TIM1 appears to not stop

Question

I am using TIM1 to drive a DMA channel which is sending bytes to GPIOF on an STM32F407. TIM1 is started by an OC channel on a different timer. In the DMA stream interrupt handler, I am attempting to stop TIM1 and also reset the counter by generating an Update event. For troubleshooting purposes I have configured an OC channel on TIM1 so I can confirm it is starting and stopping as expected.

It does appear to start and stop at the correct times when viewing the pin output using a logic analyzer, but when stepping through the interrupt handler in the debugger I see that the CEN bit is still set after clearing it. Also, TIM1->CNT continues to increment as I step through each line.

The interrupt handler is as follows:

void __attribute__((interrupt("IRQ"))) DMA2_Stream5_IRQHandler(void)
{
    // Clear interrupt flags
    do
    {
        DMA2->HIFCR = DMA_HIFCR_CTCIF5 | DMA_HIFCR_CHTIF5;
    }
    while(DMA2->HISR & DMA_HISR_TCIF5);

    // Stop TIM1, TIM5 will re-enable it
    //TIM1->CR1 &= (u16)~TIM_CR1_CEN;
    do
    {
        TIM1->CR1 &= (u16)~TIM_CR1_CEN;
    }
    while(TIM1->CR1 & TIM_CR1_CEN);
    TIM1->EGR = TIM_EventSource_Update;     // Trigger an Update to reset the counter

    // Re-enable DMA for next TIM5 event
    DMA2_Stream5->CR |= DMA_SxCR_EN;
}

The code does exit the loop which clears CEN, but afterwards viewing the TIM1 registers shows CEN is still set. How can I ensure the timer is really stopped and will start counting from 0 next time it is started?

Answer 1

It appears that clearing the enabled flag has no effect when the timer is in slave mode. I have changed my interrupt handler as follows, and now it is working as expected. The timer is stopped and reset, until started again by TIM5.

// Clear interrupt flags
DMA2->HIFCR = DMA_HIFCR_CTCIF5 | DMA_HIFCR_CHTIF5;

// Disable slave move, can't stop the timer unless we do this first
TIM1->SMCR = 0;

// Stop TIM1, TIM5 will re-enable it
TIM1->CR1 = 0;// &= (u16)~TIM_CR1_CEN;
TIM1->EGR = TIM_EventSource_Update;     // Trigger an Update to reset the counter

// Re-enable slave mode
TIM1->SMCR = TIM_SlaveMode_Trigger;

Answer 2

1. it is generally a (very very) bad practice to loop around in an isr.

2. no point in looping around to clear CEN. if it comes back set, somewhere else your code did it.

3. flip a pin in the isr to see timing.