DisplayPort AUX Pull-Up and Pull-Down Resistors

The DisplayPort Auxiliary (AUX) channel is a bidirectional differential bus responsible for link management, configuration, and status monitoring between a Source and a Sink. The internal pull-up and pull-down resistors on the AUX differential pair (AUX+ and AUX-) serve essential electrical functions, primarily establishing a defined idle state, providing DC common-mode biasing for AC-coupled signaling, and facilitating device and adapter detection such as Dual-Mode DisplayPort (DP++).

Establishing the Bus Idle State

The DisplayPort AUX channel relies on AC-coupled Manchester II encoding. When the AUX channel is not transmitting data, the differential pair enters an idle state. Without termination or bias, the high-impedance inputs of the differential transceivers would float, causing random switching noise and false packet detection. Internal pull-up and pull-down resistors create a slight differential offset voltage between AUX+ and AUX- during idle periods. This differential offset keeps the line in a known, stable logical state (typically interpreted as a squelch or quiet state) so the receiver does not falsely trigger.

DC Biasing for AC Coupling

The DisplayPort specification mandates AC-coupling capacitors (typically 75 nF to 200 nF) along the AUX lines to isolate the DC voltages between the Source and the Sink. Because these capacitors block DC current, each transceiver must establish its own operating common-mode voltage. The internal pull-up (connected to the power rail, typically 3.3V) and pull-down (connected to ground) resistors form a voltage divider that sets the appropriate DC operating point for the differential transmitters and receivers on their respective sides of the AC-coupling capacitors.

Cable and Dual-Mode (DP++) Detection

DisplayPort supports Dual-Mode operation (DP++), allowing the physical connector to output TMDS signals for HDMI and DVI compatibility via passive level shifters. In standard DisplayPort mode, the AUX pair handles differential native AUX packets. In DP++ mode, the AUX lines are reconfigured to carry standard I²C-based Display Data Channel (DDC) signals (I²C Clock and I²C Data). The internal resistor network enables the Source to detect whether a native DisplayPort monitor or a passive DP++ adapter is attached:

• Native DisplayPort: The standard resistor configuration applies specific pull-up/pull-down values that indicate differential AUX signaling is expected.
• DP++ Mode: A passive adapter pulls the AUX lines (specifically CONFIG2/AUX-) to ground or a predetermined logic level, signaling the DisplayPort controller to disable native AUX transceivers and switch the pins into DDC/I²C mode.

Preventing Floating Logic Inputs

During system initialization, sleep states, or hot-unplug events, the differential lines may be temporarily disconnected from an active transmitter. The internal pull-up and pull-down resistors ensure that neither line floats to an intermediate voltage level that could cause shoot-through currents in the complementary CMOS input stages, thereby protecting the hardware and minimizing idle power consumption.