Synchronizing DMA SAADC with PWM using nRF52

[Jarmo]

We have an optical measurement system where we need to pulse LED and sample a phototransistor precisely during the LED pulse. The rate of sampling is 1 kHz and we plan to use 1:10 pulse ratio for the LED.

This far, I have written a working system than samples at 1 kHz using SAADC and DMA (using four channels, actually). The LED is not pulsed yet, but is on all the time.

I guess it is relatively simple to use pulse width modulation (PWM) to pulse the LED. However, I need to sychronize ADC with the PWM pulse. In another words, analog-to-digital conversion must happen about 100 us after the LED is put on.

My question is how to get this done? It would be best to stay in DMA based ADC, but is the synchronization with PWM possible? Or do I need to switch to interrupt based ADC (hopefully not)?

[Jarmo]

Okay, I am now trying to set up that system. However, it seems like there's some confusion when trying to map your example into the software API instead of poking directly the hardware. Sorry to say, but most of your APIs are nothing but easy to understand. My code looks like below, I have commented away your original lines and tried to replace them with the API calls:

    nrf_ppi_channel_t ch;
    uint32_t timerEvent = nrf_drv_timer_compare_event_address_get(&m_timer, NRF_TIMER_CC_CHANNEL0);
    nrf_drv_ppi_channel_alloc(&ch);
    // Configure GPIOTE to toggle debug pin 
    NRF_GPIOTE->CONFIG[0] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                            MCU_DEBUG_LED << GPIOTE_CONFIG_PSEL_Pos | 
                            GPIOTE_CONFIG_OUTINIT_Low << GPIOTE_CONFIG_OUTINIT_Pos;
    // Set up PPI to connect the timer compare event with the GPIOTE toggle task
    nrf_drv_ppi_channel_assign(ch, timerEvent, 0); // ???
    // NRF_PPI->CH[ch].EEP = (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[0];
    / /NRF_PPI->CH[ch].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[0];
    nrf_drv_ppi_channel_enable(ch);
    // NRF_PPI->CHENSET = PPI_CHENSET_CH0_Enabled << PPI_CHENSET_CH0_Pos;

As you might guess, this does not work. I am wondering if the index zero in NRF_GPIOTE->CONFIG[0] refers to PPI channel or what is it? What is wrong with this code?

[Jarmo]

Hi and thanks. It looks like I am getting closer, led is blinking on timer PPI event and ADC seems also to work. However, now I would need a simple example how to set up PWM with say, 10% up/down ratio, and link an event from the PWM into the GPIO led toggle task. What kind of code would be that, can you help? I did not find an example of PWM with PPI.