PPI interrupt was influenced by BLE connecting event

Question

Hi, 
 I used PPI module and Timer2 to generate a 4Mhz pwm. But I founld when using softdevice(s110 v7.0), the pwm will be broke a while by BLE advertising or connecting event. So I suppose the ble event will influence the PPI module ? 
 And is there any other method to generate a 4Mhz or 8Mhz clock to driver external device?

Chris · Accepted Answer

You need to use the GPIOTE tasks to have the PPI driver the GPIO directly without any intervention. I had the same problem and had to jiggle around to get it to work.

This snippet of code is used to drive a buzzer (via PWM) with two output lines tied together and another output line at the opposite (so when the two are high, I set the other low). But you can strip out a bunch of these to just have it toggle one line.

// Configure as outputs
nrf_gpio_cfg_output(BUZZER_OUT_0);
nrf_gpio_cfg_output(BUZZER_OUT_1);
nrf_gpio_cfg_output(BUZZER_IN_0);

// Configure GPIOTE_CHANNEL_NUMBER to toggle the GPIO pin state with input.
// @note Only one GPIOTE task can be coupled to an output pin.
nrf_gpiote_task_config(0 /*GPIOTE_CHANNEL_NUMBER*/, BUZZER_OUT_0, \
                       NRF_GPIOTE_POLARITY_TOGGLE, NRF_GPIOTE_INITIAL_VALUE_LOW);
nrf_gpiote_task_config(1 /*GPIOTE_CHANNEL_NUMBER*/, BUZZER_OUT_1, \
                       NRF_GPIOTE_POLARITY_TOGGLE, NRF_GPIOTE_INITIAL_VALUE_LOW);
nrf_gpiote_task_config(2 /*GPIOTE_CHANNEL_NUMBER*/, BUZZER_IN_0, \
                       NRF_GPIOTE_POLARITY_TOGGLE, NRF_GPIOTE_INITIAL_VALUE_HIGH);

// Clear TIMER2
NRF_TIMER2->TASKS_CLEAR = 1;

// Configure TIMER2 for compare[0] event every xx 
NRF_TIMER2->PRESCALER = 0;
NRF_TIMER2->CC[0]     = 1904;
NRF_TIMER2->MODE      = TIMER_MODE_MODE_Timer;
NRF_TIMER2->BITMODE   = TIMER_BITMODE_BITMODE_16Bit;
NRF_TIMER2->SHORTS    = (TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos);

// Configure the PPI
sd_ppi_channel_assign(0 /*ch_num*/, &NRF_TIMER2->EVENTS_COMPARE[0], &NRF_GPIOTE->TASKS_OUT[0]);
sd_ppi_channel_enable_set(1 << 0 /*ch_num*/);

sd_ppi_channel_assign(1 /*ch_num*/, &NRF_TIMER2->EVENTS_COMPARE[0], &NRF_GPIOTE->TASKS_OUT[1]);
sd_ppi_channel_enable_set(1 << 1 /*ch_num*/);

sd_ppi_channel_assign(2 /*ch_num*/, &NRF_TIMER2->EVENTS_COMPARE[0], &NRF_GPIOTE->TASKS_OUT[2]);
sd_ppi_channel_enable_set(1 << 2 /*ch_num*/);
// Start it
NRF_TIMER2->TASKS_START = 1;  // Start event generation.

This value here 1904 is for about 4.1Khz. You can calculate this with the formula

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

Hope that helps, I had exactly the same problem with the BLE SD as the interrupts are the highest priority so all the nrf_pwm.c sample code had issues..