Supporting Two SuperSpeed Datapaths for USB Type-C
No need to flip out over flippability: You have the means to adjust your designs to USB Type-C even before SoCs with native USB Type-C support debut.
USB Type-C™ is not “hype-C.” USB Type-C connections are user-friendly. There is no difference between the host and device side connector; one common cable works for all products; the connector can be flipped and inserted either way; and the receptacle is small and robust, making it ideal for front-panel, space-constrained embedded systems. The addition of DisplayPort Alternate Mode and HBR-3 bitrates to the specification supports 4K/60Hz monitors simultaneously with USB 3.0 or USB 3.1 on the same connector.
Designers using USB Type-C must support two SuperSpeed datapaths matching either one or the other orientation of the USB Type-C connector. The USB Type-C connector is symmetrical and duplicates most of the signals to support flippability, as shown in Figure 1.
This duplication requires datapath multiplexers for SuperSpeed USB products and datapath crossbar switches for Alternate Mode products. Designers have two implementation choices for a USB Type-C datapath if the SoC does not natively support USB Type-C: using an external datapath switch or using two USB ports.
External Datapath Switch
External datapath switches (or multiplexers, external to the SoC or USB chipset; see Figure 2) are commonly used in commercial products supporting USB Type-C. Existing high frequency analog switches originally designed for PCIe, Ethernet, SATA, DisplayPort and other standards have been repurposed for USB Type-C. The main advantage of the external switch solution is fastest time-to-market. The disadvantages are cost, PCB area and (implementation dependent) reduction of signal quality.
Loss in the datapath switch affects the channel loss budget from the USB port to USB Type-C connector. The channel loss budget for USB 3.0 Type-C is 6.5dB, minus the switch loss. The switch loss (including package losses) is typically 1.5dB. The remaining 5dB channel loss budget reduces maximum length of PCB routing from USB port to connector to 6-7 inches, depending on the quality of the PC and the PCB layout. The channel budget for USB 3.1 (SuperSpeed USB 10 Gbps) is 8.5dB from die to connector for all connector types. Typical PCB routing distance is 4 inches, but adding an external switch can reduce PCB routing to a 2-inch length or less.
It is important to remember that USB 3.0 with standard-A connector was allowed a 10dB channel loss budget; direct conversion of an existing USB 3.0 standard-A system design to USB Type-C might not be possible. External USB Type-C specific switches with built-in analog re-drivers can compensate for some switch and PCB routing loss. This is appropriate for USB 3.0; however, the specified re-driver for 10 Gbps operation is a complete re-timer (Figure 3). This consists of two complete PHYs plus some digital circuitry and has a detrimental effect on both cost and power consumption for SuperSpeed USB 10 Gbps solutions that require a compliant re-driver.
Two USB Ports Solution
Using two USB 3.0 or USB 3.1 ports is an alternate solution to the USB Type-C switch. One or the other port is active depending on the USB Type-C connector orientation. The other port is in a low-power state. Since there is no datapath switch, there is no loss of signal quality.
Multiple commercial products with multiport USB host capability take advantage of this solution; two existing ports for Standard-A can be used to create one USB Type-C port. Figure 4 shows a common 4-port USB 3.0 host controller chip used ‘as is’ in a 2-port USB Type-C host controller plug-in board. Two SuperSpeed USB RX and TX pairs are used for one USB Type-C connector. To preserve signal integrity, the SuperSpeed signals are routed directly from the controller to the USB Type-C connector. The routing is clearly visible on the visible side of the PCB. Two USB 2.0 signal pairs are also routed from the controller to the USB Type-C connector. USB 2.0 PCB routing is less critical, and the routing continues on the opposite side of the PCB to avoid interfering with the optimal routing of the SuperSpeed USB PCB traces.
The USB Type-C specification requires that Vbus is not enabled until a device is attached. This prevents the 5V power supplies of two USB hosts to be shorted together; the USB Type-C connector allows this to happen. A load switch and large capacitor can be seen next to the USB Type-C connector. The host has one Pullup resistor Rp on each Configuration Channel (CC) pin. When a device is connected, the device Pulldown resistor Rd causes the voltage on one of the CC pins to go low. This causes the load switch to enable Vbus. When a host is connected, the load switch is not enabled.
Normally for USB Type-C, CC pin detection is used to determine orientation. For this design in Figure 4, no orientation detection is required. Both ports are active, but only one port will detect the device. The other port stays active but unused. There is a small power penalty for the two USB ports approach, but the penalty is often acceptable.
Single-port SuperSpeed USB hosts can use a USB 3.0 hub chip to create two internal SuperSpeed USB ports to connect, for instance, a USB to Ethernet controller and a memory card reader. The remaining two ports can implement an external USB Type-C connection, as described above.
Until SoCs with native USB Type-C support become available, system designers can adopt their existing designs for USB Type-C using an external datapath switch or two USB ports. For most designs, the required Type-C Configuration Channel circuitry can be easily implemented with discrete components and Type-C Port Manager (TCPM) hardware, or Type-C software is normally not required.
When sourcing or specifying SoCs, keep in mind that an optimized USB Type-C PHY or USB/DisplayPort Alternate mode PHY IP, such as those available from Synopsys, removes the need for expensive external switches or the need to use multiple ports, add-on USB host controller or hubs to solve the SuperSpeed dual datapath challenge.
Synopsys website: DesignWare USB IP
Article: Implementing USB Type-C in High-Speed USB Products
Article: Converting Existing USB Designs to Support USB Type-C Connections
Webinar: Designing SoCs for USB Type-C Products
White paper, Addressing Three Critical Challenges of USB Type-C Implementation
Morten Christiansen is the technical marketing manager for Synopsys’ DesignWare USB and DisplayPort IP. Prior to joining Synopsys, Christiansen was a principal system designer at ST-Ericsson and Ericsson, designing mobile phone and modem chipsets for 19 years. He was also member of technical Staff at ST-Ericsson.
Christiansen has contributed to more than 20 USB standards, including USB 3.1, Battery Charging, HSIC and SSIC, as well as communication standards including WMC, EEM, NCM and MBIM, which are used in billions of USB products. In addition to the non-patented USB standards contributions, Morten holds five international patents for other USB inventions. He holds a Master of Science degree from The Norwegian Institute of Technology 1983.