PCF2123 datasheet(20/54 Pages) NXP

20 / 54 page

PCF2123_1

Product data sheet

Rev. 01 — 19 November 2008

20 of 54

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

To prevent the timer ﬂags being overwritten while clearing AF, a logical AND is performed

during a write access. Writing a logic 1 will cause the ﬂag to maintain its value, whereas

writing a logic 0 will cause the ﬂag to be reset.

The following tables show what instruction must be sent to clear bit AF. In this example, bit

MSF and bit TF are unaffected.

9.8 Timer functions

The countdown timer has four selectable source clocks allowing for countdown periods in

the range from 244

µs to 4 h 15 min. There are also two pre-deﬁned timers which can be

used to generate an interrupt once per second or once per minute. For periods greater

than 4 hours, the alarm function can be used.

Registers 01h, 0Eh and 0Fh are used to control the timer function and output.

[1]

Values of COF[2:0] see Table 35.

Table 22.

Flag location in register Control_2

Bit

Control_2

MSF

Table 23.

Example to clear only AF (bit 3) in register Control_2

Bit

Control_2

Table 24.

Bit

Symbol

Value

Description

Reference

unused

6 to 4 COFx

[1]

CLKOUT control

Section 9.10

countdown timer is disabled

Section 9.8.2

countdown timer is enabled

unused

1 to 0 CTD

4.096 kHz countdown timer source clock

64 Hz countdown timer source clock

1 Hz countdown timer source clock

⁄60 Hz countdown timer source clock

Table 25.

Bit

Symbol

Value

Description

Reference

7 to 0 COUNTDOWN_

TIMER

0h to FFh

countdown value = n;

Section 9.8.2

CountdownPeriod

SourceClockFrequency

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