Power Control; Power Button - HP Compaq dc7900 USDT Technical Reference Manual

Power and Signal Distribution
7.3

Power Control

The generation of +3, +5, and +12 VDC is controlled digitally with the PS On signal. When the
PS On signal is asserted, all DC voltages are produced. When PS On is de-asserted, only
auxiliary power (+5 AUX) is generated. The +5 AUX voltage is always produced as long as the
system is connected to a live AC source.

7.3.1 Power Button

The PS On signal is typically controlled through the Power Button which, when pressed and
released, applies a negative (grounding) pulse to the power control logic on the system board.
The resultant action of pressing the power button depends on the state and mode of the system at
that time and is described as follows:
System State
Off
On, ACPI Disabled
On, ACPI Enabled
7-6
Table 7-4.
Power Button Actions
Pressed Power Button Results In:
Negative pulse, of which the falling edge results in power control logic
asserting PS On signal to Power Supply Assembly, which then initializes. ACPI
four-second counter is not active.
Negative pulse, of which the falling edge causes power control logic to
de-assert the PS On signal. ACPI four-second counter is not active.
Pressed and Released Under Four Seconds:
Negative pulse, of which the falling edge causes power control logic to
generate SMI-, set a bit in the SMI source register, set a bit for button status,
and start four-second counter. Software should clear the button status bit within
four seconds and the Suspend state is entered. If the status bit is not cleared by
software in four seconds PS On is de-asserted and the power supply assembly
shuts down (this operation is meant as a guard if the OS is hung).
Pressed and Held At least Four Seconds Before Release:
If the button is held in for at least four seconds and then released, PS On is
negated, de-activating the power supply.
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