PCI Express* Add-in Card Considerations
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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.
3.3.3 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: 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 when 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.
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