RE: sample ADC synced with PWM

AmirZ — Thu, 24 May 2018 14:24:10 GMT

Hi,

After reading some documentation and examples, I am trying to connect SAADC with PWM using PPI, but I failed to do that.

I manage to convert the PWM but faild to trigger the ADC sampling.

Attached my code to do this,

please give me some advises or suggestion to correct it.

void pwm_init(void)
{

uint32_t out_pins[] = {18, NRF_PWM_PIN_NOT_CONNECTED, NRF_PWM_PIN_NOT_CONNECTED, NRF_PWM_PIN_NOT_CONNECTED};
static uint16_t pwm_seq[2] = {62, 62};
nrf_pwm_sequence_t const seq =
{
.values.p_common = pwm_seq,
.length = sizeof(pwm_seq)/sizeof(uint16_t),
.repeats = 0,
.end_delay = 0
};

//nrf_gpio_cfg_output(5);
nrf_pwm_pins_set(NRF_PWM0, out_pins);
nrf_pwm_enable(NRF_PWM0);
nrf_pwm_configure(NRF_PWM0, NRF_PWM_CLK_16MHz, NRF_PWM_MODE_UP, 124);
nrf_pwm_loop_set(NRF_PWM0, 0);
nrf_pwm_decoder_set(NRF_PWM0, NRF_PWM_LOAD_COMMON, NRF_PWM_STEP_AUTO);

nrf_pwm_sequence_set(NRF_PWM0, 0, &seq);

NRF_PWM0->TASKS_SEQSTART[0] = 1;

}

void saadc_init(void)
{
ret_code_t err_code;
nrf_saadc_channel_config_t channel_config =
NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(3);

err_code = nrf_drv_saadc_init(NULL, saadc_callback);
APP_ERROR_CHECK(err_code);

err_code = nrf_drv_saadc_channel_init(0, &channel_config);
APP_ERROR_CHECK(err_code);

err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0], SAMPLES_IN_BUFFER);
APP_ERROR_CHECK(err_code);

/*err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1], SAMPLES_IN_BUFFER);
APP_ERROR_CHECK(err_code);*/

}

void saadc_sampling_event_init(void)
{
ret_code_t err_code;

err_code = nrf_drv_ppi_init();
APP_ERROR_CHECK(err_code);

uint32_t pwm_pulse_event_addr = nrf_pwm_task_address_get(NRF_PWM0, NRF_PWM_EVENT_STOPPED );
uint32_t saadc_sample_task_addr = nrf_drv_saadc_sample_task_get();

#if 1
/* setup ppi channel so that timer compare event is triggering sample task in SAADC */
err_code = nrf_drv_ppi_channel_alloc(&m_ppi_channel);
APP_ERROR_CHECK(err_code);

err_code = nrf_drv_ppi_channel_assign(m_ppi_channel,
pwm_pulse_event_addr,
saadc_sample_task_addr);
APP_ERROR_CHECK(err_code);
#endif
}

void saadc_sampling_event_enable(void)
{
ret_code_t err_code = nrf_drv_ppi_channel_enable(m_ppi_channel);

APP_ERROR_CHECK(err_code);
}

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
{
ret_code_t err_code;

err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAMPLES_IN_BUFFER);
APP_ERROR_CHECK(err_code);

int i;
printf("ADC event number: %d\r\n", (int)m_adc_evt_counter);

for (i = 0; i < SAMPLES_IN_BUFFER; i++)
{
printf("%d ", p_event->data.done.p_buffer[i]);
}

printf("\n\r");
}
}

void main()

{

pwm_init();
saadc_init();
saadc_sampling_event_init();

saadc_sampling_event_enable();

}

RE: sample ADC synced with PWM

Rune Holmgren — Tue, 22 May 2018 14:18:23 GMT

Hi,

It's possible to do this. It's a quite specialized use case, so I suspect you will have to interact with hardware registers directly in accordance to the product specifications instead of using the drivers. It's not particularly hard to do, but requires spending time reading the specifications for the hardware well to understand the implementation. If you set up the PWM peripheral to generate the pulse as well as a PPI event on each pulse you can trigger a measurement by the SAADC at a frequency up to 200kHz. I recommend reading the PPI documentation linked below, but to summize PPI is Nordics system for allowing two hardware peripherals to send events from one device which trigger a task in the other device without involving the CPU. The SAADC running in continuous mode should be used with this PPI channel as the trigger, and data will accumulate for the CPU to handle whenever it is available.

Be aware that this sample rate is quite high, so you will not be able to continuously send all the data over BLE at the rate you are collecting it.

PPI documentation

SAADC documentation

PWM documentation

Note that using PPI, a timer, and the GPIO might be a suitable alternative to using the PWM peripheral. I recommended the PWM peripheral for its accuracy and flexibility, but it might be overkill to use it.

Best regards,
Rune Holmgren