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DS12887
Real-Time Clock
FEATURES
§ Drop-in replacement for IBM AT computer
clock/calendar
§ Pin-compatible with the MC146818B and
DS1287
§ Totally nonvolatile with over 10 years of
operation in the absence of power
§ Self-contained subsystem includes lithium,
quartz, and support circuitry
§ Counts seconds, minutes, hours, days, day of
the week, date, month, and year with leapyear 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 (
§ 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
§ Underwriters Laboratory (UL) recognized
IRQ )
PIN ASSIGNMENT (Top View)
NC
DS12887 24 PDIP Module (700mil)
Package Dimension Information
http://www.maxim-ic.com/TechSupport/DallasPackInfo.htm
PIN DESCRIPTION
AD0–AD7 – Multiplexed Address/Data Bus
N.C. – No Connection
MOT – Bus Type Selection
CS – Chip Select
AS – Address Strobe
W – Read/Write Input
R/
DS – Data Strobe
RESET – Reset Input
IRQ – Interrupt Request Output
SQW – Square-Wave Output
V
– +5V Supply
CC
GND – Ground
ORDERING INFORMATION
PART PIN-PACKAGE TOP MARK TEMP RANGE
DS12887 24 PDIP Module
Note: Some revisions of this device may incorporate deviations from published specifications known as errata. Multiple revisions of any device
may be simultaneously available through various sales channels. For information about device errata, click here: http://www.maxim-ic.com/errata.
DS12887 0°C to +70°C
1 of 19 073102
DS12887
TYPICAL OPERATING CIRCUIT
DESCRIPTION
The DS12887 real-time clock (RTC) plus RAM is designed to be a direct replacement for the DS1287.
The DS12887 is identical in form, fit, and function to the DS1287, 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 writeprotection circuitry are contained within a 24-pin dual in-line package. As such, the DS12887 is a
complete subsystem replacing 16 components in a typical application. The functions include a nonvolatile
time-of-day clock, an alarm, a 100-year calendar, programmable interrupt, square-wave generator, and
114 bytes of NV SRAM. The RTC is unique in that time-of-day and memory are maintained even in the
absence of power.
OPERATION
The block diagram in Figure 1 shows the pin connections with the major internal functions of the
DS12887. The following paragraphs describe the function of each pin.
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Figure 1. BLOCK DIAGRAM
DS12887
POWER-UP/DOWN CONSIDERATIONS
The RTC function continues to operate, and all of the RAM, time, calendar, and alarm memory locations
remain nonvolatile regardless of the level of the VCC input. When V
reaches a level of greater than 4.25V, the device becomes accessible after 200ms, provided that the
oscillator is running and the oscillator countdown chain is not in reset (Register A). This time period
allows the system to stabilize after power is applied. When V
internally forced to an inactive level regardless of the value of
falls below 4.25V, the chip-select input is
CC
CS at the input pin. The DS12887 is,
therefore, write-protected. When the DS12887 is in a write-protected state, all inputs are ignored and all
outputs are in a high-impedance state. When V
falls below a level of approximately 3V, the external
CC
VCC supply is switched off, and an internal lithium energy source supplies power to the RTC and the
RAM memory.
is applied to the DS12887 and
CC
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SIGNAL DESCRIPTIONS
DS12887
GND, VCC – DC power is provided to the device on these pins. V
is the +5V input. When 5V are
CC
applied within normal limits, the device is fully accessible and data can be written and read. When VCC is
below 4.25V typical, reads and writes are inhibited. However, the timekeeping function continues
unaffected by the lower input voltage. As VCC falls below 3V typical, the RAM and timekeeper are
switched over to an internal lithium energy source. The timekeeping function maintains an accuracy of ±1
minute per month at +25°C, regardless of the voltage input on the VCC pin.
MOT (Mode Select) – The MOT pin offers the flexibility to choose between two bus types. When
connected to V
, Motorola bus timing is selected. When connected to GND or left disconnected, Intel
CC
bus timing is selected. The pin has an internal pulldown resistance of approximately 20kW.
