PCF2123TS datasheet(19/55 Pages) NXP

19 / 55 page

PCF2123_2

Product data sheet

Rev. 02 — 4 December 2009

19 of 55

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

Figure 14, Table 22 and Table 23 show an example for clearing bit AF, but leaving bit MSF

and bit TF unaffected. The ﬂags are cleared by a write command, therefore bits 7, 6, 4, 1

and 0 must be written with their previous values. Repeatedly re-writing these bits has no

inﬂuence on the functional behavior.

To prevent the timer ﬂags being overwritten while clearing bit AF, logic AND is performed

during a write access. A ﬂag is cleared by writing logic 0 whilst a ﬂag is not cleared by

writing logic 1. Writing logic 1 will result in the ﬂag value remaining unchanged.

Table 23 shows 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.

9.8.1 Register Timer_clkout

[1]

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

[2]

Default value.

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.

Timer_clkout - timer control register (address 0Eh) bit description

Bit

Symbol

Value

Description

Reference

unused

6 to 4 COF[2:0]

[1]

CLKOUT control

Section 9.10

countdown timer is disabled

Section 9.8.4

countdown timer is enabled

unused

1 to 0 CTD[1:0]

4.096 kHz countdown timer source clock

64 Hz countdown timer source clock

1 Hz countdown timer source clock

11[2]

⁄60 Hz countdown timer source clock

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