nRF52840: 16 MHz Clock-Out on GPIO Pin

[Mike]

Can we get a 16MHz clock out over a GPIO pin? We could use this to clock some downstream devices.

[Vidar Berg]

Hello,

The GPIO module runs on 16M so the theoretical max. frequency is limited to 8Mhz, unfortunately. 

[Mike]

Thanks for the quick response. Would it be possible then to get an 8MHz clock out over a GPIO pin? Could you provide some example code?

[Vidar Berg]

Yes, it should be possible. I made the code below that you can use to test this.

#include <stdbool.h>
#include <stdint.h>
#include "boards.h"


void gpio_clock_8m(uint32_t pin_number)
{
    
    NRF_TIMER1->PRESCALER = 0; // 16MHz
    NRF_TIMER1->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Msk;
    NRF_TIMER1->CC[0] = 1;

    NRF_GPIOTE->CONFIG[0] = GPIOTE_CONFIG_MODE_Task | (pin_number << GPIOTE_CONFIG_PSEL_Pos) |
                            (GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos);

    /*Connect TIMER event to GPIOTE out task*/
    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   = 1;

    /*Starts clock signal*/
    NRF_TIMER1->TASKS_START = 1;
}

/**
 * @brief Function for application main entry.
 */
int main(void)
{
    
    /*Start HF crystal for improved clock tolerance*/
    NRF_CLOCK->TASKS_HFCLKSTART = 1;
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);


    gpio_clock_8m(27);


    for (;;)
    {
        __WFE();
    }
     
}

Also, in case you haven't noticed, some pins are not recommended for high-frequency I/O operation (may interfere with RADIO). See the "Pin assignments" section of the PS to see which pins this applies to.

[Mike]

Thank you, this is extremely helpful. Yes, we will double-check to ensure we don't use a Radio sensitive pin. I appreciate your very quick response.

[Vidar Berg]

No problem. I hope the clock output characteristics meet your requirements.