SQW (Square-Wave Output) – The SQW pin can output a signal from one of 13 taps provided by the
15 internal divider stages of the RTC. The frequency of the SQW pin can be changed by programming
Register A, as shown in Table 1. The SQW signal can be turned on and off using the SQWE bit in
Register B. The SQW signal is not available when V
is less than 4.25V, typically.
CC
Table 1. PERIODIC INTERRUPT RATE AND SQUARE-WAVE OUTPUT
FREQUENCY
SELECT BITS REGISTER A
RS3 RS2 RS1 RS0
0 0 0 0 None None
0 0 0 1 3.90625ms 256Hz
0 0 1 0 7.8125ms 128Hz
0011
0100
0101
0110
0 1 1 1 1.953125ms 512Hz
1 0 0 0 3.90625ms 256Hz
1 0 0 1 7.8125ms 128Hz
1 0 1 0 15.625ms 64Hz
1 0 1 1 31.25ms 32Hz
1 1 0 0 62.5ms 16Hz
1 1 0 1 125ms 8Hz
1 1 1 0 250ms 4Hz
1 1 1 1 500ms 2Hz
tPI PERIODIC
INTERRUPT RATE
122.070ms
244.141ms
488.281ms
976.5625ms
SQW OUTPUT
FREQUENCY
8.192kHz
4.096kHz
2.048kHz
1.024kHz
AD0–AD7 (Multiplexed Bidirectional Address/Data Bus) – Multiplexed buses save pins because
address information and data information time-share the same signal paths. The addresses are present
during the first portion of the bus cycle and the same pins and signal paths are used for data in the second
portion of the cycle. Address/data multiplexing does not slow the access time of the DS12887 since the
bus change from address to data occurs during the internal RAM access time. Addresses must be valid
prior to the falling edge of AS/ ALE, at which time the DS12887 latches the address from AD0 to AD6.
Valid write data must be present and held stable during the latter portion of the DS or
WR pulses. In a
read cycle the DS12887 outputs 8 bits of data during the latter portion of the DS or RD pulses. The read
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DS12887
cycle is terminated and the bus returns to a high-impedance state as DS transitions low in the case of
Motorola timing or as RD transitions high in the case of Intel timing.
AS (Address Strobe Input) – A positive-going address-strobe pulse serves to demultiplex the bus. The
falling edge of AS/ALE causes the address to be latched within the DS12887. The next rising edge that
occurs on the AS bus clears the address regardless of whether CS is asserted. Access commands should
be sent in pairs.
DS (Data Strobe or Read Input) – The DS/ RD pin has two modes of operation depending on the level
of the MOT pin. When the MOT pin is connected to VCC, Motorola bus timing is selected. In this mode,
DS is a positive pulse during the latter portion of the bus cycle and is called Data Strobe. During read
cycles, DS signifies the time that the DS12887 is to drive the bidirectional bus. In write cycles the trailing
edge of DS causes the DS12887 to latch the written data. When the MOT pin is connected to GND, Intel
bus timing is selected. In this mode the DS pin is called Read ( RD ). RD identifies the time period when
the DS12887 drives the bus with read data. The RD signal is the same definition as the output-enable
( OE ) signal on a typical memory.
R/ W (Read/Write Input) – The R/ W pin also has two modes of operation. When the MOT pin is
connected to VCC for Motorola timing, R/ W is at a level that indicates whether the current cycle is a read
or write. A read cycle is indicated with a high level on R/ W while DS is high. A write cycle is indicated
when R/ W is low during DS.
When the MOT pin is connected to GND for Intel timing, the R/ W signal is an active-low signal called
WR. In this mode, the R/ W pin has the same meaning as the write-enable signal ( WE ) on generic RAMs.
