HP 7800 Specifications download pdf (Page 86)

7-6 www.hp.com Technical Reference Guide

Power and Signal Distribution

7.3 Power Con trol

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 Bu t t on

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:

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.

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HP 7800 Specifications Page 86