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PDF DS12C887 Data sheet ( Hoja de datos )
| Número de pieza | DS12C887 | |
| Descripción | Real Time Clock | |
| Fabricantes | Dallas Semiconducotr | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de DS12C887 (archivo pdf) en la parte inferior de esta página. Total 19 Páginas | ||
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DS12C887
Real Time Clock
www.dalsemi.com
FEATURES
PIN ASSIGNMENT
Drop-in replacement for IBM AT computer
clock/calendar
MOT 1
24 VCC
Pin compatible with the MC146818B and
DS1287
Totally nonvolatile with over 10 years of
moperation in the absence of power
Self-contained subsystem includes lithium,
oquartz, and support circuitry.
.cCounts seconds, minutes, hours, days, day of
the week, date, month, and year with leap
Uyear compensation valid up to 2100
Binary or BCD representation of time,
t4calendar, and alarm
12– or 24–hour clock with AM and PM in
e12–hour mode
eDaylight Savings Time option
Selectable between Motorola and Intel bus
htiming
Multiplex bus for pin efficiency
SInterfaced with software as 128 RAM
talocations
– 15 bytes of clock and control registers
a– 113 bytes of general purpose RAM
Programmable square wave output signal
.DBus–compatible interrupt signals (IRQ)
Three interrupts are separately software
maskable and testable
w– Time–of–day alarm once/second to
wonce/day
– Periodic rates from 122 ms to 500 ms
w m– End of clock update cycle
.coCentury register
NC
NC
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
GND
2
3
4
5
6
7
8
9
10
11
12
23 SQW
22 NC
21 NC
20 NC
19 IRQ
18 RESET
17 DS
16 NC
15 R/W
14 AS
13 CS
DS12C887 24-Pin
ENCAPSULATED PACKAGE
PIN DESCRIPTION
AD0-AD7 - Multiplexed Address/Data Bus
NC - No Connect
MOT
- Bus Type Selection
CS - RTC Chip Select Input
AS - Address Strobe
R/W - Read/Write Input
DS - Data Strobe
RESET - Reset Input
IRQ
SQW
VCC
GND
- Interrupt Request Output
- Square Wave Output
- +5 Volt Main Supply
- Ground
t4UDESCRIPTION
eThe DS12C887 Real Time Clock plus RAM is designed as a direct upgrade replacement for the DS12887
ein existing IBM compatible personal computers to add hardware year 2000 compliance. A century byte
hwas added to memory location 50, 32h, as called out by the PC AT specification. A lithium energy
Ssource, quartz crystal, and write-protection circuitry are contained within a 24–pin dual in-line package.
taAs such, the DS12C887 is a complete subsystem replacing 16 components in a typical application. The
afunctions include a nonvolatile time-of-day clock, an alarm, a one-hundred-year calendar, programmable
.Dinterrupt, square wave generator, and 113 bytes of nonvolatile static RAM. The real time clock is
wdistinctive in that time-of-day and memory are maintained even in the absence of power.
ww1 of 19
020900
1 page  
DS12C887
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
DS12C887 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 VCC falls below 4.25 volts,
the chip select input is internally forced to an inactive level regardless of the value of CS at the input pin.
The DS12C887 is, therefore, write-protected. When the DS12C887 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 supplies power to
the Real Time Clock and the RAM memory.
RTC ADDRESS MAP
The address map for the DS12C885 is shown in Figure 2. The address map consists of 113 bytes of user
RAM, 11 bytes of RAM that contain the RTC time, calendar, and alarm data, and 4 bytes which are used
for control and status. All 128 bytes can be directly written or read except for the following:
1. Registers C and D are read-only.
2. Bit-7 of Register A is read-only.
3. The high order bit of the seconds byte is read-only.
DS12C887 REAL TIME CLOCK ADDRESS MAP Figure 2
5 of 19
5 Page 
DS12C887
OSCILLATOR CONTROL BITS
When the DS12C887 is shipped from the factory, the internal oscillator is turned off. This feature
prevents the lithium energy cell from being used until it is installed in a system. A pattern of 010 in bits 4
through 6 of Register A will turn the oscillator on and enable the countdown chain. A pattern of 11X will
turn the oscillator on, but holds the countdown chain of the oscillator in reset. All other combinations of
bits 4 through 6 keep the oscillator off.
SQUARE WAVE OUTPUT SELECTION
Thirteen of the 15 divider taps are made available to a 1-of-15 selector, as shown in the block diagram of
Figure 1. The first purpose of selecting a divider tap is to generate a square wave output signal on the
SQW pin. The RS0–RS3 bits in Register A establish the square wave output frequency. These
frequencies are listed in Table 1. The SQW frequency selection shares its 1-of-15 selector with the
periodic interrupt generator. Once the frequency is selected, the output of the SQW pin can be turned on
and off under program control with the square wave enable bit (SQWE).
PERIODIC INTERRUPT SELECTION
The periodic interrupt will cause the IRQ pin to go to an active state from once every 500ms to once
every 122µs. This function is separate from the alarm interrupt which can be output from once per second
to once per day. The periodic interrupt rate is selected using the same Register A bits which select the
square wave frequency (see Table 1). Changing the Register A bits affects both the square wave
frequency and the periodic interrupt output. However, each function has a separate enable bit in Register
B. The SQWE bit controls the square wave output. Similarly, the periodic interrupt is enabled by the PIE
bit in Register B. The periodic interrupt can be used with software counters to measure inputs, create
output intervals, or await the next needed software function.
PERIODIC INTERRUPT RATE AND SQUARE WAVE OUTPUT FREQUENCY
Table 2
EXT. REG. B
E32K
SELECT BITS REGISTER A
RS3 RS2 RS1 RS0
tPI PERIODIC INTERRUPT
RATE
SQW OUTPUT
FREQUENCY
0 0 0 0 0 None
None
0 0 0 0 1 3.90625 ms
256 Hz
0 0 0 1 0 7.8125 ms
128 Hz
0 0 0 1 1 122.070 µs
8.192 kHz
0 0 1 0 0 244.141 µs
4.096 kHz
0 0 1 0 1 488.281 µs
2.048 kHz
0 0 1 1 0 976.5625 µs
1.024 kHz
0 0 1 1 1 1.953125 ms
512 Hz
0 1 0 0 0 3.90625 ms
256 Hz
0 1 0 0 1 7.8125 ms
128 Hz
0 1 0 1 0 15.625 ms
64 Hz
0 1 0 1 1 31.25 ms
32 Hz
0 1 1 0 0 62.5 ms
16 Hz
0 1 1 0 1 125 ms
8 Hz
0 1 1 1 0 250 ms
4 Hz
0 1 1 1 1 500 ms
2 Hz
11 of 19
11 Page | ||
| Páginas | Total 19 Páginas | |
| PDF Descargar | [ Datasheet DS12C887.PDF ] | |
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