Stuck in Timer Interrupt Handler

Question

I keep getting stuck in the TIMER1_IRQHandler function.

When I debug the code, I get to line 20 (exiting interrupt function scope) and then the assembly code hits a nop instruction. This then brings the debugger back up to line 17.

Why won't the code enter my if statement?

/** @file
* @brief Example template project.
* @defgroup nrf_templates_example Example Template
*
* NOTE: To get build to work, I right clicked nRF_micro-ecc folder and excluded it from build
*/

#include "main.h"

NRF_TIMER_Type* TIMER0 = (NRF_TIMER_Type*) TIMER0_BASE; // NRF_TIMER_Type has register structure
NRF_TIMER_Type* TIMER1 = (NRF_TIMER_Type*) TIMER1_BASE; // NRF_TIMER_Type has register structure
NRF_GPIOTE_Type* GPIOTE = (NRF_GPIOTE_Type*) GPIOTE_BASE; // NRF_GPIOTE_Type has register structure

int timer_flag = false;

void TIMER1_IRQHandler(void) {
    timer_flag = true;
    TIMER0 -> TASKS_STOP = 1;
    TIMER0 -> EVENTS_COMPARE[0] = 0;
}

void timer_init(hx711_mode_t number_of_pulses) {
    /*
      Setup timer for PWM
    */
    TIMER0 -> MODE &= 0; // Timer mode [DEFAULT]
    TIMER0 -> CC[0] |= 0x10; // Set value in CC[0] to 10
    TIMER0 -> PRESCALER |= 0x4; // Counts at 1 MHz [DEFAULT]
    TIMER0 -> SHORTS |= 1; // On compare[0] event, CLEAR timer
    
    /*
      Setup counter to stop PWM after number_of_pulses
    */
    TIMER1 -> MODE |= 0x2; // Timer set to low power counter mode
    TIMER1 -> CC[0] |= number_of_pulses; // Counter set to mode
    TIMER1 -> INTENSET |= (1 << 16); // initialise interrupt
    TIMER1 -> SHORTS |= (1 << 8); // On compare[0] event, STOP timer
    NVIC_SetPriority(TIMER1_IRQn, 0); // Priority 0 (lowest)
    NVIC_ClearPendingIRQ(TIMER1_IRQn);
    NVIC_EnableIRQ(TIMER1_IRQn); // Enables interrupt in NVIC interrupt controllers

    /*
      Use PPI for PWM generation
      Mode: task, Pin: PD_SCK, Polarity: Toggle, Initial pin value: LOW
    */
    GPIOTE-> CONFIG[0] = (0x3 << 0) |
                          (PD_SCK << 8) |
                          (0x3 << 16)|
                          (0 << 20);    

    NRF_PPI -> CH[0].EEP = (uint32_t) &TIMER0 -> EVENTS_COMPARE[0]; // Connect TIMER0 event to PPI channel 0
    NRF_PPI -> CH[0].TEP = (uint32_t) &GPIOTE-> TASKS_OUT[0]; // Connect toggle task to PPI channel 0
    NRF_PPI -> FORK[0].TEP = (uint32_t) &TIMER1-> TASKS_COUNT; // Connect toggle task to PPI channel 0
    NRF_PPI -> CHEN = (0x1 << 0); // Enable PPI channel 0
}

/**
 * @brief Function for application main entry.
 */
int main(void)
{
    timer_init(CHANNEL_A_128); // Setup Timer
    TIMER0 -> TASKS_START = 1;

    while(true) {
        if (timer_flag) {
            nrf_delay_ms(500);
            TIMER1 -> TASKS_CLEAR = 1;
            TIMER0 -> TASKS_START = 1;
            timer_flag = false;
            
        }
    }

}
/** @} */

Hakon · Accepted Answer

Doing it this way wont work either, because you are not clearing the interupt flag for the TIMER1 event when executing the TIMER1_IRQHandler. Your first attempt was correct except for the fact that you didn't force write back of EVENTS_COMPARE[0] = 0. You can do that by reading it to a dummy variable like this 
 void TIMER1_IRQHandler(void) {
 timer_flag = true;
 TIMER0 -> TASKS_STOP = 1;
	
	
	volatile uint32_t dummy;
	if (NRF_TIMER1->EVENTS_COMPARE[0] == 1)
	{
 NRF_TIMER1->EVENTS_COMPARE[0] = 0;

 // Read back event register so ensure we have cleared it before exiting IRQ handler.
 dummy = NRF_TIMER1->EVENTS_COMPARE[0];
 dummy; // to get rid of set but not used warning
	}
} 
 Hope this answers your question.

Turbo J · Answer

It wont't enter the if statement because timer_flag is not declared volatile. 
 Without the volatile, the optimizer sees an if that will never be true. 
 
 Note that the EVENTS_COMPARE[0] = 0; statement should not be the last statement in the IRQ handler due to the write buffer. This will cause the interrupt be still pending during the function return - causing tail chaining back into the handler function.

Stuck in Timer Interrupt Handler

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