[Help] Rise/fall times too slow on nanosecond-wide pulses

[2sayle]

Hello everyone :D

I am currently using the STM32H743VIT6 (LQFP-100) on a custom PCB.

My goal is to generate fast positive pulses (ideal case would be a 10ns pulse) on TIM4 CH2 (PB8). APB2 is clocked at 200MHz so I can get 5ns ticks from my timer.

Generating 100ns wide pulses is quite easy using PWM by placing "Pulse" parameter on 16bits to 19 = (20-1) since 100ns/5ns = 20.

Problem is: if I want to narrow this pulse to 50ns for example, the trace of the signal starts to become rounded and doesn't reach 3.3V on top.

I suspect it is because of the rise/fall times which are about 30ns each.

Second problem is: When I change OSPEEDR register to VERY_HIGH, the rise/fall times are only reduced by a few nanoseconds while the datasheet shows rise/fall times less than 5ns for my range of capacitive load and supply (3.3V). I have estimated the capacitive load to be approx. 40pF.

Can someone enlighten me please ? 🙏

2sayle

[2sayle]

Thank you all for your answers. Indeed, I am not used to probing on PCBs. Your advice has been invaluable! Do you have any manuals/tutorials that I can use to improve my hardware testing skills?

[AScha.3]

there are some good white papers and app notes , look at Ti, Analog Devices, Tektronix...

https://e2e.ti.com/blogs_/#pifragment-322110PostSortBy=MostComments

https://www.pmk.de/web/editor/files/High%20Speed%20SW%20Current%20measurement%20by%20Rogowski%20Coil%20Current%20Probe_Iwatsu%20AppNote.pdf

https://www.ti.com/lit/an/slyt627/slyt627.pdf

[2sayle]

Thank you !