Confusion about STM32H7S3I8T6 OTG_FS_VBUS, HSE, and VCAP

[ZitongW1]

Hi! I'm confused about the wiring for some of the pins on the STM32H7S3I8T6: 

1. Does the full-speed USB connection require an external high speed oscillator?
2. There's a GPIO that senses VBUS from USB called "OTG_FS_VBUS." Currently, we have it pulled up to VBUS so that it will read high when USB is connected. However when USB isn't connected, it is left floating. Would that be ok?

Thank you so much for the help!

[AScha.3 · Accepted solution]

Hi,

1. no extra oscillator, just the HSE 

2. Never let a cpu pin float .  Better simply use a voltage divider , lets say: usb -2k - 3k -gnd ,

so at the 3k you get 3v ( = Vdd ), when usb connected. (100% save for the cpu and pulled down by 3k to gnd , if nothing connected.)

[FBL]

Hi @ZitongW1 

1. FS OTG USB may require an HSE in host mode due to stricter clock accuracy requirements. In USB device mode, internal oscillators like HSI48 trimmed using CRS can be used without issues.

2. You may need to provide better details about your application (self powered or bus powered). I would recommend checking section 2.6 VBUS sensing detection in AN4879