware here and will ask my hardware engineer to take a look today. Thanks again.

RE: PWM with a piezo buzzer

Michael Antwih — Mon, 31 Mar 2014 14:07:14 GMT

I'm looking at your circuit. Could you explain how Q5 works? If Q5 is turned on wouldn't you be shorting pin 1 and 2 on your buzzer? So you basically are grounding out the circuit and also draining your battery. I could be looking at the circuit wrong but that seems to be a valid area to consider.

If that is the case I would remove Q5 and just ground pin 2 to ground of the buzzer. Seems to be a circuit to stop leakage from the battery when the buzzer is not in use. I would also place a cap in parallel to R4 (100pf or similar)

Hardware issue.

RE: PWM with a piezo buzzer

Michael Antwih — Mon, 31 Mar 2014 13:56:44 GMT

Eliot,

Loaded your code on a pca10001 with no softdevice using sdk 5.1.0.36092.

I placed your code inside of the uart application and just included nrf_pwm.c to the lib folder and copied your main over.

I first verified that your source would generate 2.730kHz signal and it did without any modification.

Then I added the while loop section and the duty cycle adjusted according to the formula and sin table.

I am attaching the screen shots.

tek00002 => manually setting duty tek00003 => using sin table formula tek00004 => using sin table formula

pics 00003 & 00004 show the duty cycle varying.

So your code appears to be running fine. Have you or can you try on an evaluation board. That would rule out the software and point to an hardware issue.

RE: PWM with a piezo buzzer

Eliot Stock — Mon, 31 Mar 2014 13:18:40 GMT

Here's the source I'm using, not much different from nrf51-pwm-library out of the box, and a schematic for my buzzer circuit. That line marked 'BuzzerOn' is going to pin 14 on the Nordic. I'm driving the buzzer off a lithium ion battery, rather than the Nordic pin itself, hence the transistors here.

Have just checked my battery and it's flat (2.04V)! Let me charge it and see if that helps. Will post again with multimeter output once that's done.

main_sin.c

nrf_pwm.h

nrf_pwm.c

RE: PWM with a piezo buzzer

Michael Antwih — Mon, 31 Mar 2014 12:28:09 GMT

Are you able to update your source code? Then I can run it and let you know my results.

If not I understand.

RE: PWM with a piezo buzzer

Eliot Stock — Mon, 31 Mar 2014 10:08:44 GMT

Thanks. USE_WITH_SOFTDEVICE is definitely 0 and I've tested with the nrf_pwm_set_value(0, 1465) hard coded. No joy.

Don't yet have a scope but will see what I can see with a multimeter.

RE: PWM with a piezo buzzer

Michael Antwih — Mon, 31 Mar 2014 09:53:56 GMT

Your frequency was taken into account in the formula TORBJØRN provided above ...

pwm_max_value = 16000000/ (2 * 2^0* 2730) = 2930 2930 is not in hertz. it is the max pwm value, the max counter value for the timer compare.

So between 0-2930 with a prescaler of 0 the pwm frequency will be 2730 and a variable duty cycle depending of the value to set with the nrf_pwm_set_value function.

nrf_pwm_set_value(0,1465) gives a 2730 Hz signal with 50% duty nrf_pwm_set_value(0,500) gives a 2730 Hz signal with ~6% duty

If you added the code given by TORBJØRN you should be generating the correct frequency.

have you attempted to manually set the duty cycle? nrf_pwm_set_value(0, 1465); Try to manually set it instead of using the sin table.

This will help you debug.

Do you have any means off verifying the frequency being generated on the pin? oscope? dmm? Make sure you are getting a signal first.

If you aren't using the softdevice make sure USE_WITH_SOFTDEVICE is set to 0. By default in the pwm.h I believe it is 1. This will definitely cause an app failure.

Good Luck.

RE: PWM with a piezo buzzer

Eliot Stock — Mon, 31 Mar 2014 09:31:45 GMT

I see the confusion: my buzzer requires 2,730Hz, not 2,930Hz. Half of that is 1,365. Do you mean I need to scale up the values in the sine table so that they peak at 1,365, like this?

	uint32_t value = (float)sin_table[counter]/255.0f * 1365.0f;
    nrf_pwm_set_value(0, value);

Running that produces no sound yet. I'm sure my pwm_config is wrong. Currently using this:

nrf_pwm_config_t pwm_config =
	{.num_channels  = 1,             // Was 3
	.gpio_num       = {14},          // Was 8,9,10
	.ppi_channel    = {0,1,2,3,4,5},
	.gpiote_channel = {2,3,0},
	.mode           = PWM_MODE_BUZZER_2730HZ};

RE: PWM with a piezo buzzer

Michael Antwih — Mon, 31 Mar 2014 07:56:35 GMT

Eliot,

By duty cycle of 50% he means the following ...

nrf_pwm_set_value(0, sin_table[counter]); where sin_table[counter] value what be 50% of the pwm max value. If pwm max = 2930 then half of that (50%) would give 1465.

