PWM Output on PB0 and PA8 does not Reach 0V or 3.3V (STM32F429I-Discovery)

[chachamaru]

Hello,

I am currently wor

king on connecting an MT9M001 camera module to the STM32F429I-Discovery board and displaying the captured image on a PC.
During this process, I am testing PWM output on the board, but I am experiencing an issue where the output voltage does not swing between 0 V and 3.3 V as expected.

Environment

• Board: STM32F429I-Discovery

• IDE / Firmware: STM32CubeIDE + HAL (STM32CubeF4 package)

• Pin used: PB0 / TIM3_CH3 (Alternate Function, Push-Pull, no pull-up/down)

• PWM: Generated with TIM3 (frequency and duty cycle shown in attached code/images)

• Measurement: Oscilloscope (×10 probe, GND connected to board GND)

Issue

The PWM waveform output from PB0 shows the following problems:

• The low level does not go down to 0 V

• The high level does not reach 3.3 V

Before switching to PB0, I was using PA8 (TIM1_CH1), but I observed the same issue—
the voltage could not drop to 0 V.
I received advice that PA8 might be loaded by other on-board circuitry,
so I moved the PWM output to PB0 (TIM3_CH3).
However, the same reduced voltage swing occurs on PB0 as well.

If anyone knows possible causes or has experienced a similar issue,
I would appreciate any guidance.

 

[Andrew Neil]

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@chachamaru wrote:

How can I get a clean 20–24 MHz PWM (XCLK) on this board?

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You said in this post that you understood why not:

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@chachamaru wrote:

 I now see that with a 100 MHz scope, only the 3rd harmonic of a 22 MHz square wave is fully visible and the higher harmonics are suppressed.
Therefore, the waveform I am seeing is consistent with the scope and probe limitations.

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[chachamaru]

I'm sorry, I didn’t explain it well.

What I meant is this:

If the PWM (XCLK) were being generated correctly, then VSYNC and HSYNC should also be generated correctly by the MT9M001.
So my goal is not to see a perfect waveform on the oscilloscope, but to provide a clean and reliable clock signal into the camera.

Therefore, I want to know how to output a “clear enough” PWM for the microcontroller-to-camera connection, not necessarily a perfect oscilloscope waveform.

[chachamaru]

I suspect that VSYNC and HSYNC are not being generated because the PWM clock signal is not sufficient for the camera.