Resistors on USB-C CC lines

[AWack]

Hi,

I´m designing an USB host device (with STM32U5).

Therefore I´ve ha a look at the schematics of the STM32U575I-EV. Could someone explain what the MOSFETs are for?

Best regards,

Achim

[FBL]

Hi  @AWack 

In STM32 UCPD, FRSTX1 and FRSTX2 are the dedicated FRS transmit signals to support fast role swap (FRS) as specified in USB PD spec.

 

FRS is designed to allow a sink to react quickly when the current source loses power. The source side advertises the FRS condition by pulling the active CC line low for a short, spec defined interval. This signaling is time critical and must be handled with very low latency.

As described in AN5225, the FRS signaling itself must be detected very quickly, typically within 50 µs, and once detected, the sink prepares to take over the source role. The new source must then resume VBUS delivery within the timing defined by the FRS procedure (within 150 µs), to avoid interrupting the connected device.

So those FETs are used to drive the CC lines to GND

Also in Figure 29. DRP with FRS VBUS = 5 V / 9 V / 15 V connections, the FRSTX1 and FRSTX2 fast role swap signals are wired to external MOSFETs to drive the CC lines.