草榴社区 Cloud
Cloud native EDA tools & pre-optimized hardware platforms
Request a Free Trial →
Implementing USB Type-C in High-Speed USB Products
By Morten Christiansen, Technical Marketing Manager, USB IP, 草榴社区
USB Type-C has arrived! Products have been announced in almost every conceivable product category, and many are shipping in volume. Consumers appreciate the simplicity of the new USB Type-C connector and expect most products with USB to offer Type-C connectivity in the near future.
Many of the initial products using the Type-C connector run at the USB high-speed rate of 480 Mbps. This article will help designers who plan to convert existing high-speed USB products using older ASICs or SoCs without native USB Type-C support.
Mechanical Conversion to Type-C
The Type-C receptacle, with 24 pins, is only slighter larger than a 5-pin micro-USB receptacle. Most existing products with micro-B USB can easily be modified mechanically for Type-C by mounting the receptacle low on the PCB (midmount) as shown in Figure 1.
Figure 1: Midmount receptacles support ultrathin products which may have previously used micro-B connections
Source:
High-Speed Datapath Adaptation for Type-C
Orientation-agnostic insertion of a Type-C plug is possible due to signaling duplication with symmetry around both X and Y-axis in the Type-C receptacle (Figure 2). SuperSpeed (5G) signal integrity suffers when routing the same SuperSpeed RX/TX signals to both positions in the receptacle. SuperSpeed signaling uses dual PHYs or a switch to route the active signal to the appropriate receptacle pins. The active receptacle position depends on the plug orientation.
High-speed USB requires significantly lower bandwidth than SuperSpeed or SuperSpeedPlus (10G), so high-speed D+/D- signals are centered in the receptacle. This allows the high-speed USB D+/D- pins to be routed to both positions (pins A6/A7 and B6/B7) without affecting signal integrity. This solution simplifies the conversion of high-speed, full speed or low speed products to Type-C.
Figure 2: Type-C receptacle pins show X and Y axis symmetry
Host/Device Role and Configuration Channel
For classic USB connections, the host and device roles were determined by connector type: standard-A for hosts, standard-B or micro-B for device, and micro-AB for dual role. For micro-AB, the ID-pin determines host or device. The new Type-C hosts and Type-C devices use the same receptacle. No ID pin is required because Type-C products signal host, device or accessory roles with pullup and pulldown resistors (Figure 3).
Figure 3: Type-C host/device configurations are determined with pullup and pulldown resistors
The device has pulldown resistors (Rd) on the two CC-pins in the receptacle while the host has pullup resistors (Rp). An accessory has a pulldown resistor called Ra. Hosts detect device or accessory connections because the host Rd is weaker than either the device Rd or accessory Ra. The device can (optionally) detect accessory connection because Ra is lower than Rd (Figure 3).
The symmetrical Type-C cable allows two hosts to be connected. To prevent two host Vbus power supplies to be connected, hosts do not enable Vbus until a device has been connected. Connecting two Type-C devices together is possible but will be inconsequential unless one or both devices can become a host.
High Current Capability
The standard USB current from a high-speed host is 2.5mA max in suspend, 100mA max until configured and 500mA max when configured as a high-power device. Many mobile products benefit with faster charging if they can charge their batteries with a higher current. A dedicated charger or charging downstream port (host charger) can provide between 500mA and 1.5A to the connected device. Type-C introduces even higher current capability. Different values of host pullup resistor (Rp) indicate standard USB current, guaranteed 1.5A, and guaranteed 3A currents.
To utilize these higher current levels, a device must detect the resulting voltage on CC of the combination of host Rp and device Rd. This requires precision comparators that are typically integrated with the battery charging circuitry.
How to Implement a High-Speed Device with Type-C
Creating a standard high-speed device with Type-C connectivity is easy to implement with a standard USB 2.0 PHY.
- Modify or design the PCB and enclosure for a Type-C receptacle
- Route USB D+/D- from the PHY to both pins A6/A7 and B6/B7 on the Type-C receptacle
- Connect USB Vbus and GND to all Vbus and GND pins on the receptacle
- Leave the ID pin on the PHY floating
- Connect two pulldown resistors (Rd) with nominal value 5.1kOhm to CC1 and CC2
Just as with a standard-B or micro-B device, USB operation starts when Vbus is detected.
How to Implement High-Speed Host with Type-C
A high-speed host with Type-C connectivity is more complex than the device.
- Connect the PHY ID pin to GND.
