How do I enable the legacy USB devices to be detected by a Type-C source ?
The general concept for setting up a valid connection between a Source and Sink is based on being able to detect terminations residing in the product being attached. To aid in defining the functional behavior of CC, a pull-up (Rp) and pull-down (Rd) termination model is used – actual implementation in hosts and devices may vary, the Figure below illustrates the model.
Initially, a Source exposes independent Rp terminations on its CC1 and CC2 pins, and a Sink exposes independent Rd terminations on its CC1 and CC2 pins, the Source-to-Sink combination of this circuit configuration represents a valid connection. As illustrated in the Figure above, only one CC pin is connected through the cable to establish signal orientation.
For example, below is the functional mode of an adapter that connect a Type-C source to a legacy device port. This adapter has a USB Type-C plug on one end plugged into the Source and either a USB Standard-B plug, USB Micro-B plug, USB Mini-B plug, or a USB Standard-A receptacle on the other end. The Pin A5 (CC) of the USB Type-C plug shall be connected to GND through a resistor Rd, and the recommended value of Rd in the Type-C specification is 5.1 kΩ ± 20%.
After connecting the legacy device to the Type-C source through the adapter, the source can detect the pull-down on the CC pin because of Rd, and then the source will turn on VBUS and VCONN.
Below is a prototype of the adapter to demonstrate the connection between a Type-C Source and a Legacy device port.
Connect the Pin A5 (CC) of the USB Type-C plug to GND through a resistor Rd (5.1 kΩ ± 20%), then connect Pin A6/A7 of the Type-C plug to D+/D- of USB Micro-B Plug.
All VBUS pins (A4 B4 A9 B9) and all Ground return pins (A1 B1 A12 B12) shall be connected together within the USB Type-C plug.
Connect a legacy device to the Type-C source through the adapter, it can be detected correctly, and also works well with the Type-C source.