DS1557 Maxim, DS1557 Datasheet - Page 7

• Manufacturer Part Number: DS1557
• Description: The DS1557 is a full-function, year-2000-compliant (Y2KC), real-time clock/calendar (RTC) with an RTC alarm, watchdog timer, power-on reset, battery monitor, and 512k x 8 nonvolatile static RAM
• Manufacturer: Maxim

USING THE CLOCK ALARM
The alarm settings and control for the DS1557 reside within Registers 7FFF2h-7FFF5h. Register 7FFF6h
contains two alarm enable bits: Alarm Enable (AE) and Alarm in Backup Enable (ABE). The AE and
ABE bits must be set as described below for the IRQ/FT output to be activated for a matched alarm
condition.
The alarm can be programmed to activate on a specific day of the month or repeat every day, hour,
minute, or second. It can also be programmed to go off while the DS1557 is in the battery-backed state of
operation to serve as a system wakeup. Alarm mask bits AM1 to AM4 control the alarm mode. Table 3
shows the possible settings. Configurations not listed in the table default to the once per second mode to
notify the user of an incorrect alarm setting.
Table 3. Alarm Mask Bits
When the RTC Register values match Alarm Register settings, the Alarm Flag bit (AF) is set to a 1. If
Alarm Flag Enable (AE) is also set to a 1, the alarm condition activates the IRQ/FT pin. The IRQ/FT
signal is cleared by a read or write to the Flags Register (Address 7FFF0h) as shown in Figure 2 and 3.
When CE is active, the IRQ/FT signal may be cleared by having the address stable for as short as 15 ns
and either OE or WE active, but is not guaranteed to be cleared unless t
also cleared by a read or write to the Flags Register but the flag will not change states until the end of the
read/write cycle and the IRQ/FT signal has been cleared.
Figure 2. Clearing IRQ Waveforms
C
C
E
E
AM4
,
,
1
1
1
1
0
AM3
1
1
1
0
0
AM2
1
1
0
0
0
AM1
1
0
0
0
0
ALARM RATE
Once per second
When seconds match
When minutes and seconds match
When hours, minutes, and seconds match
When date, hours, minutes, and seconds match
DS1557 4Meg, Nonvolatile, Y2K-Compliant Timekeeping RAM
7 of 17
RC
is fulfilled. The alarm flag is