M41ST87Y_08 datasheet(32/48 Pages) STMICROELECTRONICS

32 / 48 page

Clock operation

M41ST87Y, M41ST87W

32/48

Figure 22.

Calibration waveform

3.3

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

M41ST87Y/W is in the battery back-up to serve as a system wake-up call.

Bits RPT5–RPT1 put the alarm in the repeat mode of operation. Table 8 on page 33 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/OUT pin as shown in Figure 24 on page 33. To disable

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

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/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/OUT pin can also be activated in the battery backup mode. The IRQ/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 M41ST87Y/W was in the deselect mode

during power-up. Figure 24 on page 33 illustrates the backup mode alarm timing.

AI00594B

NORMAL

POSITIVE

CALIBRATION

NEGATIVE

CALIBRATION

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