I would like to know about the 4 GPIO speed settings on the STM32H743BIt6. And is it possible to measure the system clock 400Mhz from the MCO2 pin output terminal?

Question

​I am using STM32H743Bit6.#define GPIO_SPEED_FREQ_LOW     (0x00000000U) /*!< Low speed   */#define GPIO_SPEED_FREQ_MEDIUM   (0x00000001U) /*!< Medium speed */#define GPIO_SPEED_FREQ_HIGH    (0x00000002U) /*!< Fast speed  */#define GPIO_SPEED_FREQ_VERY_HIGH  (0x00000003U) /*!< High speed  */​I would like to know the maximum frequency of each.And is it possible to measure the system clock 400Mhz from the MCO2 pin output terminal?

LCE · Accepted Answer

The real world (pcb) is full of (unwanted) capacitances, driving these Cs fast takes fast and strong drivers, these are usually not incorporated in MCU IOs.

Imagine it like that: a discharged cap at 0V (even if it's only 5pF pcb stray capacitance) looks to the driver, which wants to go from low to high, in the first moment (few ps) like a short circuit, so to "fill" that cap it takes either time or very high current.

Example: even most HiSpeed USB (480Mbit) capable MCUs rather rely on external PHYs to do that job (and others).

LCE · Answer

Check the datasheet / reference manual about the GPIO speed settings. These are register settings, so you will find a description.

I am not sure if you can output an undivided system clock.

For clock related stuff I really like the CubeMX clock tree, check the path to MCO2 and if there's any dividers. But even if there are, then you should know what you get.

400 MHz output is not easy to do, and not so easy to measure.

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