Are Output Compares Hardware Interrupts? Understanding TIM8 Interrupts on STM32

Output compare functionality in timers often utilizes hardware interrupts to signal specific events. This article delves into configuring TIM8 on an STM32 microcontroller to generate interrupts using compare registers, aiming to achieve precise timing control. We’ll examine the provided code snippet and address challenges encountered while setting up the timer for interrupt-driven operation.

Decoding the Timer Initialization

The provided timerOpen() function aims to initialize TIM8 for generating interrupts every 150 microseconds using its compare registers. Let’s break down the key aspects:

• Clock Configuration: The code first calculates the APB2 clock frequency, which drives TIM8, and then determines the appropriate prescaler value to achieve a 1 MHz timer clock. This ensures that each timer tick represents 1 microsecond.

• Timer Parameters: The code sets the timer period to a large value (100000 – 1) and configures the timer to count up. The CCR1 register (Compare/Capture Register 1) is set to 150, intending to trigger an interrupt after 150 timer ticks (150 microseconds).

• Interrupt Setup: The code enables the timer’s update interrupt and configures the associated NVIC (Nested Vector Interrupt Controller) priority. However, critically, it seems to be missing the crucial step of enabling the specific output compare interrupt for channel 1 in the DIER (DMA/Interrupt Enable Register) with TIM8->DIER |= TIM_DIER_CC1IE. This is likely the primary reason the desired interrupt is not occurring. While the code comments suggest enabling the update interrupt (//timer_handle.Instance->DIER |= 0x02;), it’s commented out and thus not active.

• HAL Initialization: The code utilizes HAL (Hardware Abstraction Layer) functions for timer initialization (HAL_TIM_OC_Init and HAL_TIM_OC_Start_IT). While HAL simplifies development, it’s essential to understand the underlying register configurations it manages.

Addressing the Interrupt Issue

The key to resolving the interrupt issue lies in enabling the correct interrupt source. Modify the timerOpen() function to explicitly enable the output compare 1 interrupt:

// ... other initialization code ...

//Enable the output compare 1 interrupt
timer_handle.Instance->DIER |= TIM_DIER_CC1IE; 

// ... rest of the initialization code ...

Furthermore, ensure the interrupt handler function TIM1_IRQHandler() correctly clears the interrupt flag:

void TIM1_IRQHandler(void) {
  // Clear the OC1 interrupt flag
  if (__HAL_TIM_GET_FLAG(&timer_us_timhandle_, TIM_FLAG_CC1) != RESET) {
     __HAL_TIM_CLEAR_IT(&timer_us_timhandle_, TIM_FLAG_CC1);
     // Add your interrupt handling code here
   }
  HAL_TIM_IRQHandler(&timer_us_timhandle_); 
}

Using Other Timers (TIM2, TIM3, TIM4…)

Yes, the same principles apply to other timers on the STM32. Each timer possesses compare/capture registers and associated interrupt capabilities. You would need to adapt the code to use the appropriate timer instance, clock configuration, and interrupt vector. Ensure you consult the STM32 reference manual for specific details on each timer’s capabilities and register configurations. Remember to enable the relevant clock for the chosen timer using __HAL_RCC_TIMx_CLK_ENABLE(), replacing ‘x’ with the timer number.

Conclusion

Successfully using output compares for hardware interrupts requires meticulous attention to register configurations. Enabling the correct interrupt source in the DIER register is crucial. By understanding the interplay between HAL functions and the underlying hardware, developers can effectively leverage the power of timers for precise timing control in embedded systems. Always refer to the specific STM32 microcontroller’s datasheet and reference manual for comprehensive information.