DS1558 Datasheet, PDF (7/18 Pages) Dallas Semiconductor – Watchdog Clock with NV RAM Control

DS1558
CLOCK OPERATIONS
Table 2 and the following paragraphs describe the operation of the RTC, alarm, and watchdog functions.
Table 2. DS1558 REGISTER MAP
ADDRESS
7FFFFh
7FFFEh
7FFFDh
7FFFCh
7FFFBh
7FFFAh
7FFF9h
7FFF8h
7FFF7h
7FFF6h
7FFF5h
7FFF4h
7FFF3h
7FFF2h
7FFF1h
7FFF0h
B₇
X
X
X
X
X
OSC
W
WDS
AE
AM4
AM3
AM2
AM1
Y
WF
DATA
B₆
B₅
B₄
B₃
B₂
B₁ B₀
10 YEAR
YEAR
X
X
10 M
MONTH
X
10 DATE
DATE
FT
X
X
X
DAY
X
10 HOUR
HOUR
10 MINUTES
MINUTES
10 SECONDS
SECONDS
R
10 CENTURY
CENTURY
BMB4 BMB3 BMB2 BMB1 BMB0 RB1 RB0
Y
ABE
Y
Y
Y
YY
Y
10 DATE
DATE
Y
10 HOURS
HOURS
10 MINUTES
MINUTES
10 SECONDS
SECONDS
Y
Y
Y
Y
Y
YY
AF
0
BLF
0
0
00
FUNCTION/RANGE
YEAR
MONTH
DATE
DAY
HOUR
MINUTES
SECONDS
CONTROL
WATCHDOG
INTERRUPTS
ALARM DATE
ALARM HOURS
ALARM MINUTES
ALARM SECONDS
UNUSED
FLAGS
00–99
01–12
01–31
01–07
00–23
00–59
00–59
00–39
—
—
01–31
00–23
00–59
00–59
—
—
X = Unused, Read/Writeable Under Write and Read Bit Control
AE = Alarm Flag Enable
FT = Frequency Test Bit
Y = Unused, Read/Writeable Without Write and Read Bit Control
OSC = Oscillator Start/Stop Bit
W = Write Bit
ABE = Alarm in Backup-Battery Mode Enable
AM1–AM4 = Alarm Mask Bits
R = Read Bit
WF = Watchdog Flag
WEN = Watchdog Enable Bit
AF = Alarm Flag
BMB0–BMB4 = Watchdog Multiplier Bits
0 = Reads as a 0 and Cannot Be Changed
RB0–RB1 = Watchdog Resolution Bits
BLF = Battery Low Flag
CLOCK OSCILLATOR CONTROL
The oscillator can be turned off to minimize current drain from the battery. The OSC bit is the MSB of
the seconds register (B7 of 7FFF9h). Setting OSC to a 1 stops the oscillator; setting to a 0 starts the
oscillator. The initial state of OSC is not guaranteed. When power is applied for the first time, the OSC
bit should be enabled. Oscillator operation and frequency can be verified by setting the FT bit and
monitoring the IRQ /FT pin for 512Hz.
OSCILLATOR STARTUP TIME
Oscillator startup times are highly dependent upon crystal characteristics and layout. High ESR and
excessive capacitive loads are the major contributors to long startup times. A circuit using a crystal with
the recommended characteristics and following the recommended layout usually starts within 1 second.
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