M41ST85WMH6F datasheet(23/43 Pages) STMICROELECTRONICS

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M41ST85W

Clock operation

3.4

Setting alarm clock registers

Address locations 0Ah-0Eh contain the alarm settings. The alarm can be configured to go

off at a prescribed time on a specific month, date, hour, minute, or second, or repeat every

year, month, day, hour, minute, or second. It can also be programmed to go off while the

M41ST85W is in the battery backup to serve as a system wake-up call.

Bits RPT5–RPT1 put the alarm in the repeat mode of operation. Table shows the possible

configurations. Codes not listed in the table default to the once per second mode to quickly

alert the user of an incorrect alarm setting.

When the clock information matches the alarm clock settings based on the match criteria

defined by RPT5–RPT1, the AF (alarm flag) is set. If AFE (alarm flag enable) is also set, the

alarm condition activates the IRQ/FT/OUT pin as shown in Figure 15. To disable alarm,

write '0' to the alarm date register and to RPT5–RPT1.

Note:

If the address pointer is allowed to increment to the flag register address, an alarm condition

will not cause the interrupt/flag to occur until the address pointer is moved to a different

address. It should also be noted that if the last address written is the “Alarm Seconds,” the

address pointer will increment to the flag address, causing this situation to occur.

The IRQ/FT/OUT output is cleared by a READ to the flags register. A subsequent READ of

the flags register is necessary to see that the value of the alarm flag has been reset to '0.'

The IRQ/FT/OUT pin can also be activated in the battery backup mode. The IRQ/FT/OUT

will go low if an alarm occurs and both ABE (alarm in battery backup mode enable) and AFE

are set. The ABE and AFE bits are reset during power-up, therefore an alarm generated

during power-up will only set AF. The user can read the flag register at system boot-up to

determine if an alarm was generated while the M41ST85W was in the deselect mode during

power-up. Figure 16 on page 24 illustrates the backup mode alarm timing.

Figure 15. Alarm interrupt reset waveform

AI03664

IRQ/FT/OUT

ACTIVE FLAG

0Fh

0Eh

10h

HIGH-Z

Table 3. Alarm repeat modes

RPT5

RPT4

RPT3

RPT2

RPT1

Alarm setting

Once per second

Once per minute

Once per hour

Once per day

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