In our previous blog, “Ever Wonder How USB Type-C Works”, one of the paramount features we discussed was the Type-C connector being used with third party peripherals in-addition to USB. The mode in which the Type-C cable assembly facilitates operation of “Alternate” protocols is called Alternate Mode. USB Type-C Alternate Mode specification allows MHL, DisplayPort, HDMI, and Thunderbolt over Type-C. Alternate Mode is an option made available to the USB Hosts; however, USB should be the preeminent interface to be exposed over Type-C assembly, justifying the tag ‘Alternate’.
Alternate Mode achieves the purpose by reconfiguring groups of available pins. It should be noted that all the pins are not available for reconfiguration during Alternate Mode. The pins A2, A3, B10, B11 and B2, B3, A10, A11 represent two pairs of differential signals available for USB Gen A and Gen B operation. During Alternate Mode, these pins are used for third party protocol traffic.
In addition, a pair of side buffer pins (A8, A9) are also available, however these pins are of no significance to USB. For example, per VESA Alt Mode specification for DisplayPort, differential pairs represent Main Link (Lane1-4) and side buffers are used to connect the Auxiliary channel.
How to Enter ‘Alternate Mode’
Downstream Facing Port (DFP) and Upstream Facing Ports (UFP) use Power Delivery’s Structured Vendor Defined Messages (VDMs) to discover, configure, enter or exit “Alternate” Modes. The Power Delivery traffic on CC pin, in the form of VDMs, could identify the alternate standards using 32-bit SVIDs (Standard or Vendor ID). SVIDs are allocated by the USB Implementers Forum and each SVID represents a unique protocol. A standard flow must be followed to recognize the Alternate Modes, so a UFP can support them. The minimum criteria before exchanging VDM messages with UFP is that the power contract between DFP and UFP should have been established.
A unique SVID can represent multiple modes.
DFP can probe UFP for available modes.
UFP will respond with VDOs (VDM have data objects called VDOs), which have information about the modes available.
DFP can request mode entry/exit. UFP will respond with an ACK or NAK depending on if it accepts or rejects the mode entry/exit. Once ‘Enter Mode’ message exchange is complete, the Pins available for reconfiguration are assigned to the peripheral for which the mode entry was requested.
Type-C with Alternate Mode, a connector which is light and diminutively “form factored”, provides convenience to High Definition audio video consumers. With USB literally being universal and USB 3.2 on the horizon with an expected 20Gbps data transfer at its disposal, we can only imagine what wonders the Type-C connector can do using Alternate Mode across a broad spectrum of protocols and applications.
Synopsys’ USB Type-C™ Subsystem Verification Solution is being adopted rapidly and in production by many Synopsys customers. Read more about Adoption of Synopsys’ USB Type-C Subsystem Verification Solution by ASIX. For more information on Synopsys’ VIP for USB 3.1, 3.0, 2.0, USB PD, and verification subsystem solution for USB Type-C, please visit: www.synopsys.com/vip.
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Authored by Prishkrit Abrol.