Hope that helps.

RE: PWM with a piezo buzzer

Eliot Stock — Fri, 28 Mar 2014 19:33:07 GMT

Sorry, I'm not following entirely. I've modified nrf_pwm_init() to use your extra case, thanks for that. But don't understand what you mean by "for 50% duty cycle use 1465". What should the pwm_config look like?

nrf_pwm_config_t pwm_config =
	{.num_channels  = 3,
	.gpio_num       = {8,9,10},
	.ppi_channel    = {0,1,2,3,4,5},
	.gpiote_channel = {2,3,0},
	.mode           = PWM_MODE_BUZZER_2730HZ};

My buzzer has one pin on the Nordic's pin 14 and the other on ground, effectively.

RE: PWM with a piezo buzzer

ovrebekk — Fri, 28 Mar 2014 17:13:38 GMT

Hi Eliot

Did you figure this out?

You can modify the nrf51-pwm-library to run at a base frequency of about 2730 Hz quite easily, you just have to change the max value of the PWM using the following formula:

pwm_max_value = 16.000.000 / (2 * 2^PRESCALER * FREQUENCY)

With a prescaler of 0 this gives you a max value of 2930, and I implemented a new PWM mode in nrf_pwm.c to test this out:

    case PWM_MODE_BUZZER_2730HZ:
        PWM_TIMER->PRESCALER = 0;
        pwm_max_value = 2930;
        break;

Now you just need to set the channel according to the duty cycle you want on your buzzer signal, for 50% duty cycle use 1465.

Best regards Torbjørn

RE: PWM with a piezo buzzer

Eliot Stock — Tue, 25 Mar 2014 00:09:49 GMT

Thanks Steven. I just tried running this. No sound yet. What would a typical value be for MAX_SAMPLE_LEVELS? How would I aim for 2,730Hz?

RE: PWM with a piezo buzzer

Steven Wik — Mon, 24 Mar 2014 14:55:26 GMT

I should add that you can run other code while the buzzer is running since it will be toggled on the Timer - you may have to be clever about it if you want to change the tone frequency to create a tune, but it's doable.

RE: PWM with a piezo buzzer

Steven Wik — Mon, 24 Mar 2014 14:52:25 GMT

Eliot,

I'm not familiar with the code you posted above or the library that it uses, however I have implemented code to control a piezo buzzer in much the same way that you require, which I'll provide snippets of below. Some of this code may be in the library you use, again I haven't looked at it.

First, you can configure the pin to the buzzer as an output like below:

// Audio / PWM Pin Setup
  nrf_gpio_cfg_output(PWM_OUTPUT_PIN_NUMBER);		// Connect GPIO input buffers and configure PWM_OUTPUT_PIN_NUMBER as output
  nrf_gpiote_task_config(0, PWM_OUTPUT_PIN_NUMBER, NRF_GPIOTE_POLARITY_TOGGLE, NRF_GPIOTE_INITIAL_VALUE_LOW);
  NRF_PPI->CH[1].EEP = (uint32_t)&NRF_TIMER2->EVENTS_COMPARE[1];  // Configure PPI channel 1 to toggle PWM_OUTPUT_PIN on every TIMER2 COMPARE[1] match
  NRF_PPI->CH[1].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[0];
  NRF_PPI->CHEN = (PPI_CHEN_CH1_Enabled << PPI_CHEN_CH1_Pos);  // Enable PPI channels 0-2

Then you can configure the Timer to control the PWM output as below - this code will cause the PWM pin to toggle whenever the number in CC[1] is hit:

// Configure Timer for Buzzer
	NRF_TIMER2->MODE = TIMER_MODE_MODE_Timer;
  NRF_TIMER2->PRESCALER = 4; //1us resolution
  NRF_TIMER2->TASKS_CLEAR = 1;
  NRF_TIMER2->CC[1] = MAX_SAMPLE_LEVELS;
  NRF_TIMER2->INTENSET = (TIMER_INTENSET_COMPARE1_Enabled << TIMER_INTENSET_COMPARE1_Pos);
  NVIC_EnableIRQ(TIMER2_IRQn); // Enable interrupt on Timer 2
  __enable_irq();

Next, you can implement a handler for the timer using code like below:

void TIMER2_IRQHandler(void)						// used for buzzer
{
  if ((NRF_TIMER2->EVENTS_COMPARE[1] != 0) && ((NRF_TIMER2->INTENSET & TIMER_INTENSET_COMPARE1_Msk) != 0))
  {
    // Sets the next CC1 value
    NRF_TIMER2->EVENTS_COMPARE[1] = 0;
    NRF_TIMER2->CC[1] = (NRF_TIMER2->CC[1] + MAX_SAMPLE_LEVELS);
  }
}

To start the buzzer, simply start the Timer, as below:

NRF_TIMER2->TASKS_START = 1;

Try changing the value of MAX_SAMPLE_LEVELS around to achieve different tones.

I hope this helps!

Cheers, Steve