What is "USB_CRS_SYNC" functionality?

[moez0102]

Could you please explain to me "USB_CRS_SYNC" functionality? And, is it necessary to be used with "USB_DM" and "USB_DP" please?

[JQuan]

I have not used this before, but I am pretty sure that it is related to the clock recovery system (CRS) and is not required for normal USB operation.

3.36 Clock recovery system (CRS)

The devices embed a special block which allows automatic trimming of the internal 48 MHz

oscillator to guarantee its optimal accuracy over the whole device operational range. This

automatic trimming is based on the external synchronization signal, which could be either

derived from USB SOF signalization, from LSE oscillator, from an external signal on

CRS_SYNC pin or generated by user software. For faster lock-in during startup it is also

possible to combine automatic trimming with manual trimming action.

(from datasheet for STM32G431)

See also https://www.st.com/content/ccc/resource/training/technical/product_training/group0/35/e9/34/32/d3/68/41/24/STM32H7-System-Clock_Recovery_System_CRS/files/STM32H7-System-Clock_Recovery_System_CRS.pdf/_jcr_content/translations/en.STM32H7-System-Clock_Recovery_System_CRS.pdf