DS12B887资料
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ECopyright 1995 by Dallas Semiconductor Corporation.All Rights Reserved. For important information regardingpatents and other intellectual property rights, please refer toDallas Semiconductor data books.DS12B887
Real Time ClockDS12B887
080895 1/16FEATURES
•Drop-in replacement for IBM AT computer clock/cal-endar
•Pin compatible with the MC146818B and DS1287
•Totally nonvolatile with over 10 years of operation inthe absence of power
•Self-contained subsystem includes lithium, quartz,and support circuitry
•Counts seconds, minutes, hours, days, day of theweek, date, month, and year with leap year com-pensation
•Binary or BCD representation of time, calendar, andalarm
•12- or 24-hour clock with AM and PM in 12-hour mode
•Daylight Savings Time option
•Multiplex bus for pin efficiency
•Interfaced with software as 128 RAM locations
–14 bytes of clock and control registers
–114 bytes of general purpose RAM
•Programmable square wave output signal
•Bus-compatible interrupt signals (IRQ)
•Three interrupts are separately software-maskableand testable
–Time-of-day alarm once/second to once/day
–Periodic rates from 122 µs to 500 ms
–End of clock update cyclePIN ASSIGNMENT
24 PIN ENCAPSULATED PACKAGEVCCSQW
NC
RCLR
NC
IRQ
NC
DS
NC
R/W
AS
CSNC
NC
NC
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
GND1
2
3
4
5
6
7
8
9
10
11
1224
23
22
21
20
19
18
17
16
15
14
13
PIN DESCRIPTION
AD0-AD7–Multiplexed Address/Data Bus
NC–No Connection
CS–Chip Select
AS–Address Strobe
R/W–Read/Write Input
DS–Data Strobe
IRQ–Interrupt Request Output
SQW–Square Wave Output
VCC–+5 Volt Supply
GND–Ground
RCLR–RAM Clear
DESCRIPTION
The DS12B887 Real Time Clock plus RAM is designed
to be a direct replacement for the DS1287A or
DS12887A. The DS12B887 is identical in form, fit, and
function to the DS1287A or DS12887A, with the excep-
tion of RCLR, and has an additional 64 bytes of general
purpose RAM. Access to this additional RAM space is
determined by the logic level presented on AD6 during
the address portion of an access cycle. A lithium energy
source, quartz crystal, and write-protection circuitry arecontained within a 24-pin dual in-line package. As such,
the DS12B887 is a complete subsystem replacing 16
components in a typical application. The functions
include a nonvolatile time-of-day clock, an alarm, a one-
hundred-year calendar, programmable interrupt,
square wave generator, and 114 bytes of nonvolatile
static RAM. The real time clock is distinctive in that
time-of-day and memory are maintained even in the
absence of power.元器件交易网www.cecb2b.comDS12B887
080895 2/16OPERATION
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
USER RAM114 BYTESCLOCK, CALENDAR,AND ALARM RAMREGISTERS A,B,C,DSQUAREWAVE OUTPERIODIC INTERRUPT/SQUARE WAVESELECTOROSC.
POWERSWITCHANDWRITEPROTECT
BUSINTERFACECLOCK/CALENDARUPDATE
BCD/BINARYINCREMENTSQW
IRQ
DOUBLEBUFFERED
ADO–AD7ASDS
R/WCSCS
VCC
POKVCC
VBATB8B64B64
RAMCLEARLOGICRCLR
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 VCC input. When VCC 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. WhenVCC 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 VCC falls below a level of
approximately 3 volts, the external VCC supply is
switched off and an internal lithium energy source sup-
plies power to the Real Time Clock and the RAM
memory.元器件交易网www.cecb2b.com