CS (Chip-Select Input) – The chip select signal must be asserted low for a bus cycle in the DS12887 to
be accessed. CS must be kept in the active state during DS and AS for Motorola timing and during RD
and WR for Intel timing. Bus cycles that take place without asserting CS latch addresses but no access
occur. When VCC is below 4.25V, the DS12887 internally inhibits access cycles by internally disabling
the CS input. This action protects both the RTC data and RAM data during power outages.
IRQ (Interrupt Request Output) – The IRQ pin is an active-low output of the DS12887 that can be
used as an interrupt input to a processor. The
IRQ output remains low as long as the status bit causing the
interrupt is present and the corresponding interrupt-enable bit is set. To clear the IRQ pin, the processor
program normally reads the C register. The
When no interrupt conditions are present, the
RESET pin also clears pending interrupts.
IRQ level is in the high-impedance state. Multiple
interrupting devices can be connected to an IRQ bus. The IRQ bus is an open drain output and requires an
external pullup resistor.
RESET (Reset Input) – The RESET pin has no affect on the clock, calendar, or RAM. On power-up, the
RESET pin can be held low for a time to allow the power supply to stabilize. The amount of time that
RESET is held low is dependent on the application. However, if RESET is used on power-up, the time
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DS12887
RESET is low should exceed 200ms to ensure that the internal timer that controls the DS12887 on power-
up has timed out. When RESET is low and VCC is above 4.25V, the following occurs:
A) Periodic Interrupt Enable (PEI) bit is cleared to 0.
B) Alarm Interrupt Enable (AIE) bit is cleared to 0.
C) Update Ended Interrupt Flag (UF) bit is cleared to 0.
D) Interrupt Request Status Flag (IRQF) bit is cleared to 0.
E) Periodic Interrupt Flag (PF) bit is cleared to 0.
F) The device is not accessible until RESET is returned high.
G) Alarm Interrupt Flag (AF) bit is cleared to 0.
H) H. IRQ pin is in the high impedance state.
I) Square-Wave Output Enable ( SQWE ) bit is cleared to 0.
J) Update Ended Interrupt Enable (UIE) is cleared to 0.
In a typical application RESET can be connected to VCC. This connection allows the DS12887 to go in
and out of power fail without affecting any of the control registers.
ADDRESS MAP
The address map of the DS12887 is shown in Figure 2. The address map consists of 114 bytes of user
RAM; 10 bytes of RAM that contain the RTC time, calendar, and alarm data; and 4 bytes that 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.
The contents of four registers (A, B, C, and D) are described in the Registers section.
Figure 2. ADDRESS MAP
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DS12887
TIME, CALENDAR, AND ALARM LOCATIONS
The time and calendar information is obtained by reading the appropriate memory bytes. The time,
calendar, and alarm are set or initialized by writing the appropriate RAM bytes. The contents of the 10
time, calendar, and alarm bytes can be either binary or binary coded decimal (BCD) format. Before
writing the internal time, calendar, and alarm registers, the SET bit in Register B should be written to a
logic 1 to prevent updates from occurring while access is being attempted. In addition to writing the 10
time, calendar, and alarm registers in a selected format (binary or BCD), the data mode bit (DM) of
Register B must be set to the appropriate logic level. All 10 time, calendar, and alarm bytes must use the
same data mode. The set bit in Register B should be cleared after the data mode bit has been written to
allow the RTC to update the time and calendar bytes. Once initialized, the RTC makes all updates in the
selected mode. The data mode cannot be changed without reinitializing the 10 data bytes. Table 2 shows
the binary and BCD formats of the 10 time, calendar, and alarm locations. The 24–12 bit cannot be
changed without reinitializing the hour locations. When the 12-hour format is selected, the high-order bit
of the hours byte represents PM when it is a logic 1. The time, calendar, and alarm bytes are always
accessible because they are double buffered. The 10 bytes are advanced once per second by 1 second and
checked for an alarm condition. If a read of the time and calendar data occurs during an update, a problem
exists where seconds, minutes, hours, etc., might not correlate. The probability of reading incorrect time
and calendar data is low. Several methods of avoiding any possible incorrect time and calendar reads are
covered later in this text.
