Signal Conditioning and Routing

Display Data Channel (DDC)

The compute module DDC lines are open-drain 1.8 V signals.

Board designers must level-shift the DDC signals to 1.8 V levels before they reach the module. Level-shifting can be accomplished using several methods including discrete circuitry or with an active level shifting device. The reference expansion board performs the level-shifting of the DDC signals with an HDMI re-driver device.

Note: The HDMI source side DDC signals on some HDMI re-driver solutions may operate at 3.3 V. Be aware that if one of these devices is selected, the DDC signals will require additional level-shifting to 1.8 V levels.

Hot-Plug Detect (HPD)

The compute module hot-plug detect (HPD) is an active-low 1.8 V signal.

Board designers must invert the logic of, and level-shift the HPD signal to 1.8 V levels before it reaches the module. Level-shifting can be accomplished using several methods including discrete circuitry or with active level-shifting and signal inversion devices. The reference expansion board performs the level-shifting of the HPD signal with an HDMI re-driver device and uses an N-FET to complete the logic inversion.

Note: The HDMI source side HPD signal on some HDMI re-driver solutions may operate at 3.3 V. Be aware that if one of these devices is selected, the HPD signal will require additional level-shifting to 1.8 V levels.

Additional Recommendations

  • Add series capacitors near the module connector on the HDMI clock and data lines.
  • Include common-mode chokes with built-in ESD protection on the HDMI clock and data lines between the HDMI connector and the re-driver or termination circuitry.
  • Follow general high-speed layout guidelines when routing HDMI clock and data lines. See Differential Trace Information for module specific routing information.
  • Add ESD protection diodes to the HPD and DDC signals near the HDMI connector if not provided by a re-driver component.
  • The DDC and 5 V lines from the the HDMI connector should be isolated from the board's 5 V rail with backdrive prevention diodes.
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