Ever thought about USB PD (power delivery) and how it really works? Below you can see simple example of power supply to device communication, where device requests 20V @ 1.5A.
All bottom data was captured with logic analyzer with some home-brewed circuit. Voltage levels on USB CC lines are 0.9 V, therefore you need some kind of level shifter to capture the data. Comparator circuit works the best.
Complete transmission cycle with voltage output. At the beginning it all starts with 5V, but after approx 0.5s, it goes up slowly and controlled to 20V.
Part 1 – 3x SOURCE CAP
Power supply tells the device which options are available 3 times.
Part 2 (after 200ms) – SOURCE CAP again
Repeats available options.
Part 2 – Device request + CRC
Then device requests what is needed.
Part 2 – Power supply accepts request + CRC
Power supply can accept this request (like in this example) or reject it. If rejected, then device will get 5V and 0.9A only.
Part 3 (after changed to new voltage / power) – PS RDY
If all good, when new voltage is set and stable, power supply tells that power is good and ready to use.
