PC87307 datasheet(56/218 Pages) NSC

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Real-Time Clock (RTC) and Advanced Power Control (APC) (Logical Device 2)

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• Type of system recovery after a Power Failure state

by setting the MOAP bit. See page “The MOAP Bit” on

page 58.

• Immediate or delayed Switch Off shutdown. See “The

SWITCH Input Signal” on page 59.

• 5 or 21 second time-out fail-safe shutdown. See “The

SWITCH Input Signal” on page 59.

4.3.2

System Power States

The system power state may be No Power, Power On, Pow-

er Off or Power Failure. These states are illustrated in Fig-

ure 4-9 on page 57. Table 4-5 indicates the power-source

combinations for each state. No other power-source combi-

nations are valid.

In addition, the power sources and distribution for the entire

PC system are described in “PC87307/PC97307 Power

Supplies” on page 51.

TABLE 4-5. System Power States

WARNING:

It is illegal for VDD to be present when VCCH is absent.

No Power

This state exists when no external or battery power is con-

nected to the device. This condition will not occur once a

backup battery has been connected, except in the case of a

malfunction. The APC undergoes initialization only when

leaving this state.

Power On

This is the normal state when the PC is active. This state

may be initiated by various events in addition to the normal

physical switching on of the system. In this state, the PC

power supply is powered by external AC power and produc-

es VDD and VCCH. The PC system and the part are powered

by VDD, with the exception of the RTC logical device, which

is powered by VCCH.

VDD

VCCH

VBAT

Power State

−−

−

No Power

−−

Power Failure

−

+ or -

Power Off

+ or -

Power On

−

Illegal State

Power Off (Suspended)

This is the normal state when the PC has been switched off

and is not required to be active, but is still connected to a

live external AC input power source. This state may be ini-

tiated directly or by software. The PC system is powered

down. The RTC logical device remains active, powered by

VCCH.

Power Failure

This state occurs when the external power source to the PC

stops supplying power, due to disconnection or power fail-

ure on the external AC input power source. The RTC con-

tinues to maintain timekeeping and RAM data under battery

power (VBAT), unless the oscillator stop bit was set in the

RTC. In this case, the oscillator stops functioning if the sys-

tem goes to battery power, and timekeeping data becomes

invalid.

4.3.3

System Power Switching Logic

In the Power On state, the PC host is powered by the pow-

er-supply voltage VDD. From this state the system enters

the Power Off state, if the conditions for this state occur

(See Section 4.4.3), or the Power Failure state if external

power is removed.

In the Power Off state, the PC hosts do not receive power

from the system power supply, except for RTC and APC

which receive VCCH. The system may enter the Power On

state if the conditions for this state occur (see Section

4.4.3), or enter the Power Failure state if external power is

removed.

If the system voltage falls to the level of VBAT +500 mvolt or

less, the APC enters the Power Failure state and switches

to battery power.

When power returns after a power failure, the APC enables

power up after a delay of 1 second. The nature of the power

up depends on the MOAP bit setting (See “The MOAP Bit”

on page 58).

Knowing the system power state prior to a switch interrupt

is required for correct Switch Event interpretation. The pow-

er state is defined by the following conditions:

VDD present implies Power On

VCCH present and VDD absent implies Power Off.

If VBAT falls below 2V with VCCH absent, the oscillator, the

timekeeping functions and the APC, all stop functioning.

If no external or battery-backup power is available, the sys-

tem enters a No Power state. Upon leaving this state, the

system is initialized.

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