STM32F103CB Timer update interrupt not working

Question

I am trying to setup the timer update interrupt of a stm32f103cb. My setup and interrupt code looks as following:

void TIM2_IRQHandler(void)
{
 GPIOB->BSRR = 1UL << 1; // turn on led, works if function is called manually
 TIM2->SR = ~1;
}
void setup(void)
{
 RCC->APB1ENR |= 1; // enable clock for TIM2
 TIM2->PSC = 71;
 TIM2->ARR = 0xFFFF;
 NVIC->ISER[0] |= TIM2_IRQn; //TIM2_IRQn == 28
 NVIC->IP[7] |= 2 << 4;
 TIM2->SR = 0;
 TIM2->CNT = 0;
 TIM2->DIER = 1; // enable update interrupt
 TIM2->CR1 = 1; // enable timer
}

From my understanding TIM2_IRQHandler should be called, everytime the timer updates, but that does not happen. Some things i know:

• The rest of the programm runs fine, the interrupt is just never triggered
• The timer is running
• The update interrupt flag in TIM2->SR is getting set correctly, everytime the timer updates

Am i forgetting something in the setup or is the error somewhere else ?

• Scott Seidman: Is the timer working at all? Can you poll it and see if the value is changing?
• user5675428: I polled the timer and the value is changing
• rdtsc: Is there something like a global interrupt enable?
• Erlkoenig: Always clear the interrupt flag (i.e. set the SR) as early as possible in the ISR, or the interrupt might be triggered again endlessly.

Answer 1

I figured it out.

NVIC->ISER[0] |= TIM2_IRQn;

should be

NVIC->ISER[0] = 1UL << TIM2_IRQn;

because to enable the interrupt you want to set bit 28, not write 28 as bit value to the register. Now it works as expected.

• Erlkoenig: It should be NVIC->ISER[TIM2_IRQn/32], in case the interrupt number is larger than 31.
• the busybee: Please mark your own answer so that it is shown as "accepted" in the lists. @Erlkoenig It has to be TIM2_IRQn % 32. Since I don't use that controller I don't know, but the header file might already define the correct value even for interrupts in ISER1 and ISER2.
• Erlkoenig: @thebusybee No, the % 32 would have to go on the right side: NVIC->ISER[TIM2_IRQn/32] = (1 << (TIM2_IRQn%32));. This is is what ARM does, but using bitwise operations instead of arithmetic to make it less readable. This is an example in assembly doing the same thing.
• the busybee: @Erlkoenig Right, I wrote my comment too fast. Thanks for clarifying!