Why is 40 MHz HSE recommended for STM32MP2?

[DMårt]

In AN5489, the HSE is recommended to have 40 MHz. Why 40 MHz?

Why not 24 MHz HSE? Is 40 MHz a specific value for a certain pheripherial?

[Issamos]

Hello @DMårt 

according to the 4.1.2 of the AN5489, the 40MHz is a good choice to get accurate USB HS Clocks.

Best Regards.

II

[PatrickF · Accepted solution]

Hi,

yes, 40MHz is the one giving best clock quality (i.e. direct path from HSE to PLL, meaning lowest jitter) for PCIE/USB SuperSpeed 5Gbits/s generation while allowing flexibility for other clocks (Ethernet Gigabit, USB high-speed, etc..).

It should be noted that the trend of crystal size shrink to smaller packages mean higher frequency are 'easier' for crystal manufacturer (and so might give slightly better high volume pricing).

Nevertheless, 24 MHz is supported too. Be careful that changing default HSE frequency need many changes in the DTS frequencies and clock tree.

For USB boot, BootROM is able to autodetect 16, 20, 24, 28, 32, 36, 40 or 48 Mhz. 19.2, 25 or 26 MHz is even possible with OTP settings. 

Unless absolutely needed (e.g. network companies often want to use 25MHz of their network chipset), we recommend to stick to ST recommendation of 40 MHz.

Regards.

[DMårt]

@Issamos 

Thank you for reply. Yes, i know that.

But what's the definition of good choice?

Has the ST dev team test different values at HSE and found that 40 MHz works best. What would not work? 24 MHz?

My current settings at HSE is 24 MHz.

[DMårt]

@PatrickF 

Thank you for that good answer.

I understand now that 40 MHz is recommended due to direct path from HSE to PLL. 

The reason why I'm selecting 24 MHz is because one oscillator of 24 MHz drives both the USB hub and the MP2.

I that that for MP1, and it worked.