The three alarm bytes can be used in two ways. First, when the alarm time is written in the appropriate
hours, minutes, and seconds alarm locations, the alarm interrupt is initiated at the specified time each day
if the alarm enable bit is high. The second use condition is to insert a “don’t care” state in one or more of
the three alarm bytes. The “don’t care” code is any hexadecimal value from C0 to FF. The two most
significant bits of each byte set the “don’t care” condition when at logic 1. An alarm is generated each
hour when the “don’t care” bits are set in the hours byte. Similarly, an alarm is generated every minute
with “don’t care” codes in the hours and minute alarm bytes. The “don’t care” codes in all three alarm
bytes create an interrupt every second.
Table 2. TIME, CALENDAR, AND ALARM DATA MODES
ADDRESS
LOCATION
0 Seconds 0–59 00–3B 00–59
1 Seconds Alarm 0–59 00–3B 00–59
2 Minutes 0–59 00–3B 00–59
3 Minutes Alarm 0–59 00–3B 00–59
4
5
6
7 Date of the Month 1–31 01–1F 01–31
8 Month 1–12 01–0C 01–12
9 Year 0–99 00–63 00–99
FUNCTION
Hours, 12-hour Mode 1–12
Hours, 24-hour Mode 0–23 00–17 00–23
Hours Alarm, 12-hour 1–12
Hours Alarm, 24-hourr 0–23 00–17 00–23
Day of the Week
Sunday = 1
DECIMAL
RANGE
1–7 01–07 01–07
DATA MODE RANGE
BINARY BCD
01–0C AM,
81–8C PM
01–0C AM,
81–8C PM
01–12AM,
81–92PM
01–12AM,
81–92PM
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DS12887
NV RAM
The 114 general-purpose NV RAM bytes are not dedicated to any special function within the DS12887.
They can be used by the processor program as nonvolatile memory and are fully available during the
update cycle.
INTERRUPTS
The RTC plus RAM includes three separate, fully automatic sources of interrupt for a processor. The
alarm interrupt can be programmed to occur at rates from once per second to once per day. The periodic
interrupt can be selected for rates from 500ms to 122ms. The update-ended interrupt can be used to
indicate to the program that an update cycle is complete. Each of these independent interrupt conditions is
described in greater detail in other sections of this text.
The processor program can select which interrupts, if any, are going to be used. Three bits in Register B
enable the interrupts. Writing a logic 1 to an interrupt-enable bit permits that interrupt to be initiated when
the event occurs. A 0 in an interrupt-enable bit prohibits the IRQ pin from being asserted from that
interrupt condition. If an interrupt flag is already set when an interrupt is enabled, IRQ is immediately set
at an active level, although the interrupt initiating the event may have occurred much earlier. As a result,
there are cases where the program should clear such earlier initiated interrupts before first enabling new
interrupts.
When an interrupt event occurs, the relating flag bit is set to logic 1 in Register C. These flag bits are set
independently of the state of the corresponding enable bit in Register B. The flag bit can be used in a
polling mode without enabling the corresponding enable bits. The interrupt flag bit is a status bit that
software can interrogate as necessary. When a flag is set, an indication is given to software that an
interrupt event has occurred since the flag bit was last read; however, care should be taken when using the
flag bits as they are cleared each time Register C is read. Double latching is included with Register C so
that set bits remain stable throughout the read cycle. All bits that are set (high) are cleared when read and
new interrupts that are pending during the read cycle are held until after the cycle is completed. One, two,
or three bits can be set when reading Register C. Each used flag bit should be examined when read to
ensure that no interrupts are lost.
