FM3130-G datasheet(7/22 Pages) RAMTRON

7 / 22 page

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FM3130 Integrated RTC/Alarm with 64Kb FRAM

Rev. 3.2

Sept. 2011

Page 7 of 22

second. Setting the match select bit for seconds to „0‟

causes the logic to match the seconds alarm value to

the current time of day. Since a match will occur for

only one value per minute, the alarm occurs once per

minute. Likewise setting the seconds and minutes

match select bits causes an exact match of these

values. Thus, an alarm will occur once per hour.

Setting seconds, minutes, and hours causes a match

once per day. See Table 4 for other alarm setting

examples.

Function of the ACS Pin

The ACS pin is a multifunction pin. The alarm,

calibration, and square wave functions all share this

output. There are two ways a user can detect an alarm

event, by reading the AF flag or by monitoring the

ACS pin. An interrupt pin on the host processor may

be used to detect an alarm event. The AF flag in the

match has occurred. When a match occurs, the AF

bit will be set to „1‟ and the ACS pin will drive low.

The flag and ACS pin will remain in this state until

the RTC/Alarm Control register is read which clears

the AF bit.

Table 3 that shows the relationship between register

control settings and the function of the ACS pin.

Table 3. Control Bit Settings for ACS Pin

State of Register Bit

Function of

ACS pin

CAL

AEN

AL/SW

/Alarm

Sq Wave out

512 Hz out

Hi-Z

Cal Output/SquareWave Output

When the RTC calibration mode is invoked by

setting the CAL bit (register 00h, bit 2), the ACS

output pin will be driven with a 512 Hz square wave

and the alarm will continue to operate. Since most

users only invoke the calibration mode during

production, this should have no impact on the

otherwise normal operation of the alarm.

The ACS output may also be used to drive the system

with a continuous frequency. The AL/SW bit

(register 0Eh, bit 7) must be a „0‟. A user-selectable

frequency is provided by F0 and F1 (register 0Eh,

bits 5 and 6). The frequencies are 1, 512, 4096, and

32768 Hz.

If a continuous frequency output is

enabled by using the 512Hz or SquareWave out

functions, the alarm function will not be available.

Figure 4. ACS Pin Requires Pullup

The ACS pin is an open-drain output that needs to be

pulled up to a supply.

The ACS pin and pullup

resistor draws current only when the alarm is

triggered.

Table 4. Alarm Match Bit Examples

Seconds

Minutes

Hours

Date

Months

Alarm condition

No match required = alarm 1/second

Alarm when seconds match = alarm 1/minute

Alarm when seconds, minutes match = alarm 1/hour

Alarm when seconds, minutes, hours match = alarm 1/date

Alarm when seconds, minutes, hours, date match = alarm 1/month

ACS

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MCU

FM3130

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