DisplayPort HPD Pulse Width for AUX Status Checks

In the VESA DisplayPort standard, an interrupt request generated by the sink device over the Hot Plug Detect (HPD) line informs the source device that an auxiliary (AUX) status check is required. To be legally recognized as an interrupt rather than a line glitch or a physical cable disconnect, the HPD line must be pulled low for a duration strictly between 0.5 milliseconds and 1.0 millisecond.

The IRQ_HPD Timing Window

The DisplayPort specification defines three distinct behavioral regimes for an active-low pulse on the HPD line:

  • Glitch / Noise Range (< 0.5 ms): Any low pulse narrower than 0.5 ms is designated as an electrical glitch. The DisplayPort source device is required to filter out and ignore pulses below this threshold to prevent transient noise on the line from triggering spurious read operations.
  • Legal IRQ_HPD Range (0.5 ms – 1.0 ms): A sink device requesting attention from the source must pull the HPD signal low for a minimum of 0.5 ms and a maximum of 1.0 ms before driving it high again. When the source detects a low-going pulse within this timing window, it registers the event as a valid IRQ_HPD.
  • Hot Unplug Range (> 2.0 ms): If the HPD signal remains low for longer than 2.0 ms, the source device interprets the state not as an interrupt, but as a cable disconnect or physical unplug event. In response, the source terminates active video transmission and tears down the main link.

Pulses falling between 1.0 ms and 2.0 ms occupy an undefined boundary margin. While some source hardware may still process pulses in this gap as interrupts, standard-compliant sink devices must strictly adhere to the 0.5 ms to 1.0 ms window to prevent being misinterpreted as an unplug event.

Once a source validates that the low pulse duration was between 0.5 ms and 1.0 ms, it must initiate an AUX transaction to query the sink's DisplayPort Configuration Data (DPCD).

Specifically, the source reads the DPCD registers located from address 00200h to 00205h (the Sink/Link Status and Service IRQ Vector registers). This status check allows the source to determine the cause of the interrupt, which may include:

  • A request for link retraining due to clock recovery or channel equalization loss.
  • High-bandwidth Digital Content Protection (HDCP) authentication state changes.
  • Content Protection downstream port status updates.
  • Changes in automated test states or display configuration capabilities.

Following the AUX read, the source typically writes back to the corresponding DPCD registers to clear the interrupt flag, completing the handling sequence initiated by the legal HPD pulse.