Implementing USB Type-C Functionality on SoCs

USB Type-C’s ease of use, flexibility and adaptability comes at a cost for designers of products that will incorporate USB Type-C. While future USB PHYs and controllers will have full USB Type-C support and will be used to natively implement USB Type-C in new ASIC and SoC designs, consumers want USB Type-C now and will expect companies to immediately introduce products supporting it. Therefore, designers may need to provide immediate support for the USB Type-C connector by converting their existing designs to USB Type-C. They will need to understand orientation agnostic receptacles, specific actions for converting SuperSpeed and High-Speed USB designs, implementing Power Delivery, and implementing data bus routing.

Orientation Agnostic Receptacles 

The USB Type-C receptacle is fully symmetrical. All power, ground, and signal pins are duplicated about the symmetry axis. This duplication allows the USB Type-C connector to be flipped in the USB Type-C receptacle.

Figure 2: USB Type-C power, ground, and signal pins (front view)

Pull-Down and Pull-Up Resistors & Determining Device Orientation

The USB Type-C specification describes how the USB device uses Pull-Down resistors (Rd) on Configuration Channel pins CC1 and CC2 to signify that it is a device. The USB host is required to have Pull-Up resistors (Rp) on CC1 and CC2. The USB Type-C specification allows Rp and Rd to be implemented as current source and voltage clamp. Actual function is the same.

The host identifies a device is connected by detecting one of the device Pull-Down resistors. Both host and device can determine the cable orientation, as only one of the CC pins is wired in the cable.

Figure 3: Pull-up/pull-down CC model for connection and orientation

Converting USB 2.0 Device and Host SoCs to USB Type-C

To convert an existing USB 2.0 device to USB Type-C, the designer can short the two CC pins, add one Pull-Down resistor and route the USB D+/D- signals to both positions on the USB Type-C receptacle. No other changes are required. The device waits for Vbus to be valid, enables its D+ Pull-Up resistor (for a Full Speed or High Speed product), chirps (if a High Speed device) and USB enumeration occurs as normal.

The USB Type-C cable allows two hosts to be connected together. A USB Type-C host cannot enable Vbus at all times like a Type-A host. Enabling Vbus at all times will cause two host Vbus supplies to be shorted. Only when a device Pull-Down resistor is detected does the host enable Vbus to the device. To convert an existing USB 2.0 host to USB Type-C, Vbus for each port must be switchable.

Additionally, Pull-Up resistor(s) and the capability to detect one of the device Pull-Down resistors must be added. Some host PHYs can use the VbusValid detector for detecting devices. Some host designs can use existing ASIC or SoC GPIOs with suitable over-voltage protection. Other host designs must investigate the use of external USB Type-C ‘add-on’ chips or modify the Power Management IC (PMIC) to support device attach detection.

This simplified USB 2.0 Type-C device and host implementation does not support high current charging, active cables, power delivery, analog audio adapter or debug accessories. The advantage is it is a cost-effective approach that does not require ASIC or SoC redesign and minimizes time-to-market.