PC87591L datasheet(360/437 Pages) NSC

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Host-Controlled Modules and Host Interface (Continued)

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360

Revision 1.07

6.2.9

Power Supply

The PC87591x is supplied from four supply voltages, as shown in Figure 116 (see Section 3.1 on page 62 for more detailed

description):

●

System power supply voltage, VDD

●

System analog power supply voltage, AVCC

●

System standby power supply voltage, VCC

●

Backup voltage, from low-capacity Lithium battery

A standby voltage (VCC) from the external AC/DC power supply powers the RTC under normal conditions.

Figure 116. Power Supply Connections

Figure 117 shows a typical battery configuration. No external diode is required to meet the UL standard, because of the in-

ternal switch and internal serial resistor RUL.

Figure 117. Typical Battery Conﬁguration

RTC power is supplied from either VCC or VBAT voltage source, according to the sources’ levels. An internal voltage com-

parator delivers the control signals to a pair of switches. When VCC is active, the RTC power is drawn from it. Battery backup

voltage (VBAT) maintains the correct time and saves the CMOS memory when the VCC voltage is absent due to power failure

or disconnection of the main battery. Figure 117 illustrates the mechanism whereby VCC or VBAT is selected.

To assure that the module uses power from VCC and not from VBAT, the VCC voltage should be maintained above its

minimum (VCC2PP), as detailed in Section 7 on page 372. Figure 118 illustrates the switching between VCC (thick green

line) and VBAT (thin red line) and vice-versa for generating VPP (thin blue line).

The actual voltage point where the PC87591x switches from VBAT to VCC is lower than the minimum workable battery volt-

age but high enough to guarantee the correct functionality of the oscillator and the CMOS RAM.

The Valid RAM and Timer bit (VRT) in CRD register indicates the state of non-interrupted power supplied to the

PC87591x RTC module. VRT is cleared once VPP power is lost, i.e., when VCC and VBAT are both below VLOWBAT. The

host code should read this bit as part of the startup routine, before using the timer or the CMOS-RAM contents. In case

a value of 0 is read, the RAM and timer must be initialized before use. The VRT bit is set once read.

RTC

Power

Backup

VDD

VCC

VDD

VCC

VBAT

Battery

External AC Power

Supply

VDD Sense

VCC

VBAT

VCC

PC0

VBAT

ONCTL

ACPI Controller

AVCC

VBAT

0.1

µF

VCC

BT1

0.1

µF

VPP

RUL

VREF

RTC

1. Place a 0.1

µF capacitor on each V

CC power supply

pin as close as possible to the pin, and also on VBAT.

2. Place a 10-47

µF capacitor on the common power

supply net as close as possible to the device.

PC87591L Datasheet(PDF) 360 Page - National Semiconductor (TI)

PC87591L Datasheet(HTML) 360 Page - National Semiconductor (TI)

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