PCF2123TS datasheet(16/55 Pages) NXP

16 / 55 page

PCF2123_2

Product data sheet

Rev. 02 — 4 December 2009

16 of 55

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

During read/write operations, the time counting circuits (memory locations 02h through

08h) are blocked.

This prevents

• Faulty reading of the clock and calendar during a carry condition

• Incrementing the time registers during the read cycle

After this read/write access is completed, the time circuit is released again and any

pending request to increment the time counters that occurred during the read/write access

is serviced. A maximum of 1 request can be stored; therefore, all accesses must be

completed within 1 second (see Figure 12).

As a consequence of this method, it is very important to make a read or write access in

one go, that is, setting or reading seconds through to years should be made in one single

access. Failing to comply with this method could result in the time becoming corrupted.

As an example, if the time (seconds through to hours) is set in one access and then in a

second access the date is set, it is possible that the time may increment between the two

accesses. A similar problem exists when reading. A roll over may occur between reads

thus giving the minutes from one moment and the hours from the next. Therefore it is

advised to read all time and date registers in one access.

9.7 Alarm function

When one or more of these registers are loaded with a valid minute, hour, day, or weekday

and its corresponding alarm enable bit (AE_x) is logic 0, then that information will be

compared with the current minute, hour, day, and weekday.

9.7.1 Register Minute_alarm

[1]

Default value.

Fig 12. Access time for read/write operations

t < 1 s

013aaa222

COMMAND

data bus

chip enable

DATA

Table 18.

Minute_alarm - minute alarm register (address 09h) bit description

Bit

Symbol

Value

Place value Description

AE_M

minute alarm is enabled

1[1]

minute alarm is disabled

6 to 4 MINUTE_ALARM

0 to 5

ten’s place

minute alarm information coded in BCD

format

3 to 0

0 to 9

unit place

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