Technical Reference Guide for HP Compaq 8200 Elite Series Business PCs

Technical Reference Guide www.hp.com 7-5
Power and Signal Distribution
7.3 Powe r Co nt ro l
System power is controlled through the power button and external events.
7.3.1 Power Bu t ton
Pressing and releasing the power button 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:
Table 7-4.
Power Button Actions
System State
Pressed Power Button Results In:
Off 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.
On, ACPI Disabled 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.
On, ACPI Enabled 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.