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Converting Existing USB Designs to Support Type-C Connections
By Morten Christiansen, Technical Marketing Manager, USB IP, 草榴社区
USB is the most successful wired interconnect standard. For almost 20 years, the classic USB connectors defined for Type-A host and Type-B device configurations made USB the ‘if it fits; it will work’ connection. This was a huge improvement over previous standards like RS232. The new USB Type-C connector is designed to take USB through the next 15-20 years, ensuring consumers continue to say “Sure it works; it’s USB.”
USB Type-C in Brief
The USB Type-C receptacle is about the same size as a high-speed-only Micro-B receptacle, and is considerably smaller than standard-A receptacles. USB Type-C cables are fully symmetrical. There is no host- or device-specific end of the cable. In addition, the connector can be flipped in the host or device USB Type-C receptacle. Users will love this improvement to USB.
Figure 1: Comparison of USB Standard-A, Micro-B and USB Type-C cables
USB Type-C is more than just a new connector. It defines a number of new capabilities starting with support for Power Delivery. The USB Power Delivery feature allows products like laptops to be powered from a monitor. Power Delivery could end the tyranny of “9 different power supplies for 11 different products” in the same way that standardized USB charger is now used to recharge many phones, tablets, e-readers, and other portable devices. USB Type-C connectors also support analog audio and can replace a standard headphone connector. Lastly, the new USB Type-C alternate mode allows TV sets, displays and docking stations to share or take over the USB cable. The “U” in USB does indeed mean “Universal.”
The USB Type-C specification was released at the same time as a major update to the USB 3.1 specification. However, USB Type-C is a separate specification and is applicable to all USB products and operating speeds. USB Type-C is not only a new mechanical connector. USB Type-C is a framework for the future and continued success, usefulness and relevance of USB for the next 10 to 20 years.
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 Type-C, the designer must add two pull-down resistors to the CC pins and route the USB D+/D- signals to both positions on the Type-C receptacle. No other changes are required. The device waits for Vbus to be valid, enables it’s D+ pull-up resistor (for a Full Speed or High Speed product), Chirps (if 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.
Converting USB 2.0 DRD SoCs to USB Type-C
To convert an existing DRD product to USB Type-C, designers must add the capability to enable either Pull-Up or Pull-Down resistors and to detect Pull-Down resistors. Some DRD products only need host mode for certain use-cases like test and verification.
When converting an existing DRD product to USB Type-C, keep in mind that the ID pin in the Micro Type-AB connector is used to signify if the DRD shall be a host or device. As an example, a phone or tablet is a USB device when connected to a USB host using a Type-A to Micro-B cable. When connected to a USB thumb drive, USB keyboard, or even a USB hub using an ‘OTG adapter cable’, the phone or tablet becomes a host. However, the ID pin does not exist in the USB Type-C connector. To convert an existing DRD product to USB Type-C Dual Role Port (DRP), the host or device role must be determined by other means.
The appropriate solution to determine host or device roles is application-dependent. Some examples:
- A tablet could automatically enable host mode unless already connected as a device when the user enables file browsing. The file browser will detect if an external mass storage device is connected.
- The product can default to device mode. If the user selects host mode, host mode is enabled unless the product is already in device mode and connected to a host.
- An application-specific setting like ‘enable external keyboard’ option can enable host mode.
- The product can default to host mode and detect when a USB device is connected. With additional circuitry, it can detect connection to another USB host and enable device mode.
- The device can alternate (randomly) between host and device mode.
Each design’s requirements will determine the appropriate solution and whether DRP is necessary and implementable for their specific USB Type-C product.
Converting USB 2.0 OTG SoCs to USB Type-C
While existing Device, Host, and DRD devices can be converted to support the USB Type-C connection with some effort, it is not practical to convert existing OTG SoCs. OTG requires a new state machine using the configuration channel to support USB Type-C. Host Negotiation Protocol is used to swap host and device role for OTG, while Power Delivery communication is used to change roles for USB Type-C. Combined, these challenges preclude simple upgrades of OTC SoCs to USB Type-C.
Rather than converting an existing OTG SoC, system architects can use an external Dual Role Port USB Type-C support chip or redesigned Power Management IC (PMIC) to support DRD or OTG. The redesigned PMIC supports functions such as Power Delivery communication, Vbus switching, configuration channel signaling and device attach detection.
Converting an existing SuperSpeed DRD or OTG product to USB Type-C requires appropriate partitioning of functionality between the old controller, PHY and software, and the new USB Type-C-specific additions. The impact on low level software, drivers, and applications when properly supporting DRD or OTG should not be underestimated. The end user expects that applications using USB are just as user-friendly as the USB Type-C connector itself.
USB Power Delivery
USB Power Delivery communication over USB Type-C uses baseband signaling on the CC pin. Power Delivery communication might be required even if the product is not a Power Delivery provider or a consumer. Power Delivery is optional for High Speed products and mandatory for SuperSpeed hosts, Dual Role Device and OTG products. The Power Delivery controller can be integrated with PMIC. External PD controllers are available from multiple suppliers.
SuperSpeed USB Type-C Datapath
Enhanced SuperSpeed USB products operating in either Gen1 or Gen2 mode must transmit and receive SuperSpeed signals on one of two signal pairs in the receptacle, depending on the cable orientation. This requires some sort of switch and/or multiplexer. The switch position is determined by the cable orientation as determined by the CC pins as described earlier.
Regarding these switches, the USB Type-C specification states: "The functional requirements for implementing SuperSpeed USB data bus routing for the USB Type-C receptacle are not included in the scope of this specification. There are multiple host, device and hub architectures that can be used to accomplish this which could include either discrete or integrated switching, and could include merging this functionality with other USB 3.1 design elements, e.g. a bus repeater."
Multiple suppliers offer external high bandwidth switches that are useful when converting an existing SuperSpeed product to USB Type-C. Switch suppliers also offer complete USB Type-C chips that include Power Delivery and Configuration Channel functionality in addition to the SuperSpeed datapath switch. System architects should note that the external USB Type-C switch or chip adds power, area and cost. Designers must determine if this is appropriate for their product to meet time-to-market requirements.
Summary
USB Type-C is not just a new USB connector. It is a framework for the future. Dedicated USB Type-C PHYs and controllers will be available. This will allow ASICs and SoCs to be redesigned to natively support USB Type-C. Until then, multiple solutions exist for converting existing product designs to USB Type-C. 草榴社区 range from trivial to challenging depending on the required USB Type-C product capabilities.
Additional Resources
- Learn more about USB in 草榴社区 Interface IP University