PCA2125 datasheet(13/36 Pages) NXP

13 / 36 page

PCA2125_1

Product data sheet

Rev. 01 — 28 July 2008

13 of 36

NXP Semiconductors

PCA2125

SPI Real-time clock/calendar

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

during a write access. The ﬂag is reset by writing a logic 0 but its value is not affected by

writing a logic 1.

Table 23 shows what instruction must be sent to clear bit AF. In this example, bit MSF and

bit TF are unaffected.

8.6 Timer functions

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

the range from less than 1 ms to more than 4 hours. There are also two pre-deﬁned timers

which can be used to generate an interrupt once per second or once per minute.

Registers Control_2 (01h), Timer_control (0Eh) and Countdown_timer (0Fh) are used to

control the timer function and output.

8.6.1 Second and minute interrupt

The second and minute interrupts (bits SI and MI) are pre-deﬁned timers for generating

periodic interrupts. The timers can be enabled independently of one another, however a

minute interrupt enabled on top of a second interrupt will not be distinguishable since it will

occur at the same time; see Figure 9.

Table 22.

Flag location in register Control_2

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Control_2

MSF

Table 23.

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

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Control_2

Table 24.

Bit

Symbol

Value

Description

Reference

countdown timer is disabled

Section 8.6.2

countdown timer is enabled

6 to 2

unused

1 to 0

CTD[1:0]

4096 Hz 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[7:0]

00h to FFh

countdown value = n.

Section 8.6.2

CountdownPeriod

SourceClockFrequency

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