The second flag bit usage method is with fully enabled interrupts. When an interrupt flag bit is set and the
corresponding interrupt-enable bit is also set, the
IRQ pin is asserted low. IRQ is asserted as long as at
least one of the three interrupt sources has its flag and enable bits both set. The IRQF bit in Register C is
a 1 whenever the
IRQ pin is being driven low. Determination that the RTC initiated an interrupt is
accomplished by reading Register C. A logic 1 in bit 7 (IRQF bit) indicates that one or more interrupts
have been initiated by the DS12887. The act of reading Register C clears all active flag bits and the IRQF
bit.
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DS12887
OSCILLATOR CONTROL BITS
When the DS12887 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 turns the oscillator on and enables the countdown chain. A pattern of 11X turns 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 causes the IRQ pin to go to an active state from once every 500ms to once every
122ms. 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 (Table 1). Changing the Register A bits affect 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.
UPDATE CYCLE
The DS12887 executes an update cycle once per second regardless of the SET bit in Register B. When
the SET bit in Register B is set to 1, the user copy of the double-buffered time, calendar, and alarm bytes
is frozen and will not update as the time increments. However, the time countdown chain continues to
update the internal copy of the buffer. This feature allows time to maintain accuracy independent of
reading or writing the time, calendar, and alarm buffers and also guarantees that time and calendar
information is consistent. The update cycle also compares each alarm byte with the corresponding time
byte and issues an alarm if a match or if a “don’t care” code is present in all three positions.
There are three methods that can handle access of the RTC that avoid any possibility of accessing
inconsistent time and calendar data. The first method uses the update-ended interrupt. If enabled, an
interrupt occurs after every update cycle that indicates that over 999ms are available to read valid time
and date information. If this interrupt is used, the IRQF bit in Register C should be cleared before leaving
the interrupt routine.
A second method uses the update-in-progress bit (UIP) in Register A to determine if the update cycle is in
progress. The UIP bit pulses once per second. After the UIP bit goes high, the update transfer occurs
244ms later. If a low is read on the UIP bit, the user has at least 244ms before the time/calendar data is
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DS12887
changed. Therefore, the user should avoid interrupt service routines that would cause the time needed to
read valid time/calendar data to exceed 244ms.
The third method uses a periodic interrupt to determine if an update cycle is in progress. The UIP bit in
Register A is set high between the setting of the PF bit in Register C (Figure 3). Periodic interrupts that
occur at a rate of greater than t
of the periodic interrupt. The reads should be complete within one (t
allow valid time and date information to be reached at each occurrence
BUC
PI/2
+ t
) to ensure that data is not
BUC
read during the update cycle.
Figure 3. UPDATE-ENDED AND PERIODIC INTERRUPT RELATIONSHIP
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DS12887
REGISTERS
The DS12887 has four control registers that are accessible at all times, even during the update cycle.
REGISTER A
MSB LSB
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
UIP DV2 DV1 DV0 RS3 RS2 RS1 RS0
UIP – The update-in-progress (UIP) bit is a status flag that can be monitored. When the UIP bit is a 1, the
update transfer occurs soon. When UIP is a 0, the update transfer does not occur for at least 244s. The
time, calendar, and alarm information in RAM is fully available for access when the UIP bit is 0. The UIP
bit is read-only and is not affected by RESET . Writing the SET bit in Register B to a 1 inhibits any update
transfer and clears the UIP status bit.
DV0, DV1, DV2– These three bits are used to turn the oscillator on or off and to reset the countdown
chain. A pattern of 010 is the only combination of bits that turn the oscillator on and allow the RTC to
keep time. A pattern of 11X enables the oscillator but holds the countdown chain in reset. The next
update occurs at 500ms after a pattern of 010 is written to DV0, DV1, and DV2.
RS3, RS2, RS1, RS0 – These four rate-selection bits select one of the 13 taps on the 15-stage divider or
disable the divider output. The tap selected can be used to generate an output square-wave (SQW pin)
and/or a periodic interrupt. The user can do one of the following:
1) Enable the interrupt with the PIE bit;
2) Enable the SQW output pin with the SQWE bit;
3) Enable both at the same time and the same rate; or
4) Enable neither.
