Can make a 50MHz PWM output?

jhwan.2 · ‎2022-11-13

MCU : STM32H7, PB0 PWM CH2N

TIM1 CLK : 200MHz

Prescaler : 0

Counter Period : 3

Pulse : 2

GPIO Speed : Very High

I'm testing a 50MHz PWM output.

And my test conditions are written at the top of this question.

As a result, I decided that a 50MHz PWM output was impossible.

This is because the square wave is distorted by the slew rate as shown in the attached picture.

However, the output buffer timing characteristics of the datasheet states that frequencies up to 220 MHz are possible.

So I leave a question to ask if I've missed anything.

thank you

AScha.3 · ‎2022-11-15

you have to use probe in 10:1 setting. (1:1 is useless with hi-speed cmos circuits ! )

cpu is stm32H743vit , at 200MHz core clk.

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Javier1 · ‎2022-11-13

>>GPIO Speed : Very High

are you just toggling a GPIO or using a proper PWM timer?

What about unwanted capacitances? show us your hardware

we dont need to firmware by ourselves, lets talk

QSHAO.1 · ‎2022-11-14

the capacitance load on PB0 may be a reason of longer rising time of PMW signal, so you may check this point firstly.

jhwan.2 · ‎2022-11-14

hi. Javier

I use TIM1 for PWM CH2N.

Nothing is connected to the PWM output.

I'm just checking the waveform with an oscilloscope.

LCE · ‎2022-11-14

... and as your oscilloscope looks exactly like my Tektronix with only 100 MHz Bandwidth:

to check a 50 MHz square wave, you will need much higher bandwidth.

Not to forget the probe, and how it is connected to the test point - mind the GND connection, which should not run over a few inches of extra cable.

jhwan.2 · ‎2022-11-14

hello.

I haven't connected anything to the PWM output (PB0).

I'm just checking the PWM waveform with an oscilloscope.

Thank you for answer

jhwan.2 · ‎2022-11-14

S.Ma · ‎2022-11-14

Io speed = 3 ?

Probe in DC or AC mode?

Test MCO pin at 50+ MHz.

AScha.3 · ‎2022-11-14

and probe is 10:1 ? how many Pf input ? min. 200 MHz type?

+ square wave is adjusted ?

64Mhz out on MCO1, with my DSO, and with 100MHz probe (so no square possible, just similar sine wave):

and 100Mhz , 4Vss ...

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Javier1 · ‎2022-11-14

>>Nothing is connected to the PWM output.

you think nothing is connected to the pwm output.

Are you usign your own custom board or any nucleo/discovery boards?

Because nucleo and discovery boards have lots of caps and external components

we dont need to firmware by ourselves, lets talk