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

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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 ? :folded_hands:

2sayle

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AScha.3Best answer

Super User

ok now - basically.
probe needs adjustment (square wave / overshoot , check with probe signal at scope)
hi speed testing only possible with 10:1 tip at point to test and ground at metal ring on tip.
line to tip gnd < 3cm from gnd on board (solder blank wire there).
with "standard" long gnd line /alligator clip signal never will be "perfect" looking

like this...