nRF52 clock in and out

[Greg Nelson]

I have two related questions about nRF52 (nrf52832) which I have not been able to pinpoint from the documentation.

1. Can the 32MHz crystal connection be replaced by a 32MHz input if there is an existing clock in the system that meets the precision, tolerance, etc? On many processors, one of the two crystal pins may be used as an input while the other is left open if there is a driven external source, but I see no information about this for nRF52.

2. What is the most power-efficient way to get a high frequency clock output from the nRF52? Other ARM processors seem to offer direct "HFCLK" outputs to pins, but I can't find any related features in the nRF52 documentation. The best I've been able to guess is that the SPIM or PWM blocks could be used to provide a clock (8MHz or 16MHz respectively?) but each of those draws additional power and we don't (in this case) need the remaining functionality of the peripherals.

Thank you.

[Øyvind Karlsen]

Hi,

1. See Martin's answer.

2. The peripherals only run at 16MHz, this is why it is not possible to get HFCLK output from the nRF52. Max output is 8MHz, since you need to toggle on and off a pin. This is achieved using GPIOTE and PPI with the following code which creates a 8MHz output on pin 18:

    int main(void)
    {
    nrf_gpio_range_cfg_output(18); //Configure pin 18 as output
   
    NRF_CLOCK->TASKS_HFCLKSTART = 1; //Start high frequency clock
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0)
    {
         //Wait for HFCLK to start
    }
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0; //Clear event
    
    
    //Configure GPIOTE to toggle pin 18 
    NRF_GPIOTE->CONFIG[0] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                            18 << GPIOTE_CONFIG_PSEL_Pos | 
                            GPIOTE_CONFIG_OUTINIT_Low << GPIOTE_CONFIG_OUTINIT_Pos;
    
    //Configure timer
    NRF_TIMER1->PRESCALER = 0;
    NRF_TIMER1->CC[0] = 1;  // Adjust the output frequency by adjusting the CC.
    NRF_TIMER1->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos;
    NRF_TIMER1->TASKS_START = 1;
    
    //Configure PPI
    NRF_PPI->CH[0].EEP = (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[0];
    NRF_PPI->CH[0].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[0];
    
    NRF_PPI->CHENSET = PPI_CHENSET_CH0_Enabled << PPI_CHENSET_CH0_Pos;
    
    while (true)
    {
        // do nothing
    }
    }
   

Best regards,

Øyvind

[Øyvind Karlsen]

Well we have a pin capacitance of 4pF at each GPIO, the pulse moves at 8MHz and we know the relation X_C = 1/ ω C, this gives a equivalent resistance of approximately 5kΩ. Then you will have to add what you end up connecting to the pad. In the end the power consumed will be approximately equal to the power going through the equivalent resistor.

Alternately you can set up this on your development kit and measure the difference in current draw with and without 8MHz output. Please refer to the current measurement guide for practical information.