PC87570 datasheet(67/168 Pages) NSC

67 / 168 page

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

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6.2.11

Alarms

The timekeeping function can be set to generate an alarm

when the current time reaches a stored alarm time. After

each RTC time update (every 1 second), the seconds, min-

utes, hours, day of the week, date of month, month and year

counters are compared with their corresponding registers in

the alarm settings. If equal, bit 5 (AF) of the CRC Register

is set. If the Alarm Interrupt Enable bit was previously set

(bit 5 of the CRB Register), IRQ8 interrupt request pin will

also go low (IRQ8 = 0). (Note: This pin is an open-drain out-

put and needs an external pull-up.)

Any alarm register may be set to “Don’t Care” by setting bits

7 and 6 to 11. This combination, not used by any BCD or bi-

nary time codes, results in a periodic alarm. The rate of this

periodic alarm is determined by the registers that were set

to “Don’t Care”.

For example, if only the seconds and minutes alarm regis-

ters are set to “Care”, an interrupt will be generated every

hour at the specified minute and second. If only the sec-

onds, minutes and hours alarm registers are set to “Care”,

an interrupt will be generated every day at the specified

hour, minute and second.

6.2.12

Power Supply

The PC87570 is supplied from two supply voltages, as

shown in Figure 6-4:

System standby power supply voltage, VSB

Backup voltage, from low capacity Lithium battery.

A standby (help) voltage (VSB) from the external AC/DC

power supply powers the RTC and APC under normal con-

ditions.

Figure 6-4. Power Supply Connections

Figure 6-5 represents a typical battery configuration. No ex-

ternal diode is required to meet the UL standard, due to the

internal switch and internal serial resistor RUL.

Figure 6-5. Typical Battery Conﬁguration

The RTC/APC module is supplied from one of two power

supplies, VCC or VBAT, according to their levels. An internal

voltage comparator delivers the control signals to a pair of

switches. Battery backup voltage VBAT maintains the cor-

rect time and saves the CMOS memory when the VCC volt-

age is absent, due to power failure or disconnection of the

external AC/DC input power supply or VCC main battery.

To assure that the module uses power from VCC and not

from VBAT, the VCC voltage should be maintained above its

minimum, as detailed in Table 19-8. "Voltage Thresholds"

on page 136.

The actual voltage point where the module switches from

VBAT to VCC is lower than the minimum workable battery

voltage, but high enough to guarantee the correct function-

ality of the oscillator and the CMOS RAM.

Figure 6-6 shows typical battery current consumption dur-

ing battery-backed operation, and Figure 6-7 during normal

operation.

Figure 6-6. Typical Battery Current During Battery

Backed Power Mode

Figure 6-7. Typical Battery Current During Normal

Operation Mode

RTC

Host PC

Power

Backup

External

VDD

VSB

VDD

VSB

VBAT

ONCTL

PC87570

Battery

AC Power

Supply

VDD

VSB

VBAT

ONCTL

VCC

PC0

VBAT

and

APC

VBAT

0.1

µF

VSB

VCC

BT1

0.1

µF

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.

RTC

and

APC

VPP

RUL

PC87570

VREF

IBAT (µA)

1.0

0.9

0.8

0.7

2.4 3.0 3.6

VBAT (V)

IBAT (µA)

0.20

0.15

0.10

0.05

3.0 3.3 3.6

VCC

Note: Battery voltage in this test is 3.0V.

4.5

5.0

5.5

(V)

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