Delays using Timer in stm32

Question

I am currently trying to achieve delay of 1 uS using timer in stm32 for my application purpose.

SYSCLk is set to 16MHZ LSI and define same for timer clock.
Timer used is a 16 bit timer.
The logic I am using is this.

void delay_us (unsigned int cnt_val)
{ 
unsigned long x =0;
unsigned int cnt=0, psc_cnt = 0;
RCC->APB1ENR |= (RCC_APB1ENR_TIM6EN );
x = (unsigned int)((cnt_val * 1000) / 62);
if(x < 65536)
 {
 cnt = x;
 psc_cnt = 0;
 }
else
 {
 cnt = 0xFFFF;
 psc_cnt = x / 65536;
 }
TIM6->SR = 0x00;
TIM6->ARR = cnt;
TIM6->PSC = psc_cnt;
TIM6->CR1 |= (TIM_CR1_CEN | TIM_CR1_OPM);
while(!(TIM6->SR & TIM_SR_UIF));
RCC->APB1ENR &= ~(RCC_APB1ENR_TIM6EN); 
}

Issue is when i tried to achieve the delay of 1 us i am getting 2.5 us. I checked it by toggling the gpio.

while(1)
{
GPIOA->ODR ^= GPIO_PIN_5;
delay_us(1);
}

can't understand the reason and looking for the solution.

• Eugene Sh.: How about longer times? I guess 1us is comparable with your processor clock and very affected by the overhead of the instruction executed around.
• Elliot Alderson: Achieving accurate delays on the order of a microsecond will be difficult using microcontroller software. Why don't you tell us what you plan to do with this delay function?
• Vaibhav: @EugeneSh. in case of longer times as the error is in microseconds, output is acceptable I tried with 1 sec delay and it works good.
• Vaibhav: @ElliotAlderson curiosity as how far can i go.
• Lundin: If you need this kind of microsecond accuracy you are using the wrong tool. You'd need some beast like a fast-clocked DSP. It can be achieved with a MCU however, by simply executing a bunch of NOP. Not in a loop, but literally just NOP NOP NOP...

Answer 1

Look at your delay_us() function.
How many assembler instructions do you think that will compile to?
Don't forget the stack push/pop and return at the end.
Now, at 16MHz, how long do you think that function will take to execute, assuming the while() at the end just falls straight through.

I threw your function into my GCC-based ARM compiler and, without much optimization enabled, it produced 33 instructions.
Considering that your measured 2.5uSec corresponds to about 40 instructions at 16MHz, and adding a few extra instructions for the while(1) loop , the GPIO toggle and the call to the function in your main(), it looks to me as though it's pretty spot-on.

• Vaibhav: is there any better logic for achieving same result?. If yes a hint will be helpful.
• brhans: It depends largely on what your desired use-case is. For toggling a GPIO, as in your example, you should be directly driving the pin using a hardware module in your micro and not try to control it in the firmware. So - what are you actually trying to do?
• Vaibhav: i am trying to create a delay in microseconds, toggling gpio is just for testing that delay created working expected or not. I was confirming that reason of issue is same as i am thinking and get any better solution if any