Schematic
Depending on the MCU and the mode of operation (monitoring vs. power sink), additional electronic components are needed:
| MCU / Board | Monitor | Sink |
|---|---|---|
| STM32 F1 Family | Comparators | Comparators and resistors for interaction |
| STM32 F4 Family | Comparators | Comparators and resistors for interaction |
| STM32 L4 Family | None | Resistors for interaction |
| STM32 G0 Family | None† | None† |
| STM32 G4 Family | None† | None† |
The comparators are needed to level-shift the CC1 and CC2 signals from 1.6V to 3.3V and to combine them into a single signal. For that reason, comparators with an open-collector output are used, and wired such that they invert the signal. A suitable and easily available dual comparator for this purpose is the LM393(P).
For USB PD interaction in sink mode, several resistors are used. The 5.1kΩ resistors are pull-down resistors to indicate that this is a power sink.
The other resistors are needed to create a digital signal between 0V and about 1.2V (once per channel).
† Provided the MCU contains the UCPD peripheral.