What is the min/max skew on the counter outputs for the STM32F107RC?

AMoro.1 · ‎2020-10-01

Hello all,

I looking to use a STM32F107FC as a digital waveform generator. The system clock would be 28MHz and so would the APB bus feeding the timers. I plan to have TIM4 drive an output with a 14MHz clock signal and act as a master for TIM3. TIM3 would be (effectively) clocked by TIM4 and drive four outputs, all using PWM in TIM3 in a fixed cycle.

Of course, there's going to be some edge skew between the clock from TIM4 and the outputs driven by TIM3 (and within the TIM3 outputs), so things will need to be re-clocked. The current plan is to use the falling edge of the TIM4 output. However, this relies on the five signals having a edge skew within 25 to 30nsec (the outputs will all be high powered with less than 30pF for a load capacitance). I checked the spec and the reference manual for the STM32F107FC, but there's no mention of signal skew. I've also searched the forums and the app notes with no luck. Anyone have any ideas how to get the info or know what the values are? Otherwise I'll have to start adding parts to the design like a PLD. Using the MCU would be cleaner though.

Thanks!

waclawek.jan · ‎2020-10-01

The timers are synchronous, i.e. they are reclocked by their internal clock source (APB clock). In other words, there will be a difference between the outputs of the two timers, but it will be multiple of the APB clock, and it could be compensated by a suitable "phase difference" between TIM4 output and the TIM4 channel which clocks TIM3.

JW

AMoro.1 · ‎2020-10-07

Not really worried about the APB multiple clock period difference between TIM3 and TIM4. As waclawek.jan mentioned, that can accommodated.

The issue is the difference in propagation time from the event that the APB clock rises to the event that the appropriate output changes. So not just TIM3_Ch1 versus TIM4_CH1, but TIM3_CH1 versus TIM3_CH2 versus TIM3_CH3 versus TIM3_CH4 versus TIM4_CH1.

waclawek.jan · ‎2020-10-07

Please note that this is a primarily user-driven forum, with casual ST presence.

If you want an assurance beyond what can be found in the DS, you have to contact ST directly, through web support form or through FAE.

JW