One of the challenges of USB PD is that devices need to work correctly with any PD sources or sinks they might encounter. We frequently see shipped products that don't follow PD negotiations correctly. Testing your USB-powered device against a single charger is not enough!
One of our senior engineers was recently having trouble with a USB-powered mini PC. Fortunately, he had an Acroname USBHub3c on his desk and was able to quickly diagnose the issue using the HubTool app.
He wrote the manufacturer's customer support address demonstrating the problem, and they immediately responded thanking him for his analysis and fixed the issue. Just kidding, no response from support, unclear if anyone even reads those emails.
Rather than just let it go, we thought we should share his analysis to show how easy it is to diagnose faulty USB PD negotiations with USBHub3c
Requirements:
- USBHub3c with PD Logger add-on feature
- PD power supply of greater wattage than the device under test
Connections:
- PD power supply connected to USBHub3c PowerC port
- Device is connected to port 4
- Test computer running HubTool is connected to Control port
View PD Negotiations, Port Voltage, and Current:
To view PD negotiations, select HubTool's PD Logging tab.
- PDO’s (“Source Capabilities") are sent by the USBHub3c. This is the "Menu" of power options available from the source.
Note the Power Data Object that mentions “Voltage: 15.000" and "Max Current: 2.500A"
The Device Accepts and provides a RDO selecting: “Fixed index 3 of 7” (The 15 volt PDO) “Max Current 2.500A"
- USBHub3c issues PS Ready command and enables voltage rail
- The device immediately violates the contract by pulling 2.852 Amps. This is out of tolerance by 350mA so the port is toggled and this whole process continues indefinitely. Unfortunately after about 5-10 toggles the device reboots.
We can observe the port toggling in the Vbus voltage and current traces:
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