STM32H743II : Timer Frequency Tolerance

[keiju]

Hello,

I am working on creating a timer function using the timer peripheral of the STM32H743II, with a timer range from 1 second to 3600 seconds.

I'd like to ask a question regarding the frequency accuracy of the timer.

Let me first explain the current timer configuration I’m considering.

At this point, I am feeding a 19.2 MHz clock signal into the HSE, and have configured the APB1 Timer Clock to be 201.6 MHz.

I plan to use TIM2 and adjust the prescaler(PSC) and ARR values to achieve a timer period ranging from 1 to 3600 seconds.

Since the timer period is given by:

Timer Period = (PSC + 1) * (ARR + 1) / (APB1 Timer Clock)

If I set PSC = 20159, then:

Timer Period = 20,160 * (ARR + 1) / 201,600,000 = (ARR + 1) / 10,000

This means that by setting (ARR + 1) in the range of 10,000 to 36,000,000, I can achieve a timer period of 1 to 3600 seconds.

Here’s my question:
If the 19.2 MHz signal fed into the HSE has a frequency accuracy of ±1 ppm, can I assume that the timer I create will also have a frequency accuracy of ±1 ppm?

Best regards,
Keiju

[AScha.3]

Hi,

Yes.

The PLL will track the HSE without any drift, but you get some jitter , that's not important here, because jitter is a short time phenomenon by the PLL, but your time values are so big, it will do nothing bad here. (You might get some xx ps, picoseconds, +/- , in the time interval, but this is here same or even less, than the frequency drift and jitter of the crystal itself.) So: perfect timing.