PCA2125TS datasheet(14/40 Pages) NXP

14 / 40 page

PCA2125_00

Preliminary data sheet

Rev. 00.11 — 30 January 2007

14 of 40

NXP Semiconductors

PCA2125

SPI Real time clock / calendar

Generation of interrupts from the alarm function is described under the interrupt section,

Section 6.7.3.

6.5.1 Alarm flag

When all enabled comparisons first match, the Alarm Flag (AF) is set. AF will remain set

until cleared by software. Once AF has been cleared it will only be set again when the

time increments to match the alarm condition once more. Alarm registers which have their

bit AEn at logic 1 are ignored.

Figure 9 shows an example for clearing AF but leaving MSF and TF unaffected. Clearing

the flags is made 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 AF, a logic AND is performed

during a write access. Writing a ‘1’ will cause the flag to maintain it’s value, whilst writing a

‘0’ will cause the flag to be reset.

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

MSF and TF are unaffected.

Example where only the minute alarm is used and no other interrupts are enabled.

Fig 9.

AF timing

Table 23:

Flag location in control 2

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Control 2

-MSF

-AF

Table 24:

Example to clear only TF (bit 2)

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Control 2

-1

-0

001aaf903

minute alarm

minutes counter

INT when AIE = 1

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