STM32F030C: SPI Slave at 13.5 MHz with GPIO_Speed = Low for EMI reduction?

[MERSI]

Hello everyone,

I am currently working on a project using the STM32F030C, and I have a question regarding the GPIO speed configuration (slew rate) for SPI pins.

Here is my setup:

• MCU: STM32F030C

• Interface: SPI (configured as Slave)

• SPI Clock Frequency: 13.5 MHz

To keep EMI (Electromagnetic Interference) emissions as low as possible, I was considering setting the GPIO output speed for the SPI pins (specifically MISO, since the MCU is the slave) to GPIO_Speed_Level_Low.

However, according to the datasheet (see attached screenshot), the "Low" speed setting is specified for a maximum frequency of 2 MHz.

My questions are:

1. Is it practically possible to run a 13.5 MHz SPI signal with the GPIO speed set to Low, despite the datasheet specs?

2. Does this pose a high risk of corrupted/poor communication due to the slew rate being too slow (e.g., degraded signal integrity, failing to meet setup/hold times for the master)?

3. Has anyone here tried this approach to reduce EMI at similar frequencies, or is it strictly recommended to bump the GPIO speed to Medium/High for 13.5 MHz?

Any insights or shared experiences would be highly appreciated.

Thanks in advance!

[waclawek.jan]

> Does this pose a high risk of corrupted/poor communication due to the slew rate being too slow (e.g., degraded signal integrity, failing to meet setup/hold times for the master)?

Yes.

Note, that attempting to do this *may* appear to work; however, the pin's slew rate changes with temperature (and VDD) and also with process variation (i.e. every piece is slightly different) and the datasheet values are guaranteed worst case.

JW

[Andrew Neil]

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@MERSI wrote:

1. Is it practically possible to run a 13.5 MHz SPI signal with the GPIO speed set to Low, despite the datasheet specs?

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Think about the numbers: 

13.5 MHz has a period of 77ns: that's not going to work with 125ns rise and fall times - is it?

You could easily try it, and observe with an oscilloscope what you get ...

But note @waclawek.jan's warning about test conditions and process variations.

In general, you should always do what the datasheet tells you - those are the guaranteed performance conditions.

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@MERSI wrote:

the "Low" speed setting is specified for a maximum frequency of 2 MHz.

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and you want to go at over six times that speed ?!