USB-C DisplayPort Alt Mode CC Pin Negotiation

When a USB Type-C port connects to a DisplayPort-compatible display or adapter, the Configuration Channel (CC) pins handle the physical detection and the digital messaging required to reconfigure the interface. Using USB Power Delivery (USB-PD) protocol packets, the host and device negotiate DisplayPort Alternate Mode (DP Alt Mode) through a series of structured Vendor Defined Messages (VDMs). This negotiation allows the port to repurpose its high-speed differential lines to carry native DisplayPort audio and video signals.

1. Physical Attachment and Cable Orientation

Before any data packets are sent, the hardware detects the physical connection via the CC pins:

  • Termination Detection: The host (Downstream Facing Port, or DFP) exposes pull-up resistors (\(R_p\)) on both CC1 and CC2 pins, while the peripheral (Upstream Facing Port, or UFP) exposes pull-down resistors (\(R_d\)).
  • Orientation Resolution: Whichever CC line forms a complete \(R_p/R_d\) circuit becomes the active CC communication line.
  • VCONN Power: The remaining unused CC pin is repurposed as \(V_{CONN}\) to supply low-voltage power to the electronic marker (E-Marker) chip embedded within the cable, if present.

2. Establishing the USB-PD Communication Channel

Once physical attachment is established, the active CC line switches to digital signaling using Biphase Mark Coding (BMC) at a nominal bit rate of 300 kbps. The host and device negotiate basic operational parameters, establishing a standard USB-PD contract. All subsequent DisplayPort negotiations occur as packetized data payloads transmitted directly across this active CC wire.

3. Capability Discovery via Structured VDMs

To determine if both sides support DisplayPort, the host initiates an interrogation sequence using USB-PD Structured VDMs addressed to the device (using Start of Packet, or SOP) and optionally to the cable (SOP'):

  • Discover Identity: The host sends a Discover Identity request. The device responds with details regarding its product type and whether it supports alternate modes.
  • Discover SVIDs: The host queries which Standard or Vendor IDs (SVIDs) the device supports. The device must return the Video Electronics Standards Association (VESA) SVID, which is standardized as 0xFF01.
  • Discover Modes: The host queries the modes available under SVID 0xFF01. The device replies with a list of capabilities, including supported pin assignments (such as Pin Assignment C for 4-lane DP, or Pin Assignment D for 2-lane DP plus USB 3.x), signaling rates, and receptor characteristics.

4. Entering DP Alt Mode and Pin Configuration

Once capabilities are matched, the host commands the hardware transition:

  • Enter Mode: The host sends an Enter Mode command targeted at VESA SVID 0xFF01 and the selected mode index. The sink device acknowledges (ACK) the command.
  • DP Status Update: The devices exchange DisplayPort Status Update VDMs to communicate current states, such as adapter power state and connection status.
  • DP Configure: The host issues a DP Configure VDM, instructing the peripheral to set its internal multiplexers to a specific pin configuration (e.g., allocating either two or four high-speed pairs to the DisplayPort Main Link).

5. Signal Reallocation and Hot Plug Detect (HPD)

Once the DP Configure message is acknowledged over the CC pin:

  • The USB Type-C high-speed lanes (TX/RX) are internally switched to carry native DisplayPort Main Link lanes (ML0 through ML3).
  • The Sideband Use pins (SBU1 and SBU2) are mapped to the DisplayPort Auxiliary Channel (AUX+ and AUX-) for device configuration and link training.
  • Display Hot Plug Detect (HPD) signaling—traditionally a dedicated physical pin in standard DisplayPort connectors—is virtualized. The sink signals connection, disconnection, or interrupt requests (HPD_IRQ) by sending USB-PD Attention messages over the active CC pin back to the host.