Table 1 lists the periodic interrupt rates and the square-wave frequencies that can be chosen with the RS
bits. These four read/write bits are not affected by RESET .
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DS12887
REGISTER B
MSB LSB
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
SET PIE AIE UIE SQWE DM 24/12 DSE
SET – When the SET bit is a 0, the update transfer functions normally by advancing the counts once per
second. When the SET bit is written to a 1, any update transfer is inhibited and the program can initialize
the time and calendar bytes without an update occurring in the midst of initializing. Read cycles can be
executed in a similar manner. SET is a read/write bit that is not modified by RESET or internal functions
of the DS12887.
PIE – The periodic-interrupt enable (PIE) bit is a read/write bit that allows the periodic-interrupt flag
(PF) bit in Register C to drive the IRQ pin low. When the PIE bit is set to 1, periodic interrupts are
generated by driving the IRQ pin low at a rate specified by the RS3–RS0 bits of Register A. A 0 in the
PIE bit blocks the IRQ output from being driven by a periodic interrupt, but the periodic flag (PF) bit is
still set at the periodic rate. PIE is not modified by any internal DS12887 functions, but is cleared to 0 on
RESET .
AIE – The alarm interrupt enable (AIE) bit is a read/write bit that, when set to a 1, permits the alarm flag
(AF) bit in Register C to assert IRQ . An alarm interrupt occurs for each second that the three time bytes
equal the three alarm bytes, including a “don’t care” alarm code of binary 11XXXXXX. When the AIE
bit is set to 0, the AF bit does not initiate the IRQ signal. The RESET pin clears AIE to 0. The internal
functions of the DS12887 do not affect the AIE bit.
UIE – The update-ended interrupt enable (UIE) bit is a read/ write that enables the update-end flag (UF)
bit in Register C to assert IRQ . The RESET pin going low or the SET bit going high clears to UIE bit.
SQWE – When the square-wave enable (SQWE) bit is set to a 1, a square-wave signal at the frequency
set by the rate-selection bits RS3 through RS0 is driven out on a SQW pin. When the SQWE bit is set to
z0, the SQW pin is held low; the state of SQWE is cleared by the
RESET pin. SQWE is a read/write bit.
DM – The data mode (DM) bit indicates whether time and calendar information is in binary or BCD
format. The DM bit is set by the program to the appropriate format and can be read as required. This bit is
not modified by internal functions or RESET . A 1 in DM signifies binary data while a 0 in DM specifies
BCD data.
24/12 –
The 24/12 control bit establishes the format of the hours byte. A 1 indicates the 24-hour mode
and a 0 indicates the 12-hour mode. This bit is read/write and is not affected by internal functions of
RESET .
DSE – The Daylight Savings Enable (DSE) bit is a read/write bit that enables two special updates when
DSE is set to 1. On the first Sunday in April, the time increments from 1:59:59 AM to 3:00:00 AM. On
the last Sunday in October when the time first reaches 1:59:59 AM, it changes to 1:00:00 AM. These
special updates do not occur when the DSE bit is a 0. This bit is not affected by internal functions
RESET .
or
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DS12887
REGISTER C
MSB LSB
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
IRQF PF AF UF 0 0 0 0
IRQF– The interrupt request flag (IRQF) bit is set to a 1 when one or more of the following are true:
PF = PIE = 1
AF = AIE = 1
UF = UIE = 1
That is, IRQF = PF x PIE + AF x AIE + UF x UIE.
Any time the IRQF bit is a 1, the IRQ pin is driven low. All flag bits are cleared after Register C is read
by the program or when the RESET pin is low.
PF– The periodic-interrupt flag (PF) is a read-only bit that is set to a 1 when an edge is detected on the
selected tap of the divider chain. The RS3 through RS0 bits establish the periodic rate. PF is set to a 1
independent of the state of the PIE bit. When both PF and PIE are 1s, the IRQ signal is active and sets the
IRQF bit. The PF bit is cleared by a RESET or a software read of Register C.
