ATX Version 3.0 Multi Rail Desktop Platform Power Supply

Design Guide

ID 336521
Date 02/01/2023

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Document Table of Contents

CARD_CBL_PRES# (Optional)

This sideband signal has two functions:

  • Primary Function:
    • This sideband signal provides a signal from the Add-in Card to the PSU that an Add-in Card has been detected and is correctly attached to the 12VHPWR Auxiliary Power Connector.
    • The primary function is required for all PCIe* Gen 5.0 Add-in Cards.
  • Secondary Function:
    • This sideband signal provides communication from the Add-in Card to tell the PSU when an Add-in Card is present and can be included in the “Power Budgeting Sense Detect Registers”. This allows the system to correlate which system and power cables are used with specific PCIe* card slot.

Note that this sideband signal is for the purpose of the system power supply management support. It is not for the purpose of the Add-in Card to determine the power limits available to it. Power limits to the Add-in Card are communicated by the SENSE0/SENSE1 signals.

The CARD_​CBL_​PRES# signal is tied to a 4.7kΩ pull-down resistor to ground on the Add-in Cards. This signal is required for all PCIe* Gen 5.0 Add-in Cards, it will traverse the sideband signals of the 12VHPWR connector. If the PSU monitors the state of the CARD_​CBL_​PRES# signal, it must do so with a high impedance 3.3V logic compatible device input. This signal will be low at the Add-in Cards at all times with main power is absent. For a PSU to detect the active low presence condition of the CARD_​CBL_​PRES# signal a 100kΩ pull-up resistor to 3.3V is required.

When supporting the secondary function, the Add-in Card reads this signal on a high impedance 3.3V logic compatible input and record the logic high/low state in the “Power Budgeting Sense Detect” registers. The PSU is allowed to drive this signal high with a push-pull driver to 3.3V.

Support for this optional sideband signal does not rely on any of the other sideband signals defined for the 12VHPWR connector and can be implemented independently of other of these sideband signals.

For a power supply this sideband signal can be used to determine how many PCIe* Add-in Cards are connected to the power supply. Then the power supply could determine how much power it can provide to each 12VHPWR connector via the SENSE0/1 sideband signals. If this feature is used and the SENSE0/1 signals are dynamically changed they must be changed only when the power supply is in Standby Mode (PS_​ON# is de-asserted and Main Power rails are not on). Once PS_​ON# and the main power rails achieve their full voltages the SENSE0/1 sideband signals must not be changed.

An example for this scenario would be if a power supply has enough rated power to supply 600 Watts to one PCIe* Add-in Card it could determine the following scenarios:

  • One PCIe* Add-in Card is detected, and that one card gets 600 Watt.
  • Two PCIe* Add-in Cards are detected, each card gets 300 Watts.
  • Three or Four PCIe* Add-in Cards are detected, each card gets 150 Watts.