Comparator External Connections

Question

I am attempting to use the COMP section to build a relaxation oscillator. There appear to be a number of interactions between sections, some of which are subtly documented and some for which I find no documentation. For instance, the inability to set a GPIO output once a task is connected to a pin is documented but well-hidden. The fact that enabling a pin as a comparator input completely disables both digital inputs and digital outputs is not documented anywhere that I can find. 
 Bottom line is that I have everything working nicely as it should, except that the comparator sometimes appears to miss a transition and thereafter is hung. I fundamentally just need to be able to place the comparator output on a digital output pin, but the only way I can find to do that is via the crossing events from the comparator, feeding set and clr tasks of GPIOTE. This causes the same sort of problems as the early IBM PCs using edge-triggered interrupts: If you miss one, you're dead forever. Is there a better way to get the comparator output to appear on a pin? If not, how best can I initialize things to ensure that I don't miss a transition?

L.B. · Answer

Hi, I'm doing the same thing - I'm building an RC generator that involves the COMP module. 
 So far I haven't seen any glitches, so I decided to share my init code: 
 
 #define CONFIG_COMP_OUT_PIN 28

static nrf_ppi_channel_t m_ppi_channel;

nrfx_comp_config_t const comp_config = {
 .reference = NRF_COMP_REF_VDD,
 .ext_ref = NRF_COMP_EXT_REF_7, //P0.31
 .main_mode = NRF_COMP_MAIN_MODE_SE,
 .threshold = {
 .th_up = 34,
 .th_down = 30
 },
 .speed_mode = NRF_COMP_SP_MODE_Low,
 .hyst = NRF_COMP_HYST_50mV,
 .isource = NRF_COMP_ISOURCE_Off, 
 .input = NRF_COMP_INPUT_1, //P0.03
 .interrupt_priority = 0
};

void nrfx_comp_irq_handler(void){

}

void init(void){
	uint32_t gpiote_task_addr;
	uint32_t comp_event_addr;

	nrfx_comp_init(&comp_config, nrfx_comp_irq_handler);
	nrfx_comp_start(NRFX_COMP_EVT_EN_CROSS_MASK, 0);

	nrf_drv_ppi_init();

	nrf_drv_gpiote_out_config_t config = GPIOTE_CONFIG_OUT_TASK_TOGGLE(false);
 nrf_drv_gpiote_out_init(CONFIG_COMP_OUT_PIN, &config);

	nrf_drv_ppi_channel_alloc(&m_ppi_channel);

	comp_event_addr = nrf_drv_comp_event_address_get(NRF_COMP_EVENT_CROSS);
	gpiote_task_addr = nrf_drv_gpiote_out_task_addr_get(CONFIG_COMP_OUT_PIN);

	nrf_drv_ppi_channel_assign(m_ppi_channel, comp_event_addr, gpiote_task_addr);
	nrf_drv_ppi_channel_enable(m_ppi_channel);

	nrf_drv_gpiote_out_task_enable(CONFIG_COMP_OUT_PIN);
	nrf_drv_gpiote_set_task_trigger(CONFIG_COMP_OUT_PIN); //Start oscillations
} 
 
 I have inputs on P0.03 (-) and P0.31 (+), and an output on P0.28 (OUT) with this code. 
 Hope this helps! 
 
 Regards, 
 L. B. 
 
 p.s. My target is NRF52832 / nRF5_SDK_17.0.2_d674dde / gcc version 10.3.1 20210621 (release) (GNU Arm Embedded Toolchain 10.3-2021.07)