DS12885S ,Real-time clock, 14 bytes of clock and control registers, 114 bytes of general purpose RAMpin descriptions other than X1, X2, V , and RCLR , see the DS12887 data sheet. BAT TYPICAL OPERATIN ..
DS12887 ,Real Time ClockFEATURES Drop-in replacement for IBM AT computerclock/calendar Pin-compatible with the MC146818B ..
DS12887 ,Real Time ClockDS12887Real-Time ClockPIN ASSIGNMENT (Top View)
DS12887 ,Real Time ClockPIN DESCRIPTION Multiplex bus for pin efficiencyAD0–AD7 – Multiplexed Address/Data Bus Interfaced ..
DS12887 ,Real Time Clockblock diagram in Figure 1 shows the pin connections with the major internal functions of theDS12887 ..
DS12887+ ,Real-Time ClocksELECTRICAL CHARACTERISTICS(V = 0V, V = 3.0V, T = over the operating range, unless otherwise noted.) ..
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DS12885-DS12885Q
Real Time Clock
FEATURES § Drop-in replacement for IBM AT computer clock/calendar
§ Pin configuration closely matches MC146818B and DS1285
§ Counts seconds, minutes, hours, days, day of
the week, date, month, and year with leap-year compensation valid up to 2100
§ Binary or BCD representation of time, calendar, and alarm
§ 12-hour or 24-hour clock with AM and PM in
12-hour mode § Daylight Savings Time option
§ Selectable between Motorola and Intel bus timing
§ 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-maskable and testable
§ Time-of-day alarm once/second to once/day
§ Periodic rates from 122ms to 500ms
§ End-of-clock update cycle
§ Optional 28-pin PLCC surface mount package or 32-pin TQFP
§ Optional industrial temperature range
available
§ Underwriters Laboratory (UL) recognized
PIN ASSIGNMENT (Top View) Package Dimension Information http:///TechSupport/DallasPackInfo.htm
DS12885/DS12885Q/DS12885T
DESCRIPTION The DS12885 real-time clock plus RAM is designed to be a direct replacement for the DS1285. The
DS12885 is identical in form, fit, and function to the DS1285, 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. An external crystal and battery are the only components
required to maintain time-of-day and memory status in the absence of power. For a complete description of operating conditions, electrical characteristics, bus timing, and pin descriptions other than X1, X2,
VBAT, and RCLR, see the DS12887 data sheet.
TYPICAL OPERATING CIRCUIT
ORDERING INFORMATION
PIN-PACKAGE 24 DIP 24 DIP 24 SO 24 SO 28 PLCC 28 PLCC 28 PLCC/Tape and Reel 32 TQFP 32 TQFP 32 TQFP/Tape and Reel
DS12885/DS12885Q/DS12885T
PIN DESCRIPTION AD0–AD7 – Multiplexed Address/Data Bus
N.C. – No Connection MOT – Bus Type Selection – Chip Select AS – Address Strobe W – Read/Write Input DS – Data Strobe
RESET – Reset Input
IRQ – Interrupt Request Output (Open Drain)
SQW – Square-Wave Output
VCC – +5V Supply
GND – Ground
X1, X2 – 32.768kHz Crystal Connections
VBAT – +3V Battery Input
RCLR – RAM Clear
PIN DESCRIPTION X1, X2 – Connections for a standard 32.768kHz quartz crystal. The internal oscillator circuitry is
designed for operation with a crystal having a specified load capacitance (CL) of 6pF. The crystal is connected directly to the X1 and X2 pins. There is no need for external capacitors or resistors. Note: X1
and X2 are very high-impedance nodes. It is recommended that they and the crystal be guard-ringed with
ground and that high-frequency signals be kept away from the crystal area. For more information about crystal selection and crystal layout considerations, refer to Application Note 58 “Crystal Considerations
with Dallas Real Time Clocks.”
Oscillator startup times are highly dependent upon crystal characteristics and layout. High ESR and
escessive capactitive loads are the major contributors to long startup times. A circuit using a crystal with the recommended characteristics and following the recommended layout usually start within one second. VBAT – Battery input for any standard 3V lithium cell or other energy source. Battery voltage must be
held between 2.5V and 4V for proper operation. A maximum load of 0.5mA at +25°C in the absence of
power should be used to size the external energy source. Maximum load is measured using a recommended crystal type connected to X1 and X2. The battery should be connected directly to the VBAT pin. A diode must not be placed in series with the
battery to the VBAT pin. Furthermore, a diode is not necessary because reverse charging-current protection circuitry is provided internally to the device and has passed the requirements of Underwriters
Laboratories for UL listing.
See “Conditions of Acceptability” at http:///TechSupport/QA/ntrl.htm.
RCLR – The RCLR pin is used to clear (set to logic 1) all 114 bytes of general purpose RAM but does
not affect the RAM associated with the real-time clock. In order to clear the RAM, RCLR must be forced
to an input logic “0” (-0.3V to +0.8V) during battery-backup mode when VCC is not applied. The RCLR function is designed to be used by human interface (shorting to ground manually or by switch) and not to
be driven with external buffers. This pin is internally pulled up. Do not use an external pullup resistor on
this pin.
DS12885/DS12885Q/DS12885T CLOCK ACCURACY The accuracy of the clock is dependent upon the accuracy of the crystal and the accuracy of the match
between the capacitive load of the oscillator circuit and the capacitive load for which the crystal was trimmed. Additional error is added by crystal frequency drift caused by temperature shifts. External
circuit noise coupled into the oscillator circuit can result in the clock running fast. Refer to Application
Note 58 “Crystal Considerations with Dallas Real-Time Clocks” for detailed information.
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