nRF52, 1-4 MHz clock needed

[Ladasky]

I am using the nRF52 Preview board, PCA 10036. I need to provide a high-frequency clock signal to a peripheral. Anything between 1 and 4 MHz will work. However, I don't need and don't want the CPU to receive an interrupt on every clock pulse.

I think that the GPIOTE example in the SDK is relevant to my needs. (I'm using SDK 0.9.2.) However: the code has a function called timer_dummy_handler(). While this function does nothing, the code does connect it to a timer event, with a call to nrf_drv_timer_init(). The documentation for nrf_drv_timer_init() says that an event handler function is a required argument.

The GPIOTE example code has a timer frequency of 200 milliseconds. Calling a quick and useless function five times/second is harmless. Calling that same function 1-4 million times / second would be a nightmare.

Is that function call actually used?

Can I avoid attaching a function call to a timer event, if I program at a lower level?

[Øyvind Karlsen]

Hi,

I wrote some code for this earlier, for 4 MHz output on pin 18 set NRF_TIMER1->CC[0] = 2;.

int main(void)
{
    // Set up GPIO as output
    nrf_gpio_range_cfg_output(17, 18);
    nrf_gpio_pin_clear(17); // Light LED 1 to indicate that the code is running
    
    // Start high frequency clock
    NRF_CLOCK->TASKS_HFCLKSTART = 1;
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0)
    {
        // Wait for clock to start
    }
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    
    // 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;
                            
    // Set up timer
    NRF_TIMER1->PRESCALER = 0;
    NRF_TIMER1->CC[0] = 2; // 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;
        
    // Set up PPI to connect the timer compare event with the GPIOTE toggle 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 = PPI_CHENSET_CH0_Enabled << PPI_CHENSET_CH0_Pos;
    
    while (true)
    {
       
    }
}

The principle is well described by RKs post. You can find relevant documentation in the infocenter:

• Programmable Peripheral Interconnect (PPI)
• General Purpose Input Output (GPIO)
• GPIOTE (GPIO Tasks and Events)
• Timer

Best regards,

Øyvind

[RK]

18 lines - I guessed 20 - I must be slipping :)