USB use has grown in industrial applications: for software uploads and configuration, diagnostics, maintenance, and connecting peripheral modules such as Wi-Fi® routers, display screens and human-machine interface modules. However, since USB’s primary definition is as a consumer electronics interface, it is not inherently capable of handling the large noise disturbances, ground bounce and ground potential differences common in industrial applications. Also, in applications such as uninterrupted power supplies, controllers with the USB interface are on the high-voltage or “hot” side, needing protective isolation to the USB connector. Because of these reasons, isolating the USB interface has become necessary in a broad spectrum of applications, including factory automation, motor drives, medical equipment, e-meters, data concentrators, in-flight entertainment and gaming consoles.

The applications mentioned above often need the flexibility of connecting to a PC while acting as a peripheral (device), or to peripherals such as a Wi-Fi module or USB memory drive while acting as a host. In the past, system designers provided two different ports to support this functionality: one for the host and one for the peripheral - using two different USB isolators (also called isolated USB repeaters). This solution is expensive and takes up valuable board area. Instead, a USB Type-C® connector can implement a dual-role port (DRP) – one port that supports both host and peripheral functionality. This article discusses how to implement an isolated USB 2.0 USB Type-C DRP.

Figure 2-1 shows the traditional implementation of isolated host and peripheral ports in industrial equipment. This implementation uses a USB isolator that has fixed upstream- and downstream-facing definitions. The downstream-facing (host) port powers a 5-V supply to the VBUS and includes 15-kΩ pulldown resistors, as specified by the USB standard. The upstream-facing (peripheral) port does not provide power to the USB VBUS. Once this port connects to a host, it detects the presence of VBUS and pulls up either DP (for full speed and high-speed operation) or DM (for low-speed operation) with a 1.5-kΩ resistor. Here, DP refers to DPLUS/D+ or the positive terminal of the USB data differential pair, and DM refers to DMINUS/D-. Since a DRP must expect to either pull up DP and DM in peripheral mode (upstream facing), or offer a 15-kΩ pulldown resistor to ground and expect an external 1.5-kΩ pullup on DP and DM in host mode (downstream facing), it is clear that traditional isolated USB repeaters that have fixed upstream- and downstream-facing sides cannot support a DRP.

In USB Type-C, the role of a port is determined by the state of the CC1 and CC2 pins. A peripheral port has pulldown resistors Rd from CC1 and CC2 to ground. A host port has pullup resistors Rp to VCC. This is shown in Figure 3-1. The values of Rd and Rp are governed by the USB Type-C standard. The host can use the value of Rp to advertise current available on the VBUS pin: 0.5 A, 1.5 A or 3 A.

A DRP periodically toggles CC1 and CC2 between Rp pullup to VCC and Rd pulldown to ground over a period of 50 ms to 100 ms, as defined in the USB Type-C standard. When connecting an external host to a DRP, the connection is detected during the interval of Rd pulldown and the DRP assumes a peripheral role (upstream-facing port). When connecting an external peripheral to a DRP, the connection is detected during the interval of Rp pullup, and the DRP assumes a host role (downstream-facing port).