USB C connection to older USB devices

USB C uses a reversible connector, additional data pins and higher current capability. However, the pin usage from older USB will work just fine in many instances.

Simplest connection
Required:
VBUS  5V input
GND   0V
D+    D+
D-    D-

Power signalling:
CC1 & CC2 pin, each with a 5.1K resistor to GND. Indicates to the upstream port to provide 5V and up to 1.5A (it's a request by your device, it may not actually be able to supply that much current).

Optional:
Monitor CC and SBU pins to determine cable polarity (or leave disconnected)

CC pins

The CC1 and CC2 pins allow you to detect which orientation the cable has been inserted, because cables only have 1 wire providing the CC link. This means only 1 of the CC pins will be pulled by the remote devices resistors applied to the pin. At either end you can connect each CC pin to an AtoD and detect:

  • Which has been pulled = which orientation the connection is in
  • What resistance the remote end has applied = its basic power specs

If you don’t care about connector orientation, which will be the case if you’re just providing a basic D+ and D- connection to both of the sets D- and D+ pins, then you can link the two CC pins and apply just the one resistor and save yourself an extra AtoD input being used on your processor.

Resistor values

A DFP device (Downwards facing port / Host) either has its CC pins linked (if it doesn’t care about connector orientation) or has a resistor on each. The resistor is the Rp resistor, has a standard value (500mA/900mA current capability) of 56K when pulled up to +5V, or 36K if pulled to 3V3 (e.g. for a 3V3 CPU AtoD input). However, it can have different values for different current availability advertising and be pulled to different voltages.

A UFP (Upwards facing port / Device) either has its CC pins linked (if it doesn’t care about connector orientation) or has a resistor on each. The resistor is the Rd resistor, each has a value of 5K1 and is pulling down to 0V.

Good resources

https://microchipdeveloper.com/usb:tc-pins

Adapter PCBs

https://www.adafruit.com/product/4090

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