- Connect two pullup resistors (Rp) with a nominal value 56kOhm between Vbus and CC1/CC2.
- Alternative: Rp with nominal value 36kOhm between 3.3V and CC1/CC2.
- Detect voltage on CC1 and CC2 with two comparators. Nominal detection level is 1.60V.
- Supply Vbus to receptacle via load switch.
Unlike a standard-A host, Vbus is not provided to the receptacle at all times. Vbus can only be provided when a device is attached. After Vbus is provided, normal USB device detection, high-speed chirp and USB operation will commence. Mobile devices will save power as they do not need to enable Vbus at all times.
A high current-capable host uses lower values for Rp. Hosts can change current capability dynamically. Device attach detection level for 3A current capability changes to 2.6V. Further details can be found in the Type-C specification .
How to Implement High-Speed Dual Role Port with Type-C
Products with micro-AB receptacles can act as either host or device. The host or device role is determined by the cable orientation and the ID pin is floating for device and grounded for host role. While the host/device role can be changed dynamically using Host Negotiation Protocol (HNP), the HNP is not widely used. Most products that claim to be on the go (OTG) capable are actually dual role capable. Dual role devices (DRDs) have a static host or device role depending on cable orientation and do not support OTG with dynamic role switching. This distinction is useful to keep in mind when designing a Type-C Dual Role Port product.
As stated, Type-C cables have no orientation and the receptacle does not use an ID pin. Therefore, the host/device role is determined by pull resistors Rp and Rd. A connection between two Type-C devices requires that one device is configured as a host and the other remains as a device. For static Dual Role Port, host or device role can be decided before the cable is connected—for example, the user can configure the host mode before connecting a thumb drive, a keyboard or similar device.
To implement a Dual Role Port with Type-C functionality, designers need to consider the following in addition to the implementation points for high-speed devices and hosts.
- Enable either Pullup resistors (Rp) or Pulldown resistors (Rd) depending on host or device role
- Configure host or device role.
- Role can be based on user input, context sensitive application or other method.
- Ignore PHY ID pin signal and configure the controller for host or device role.
- Alternatively, connect the PHY ID pin to GND for host and leave “floating” for the device. This might be needed depending on DRP controller configuration and capability. With proper use of standard GPIO, no external circuitry is required.
- In host mode, detect voltage on CC1 and CC2 to determine device attach.
- In host mode, supply Vbus to receptacle via load switch.
Type-C protocol and state machines exist for determining initial host/device role if the product does not have a static configuration. Dynamic role reversal is possible using power delivery communication messages. Dynamic role switching is more user-friendly but adds complexity; however, support chips exist that reduce implementation complexity.
Notes on Implementing SuperSpeed Products with Type-C
SuperSpeed products require the same pullup, pulldown, Vbus switch etc as high-speed products. Additionally, SuperSpeed products require switching SuperSpeed signaling to either TX1/RX1 or TX2/RX2 datapaths depending on the connector orientation. The switch must be capable of switching 5Gbit/s signals with minimal loss and distortion.
SuperSpeed products with Type-C connectivity must detect the connector orientation. As there is only one CC wire in the cable, orientation is detected via the pull resistor on either CC1 or CC2. The appropriate switch position is determined by the result. A Type-C host can use the same two comparators as for device attach. A Type-C device must add two comparators with nominal detection level of 0.20V. Again, this adds cost and complexity to SuperSpeed products with Type-C.
Fortunately, converting a USB dongle or a device with a captive cable to Type-C is easier. For example, a SuperSpeed USB Thumb Drive does not need the datapath switch. Since the Type-C connector is already connected to the product, the orientation cannot be changed, nor does the orientation need to be detected by the device. The required SuperSpeed datapath switching and orientation detect is done by the host, depending on the orientation of the thumb drive. Additionally, only one pulldown resistor (Rd) is required.
Type-C Support Chips
Complexity, area and cost for discrete external circuitry to support Type-C can become prohibitive for many products even though multiple companies have introduced Type-C support chips. Functionality in these chips range from programmable resistors and comparators to complete Power Delivery communication with dynamic dual role port support. Some support chips also include the SuperSpeed dual datapath switch required for SuperSpeed Type-C products.
Each product designer must determine whether to use external support chips or discrete components.
Summary
Multiple Type-C products have been launched and are shipping in volume using standard USB PHYs that were not designed or optimized for Type-C. Converting a high-speed USB product is reasonably easy and offers fast time to market.