DS12B887 Datasheet, PDF (2/16 Pages) Dallas Semiconductor – Real Time Clock

DS12B887
OPERATION
The block diagram in Figure 1 shows the pin connec-
tions with the major internal functions of the DS12B887.
The following paragraphs describe the function of each
pin.
BLOCK DIAGRAM DS12B887 Figure 1
CS
V_CC
V_BAT
OSC.
POWER
SWITCH
AND
WRITE
PROTECT
V_CC
POK
CS
DS
R/W
AS
ADO–
AD7
BUS
INTERFACE
B8
B64
B64
PERIODIC INTERRUPT/SQUARE WAVE
SELECTOR
SQUARE
WAVE OUT
SQW
CLOCK/
CALENDAR
UPDATE
BCD/
BINARY
INCREMENT
REGISTERS A,B,C,D
CLOCK, CALENDAR,
AND ALARM RAM
IRQ
DOUBLE
BUFFERED
USER RAM
114 BYTES
RAM
CLEAR
LOGIC
RCLR
POWER-DOWN/POWER-UP
CONSIDERATIONS
The Real Time Clock function will continue to operate
and all of the RAM, time, calendar, and alarm memory
locations remain nonvolatile regardless of the level of
the V_CC input. When V_CC is applied to the DS12B887
and reaches a level of greater than 4.25 volts, the device
becomes accessible after 200 ms, provided that the
oscillator is running and the oscillator countdown chain
is not in reset (see Register A). This time period allows
the system to stabilize after power is applied. When
V_CC falls below 4.25 volts, the chip select input is inter-
nally forced to an inactive level regardless of the value of
CS at the input pin. The DS12B887 is, therefore, write-
protected. When the DS12B887 is in a write-protected
state, all inputs are ignored and all outputs are in a high
impedance state. When V_CC falls below a level of
approximately 3 volts, the external V_CC supply is
switched off and an internal lithium energy source sup-
plies power to the Real Time Clock and the RAM
memory.
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