PCF2123 datasheet(12/63 Pages) NXP

12 / 63 page

PCF2123

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Product data sheet

Rev. 5 — 27 April 2011

12 of 63

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

8.4 Time and date function

The majority of the registers are coded in the Binary Coded Decimal (BCD) format. BCD is

used to simplify application use. An example is shown for the seconds in Table 9.

8.4.1 Register Seconds

[1]

Default value.

8.4.1.1

OS flag

The PCF2123 includes a flag (OS in register Seconds, see Table 8) which is set whenever

the oscillator is stopped (see Figure 8 and Figure 9). The flag will remain set until cleared

by software. If the flag cannot be cleared, then the PCF2123 oscillator is not running. This

method can be used to monitor the oscillator and to determine if the supply voltage has

reduced to the point where oscillation fails.

Table 8.

Seconds - seconds register (address 02h) bit description

Bit

Symbol

Value

Place value Description

clock integrity is guaranteed

1[1]

clock integrity is not guaranteed;

oscillator has stopped or has

been interrupted

6 to 4

SECONDS

0 to 5

ten’s place

actual seconds coded in BCD

format, see Table 9

3 to 0

0 to 9

unit place

Table 9.

Seconds coded in BCD format

Seconds value

(decimal)

Upper-digit (ten’s place)

Digit (unit place)

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

0000

0001

0000

0010

::::

0000

1001

0001

0000

::::

0101

1000

0101

1001

Fig 8.

OS set by failing VDD

001aai561

VDD

VOSC(MIN)

battery operation

main supply

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