AF– A 1 in the alarm-interrupt flag (AF) bit indicates that the current time has matched the alarm time. If
the AIE bit is also a 1, the IRQ pin goes low and a 1 appears in the IRQF bit. A RESET or a read of
Register C clears AF.
UF – The update-ended interrupt flag (UF) bit is set after each update cycle. When the UIE bit is set to 1,
the one in UF causes the IRQF bit to be a 1, which asserts the IRQ pin. UF is cleared by reading Register
C or a RESET .
BIT 0, BIT 1, BIT 2, BIT 3 – These are unused bits of the status Register C. These bits always read 0
and cannot be written.
REGISTER D
MSB LSB
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
VRT0000000
VRT – The valid RAM and time (VRT) bit is set to the 1 state prior to shipment. This bit is not writable
and should always be a 1 when read. If a 0 is ever present, an exhausted internal lithium energy source is
indicated and both the contents of the RTC data and RAM data are questionable. This bit is unaffected by
RESET .
BIT 6, BIT 5, BIT 4, BIT 3, BIT 2, BIT 1, BIT 0– The remaining bits of Register D are not usable.
They cannot be written and, when read, they always read 0.
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DS12887
ABSOLUTE MAXIMUM RATINGS*
Voltage Range on Any Pin Relative to Ground -0.3V to +7.0V
Operating Temperature Range 0°C to +70°C
Storage Temperature Range -40°C to +70°C
Soldering Temperature See IPC/JEDEC J-STD-020A (Note 7)
*This is a stress rating only and functional operation of the device at these or any other conditions beyond those indicated in the operation
sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods of time can affect reliability.
RECOMMENDED DC OPERATING CONDITIONS (T
= 0°C to +70°C)
A
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Power Supply Voltage V
Input Logic 1 V
Input Logic 0 V
CC
IH
IL
DC ELECTRICAL CHARACTERISTICS (V
4.5 5.0 5.5 V 1
2.2 VCC + 0.3 V 1
-0.3 0.8 V 1
= 4.5 to 5.5V, 0°C to +70°C)
CC
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Power Supply Current I
Input Leakage I
I/O Leakage I
Input Current I
Output at 2.4V I
Output at 0.4V I
Write Protect Voltage V
CC1
IL
LO
MOT
OH
OL
TP
-1.0 +1.0
-1.0 +1.0
-1.0 +500
-1.0 mA 1, 5
4.0 4.25 4.5 V
715mA 2
mA
mA
mA
4.0 mA 1
CAPACITANCE (T
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Input Capacitance C
Output Capacitance C
IN
OUT
5pF
7pF
3
4
3
= +25°C)
A
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DS12887
AC ELECTRICAL CHARACTERISTICS (V
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Cycle Time t
Pulse Width, DS/E Low or RD/ WR High
Pulse Width, DS/E High or RD/ WR Low
CYC
PW
PW
Input Rise and Fall Time tR, t
R/ W Hold Time
R/ W Setup Time Before DS/E
Chip-Select Setup Time Before DS, WR ,
t
RWH
t
RWS
t
CS
or RD
Chip-Select Hold Time t
Read-Data Hold Time t
Write-Data Hold Time t
Muxed Address Valid Time to AS/ALE
Fall
Muxed Address Hold Time t
Delay Time DS/E to AS/ALE Rise t
CH
DHR
DHW
t
ASL
AHL
ASD
Pulse Width AS/ALE High PW
Delay Time, AS/ALE to DS/E Rise t
Output Data Delay Time From DS/E or
RD
Data Setup Time t
Reset Pulse Width t
IRQ Release from DS
IRQ Release from RESET
ASED
t
DDR
DSW
RWL
t
IRDS
t
IRR
EL
EH
F
ASH
385 DC ns
150 ns
125 ns
10 ns
50 ns
20 ns
0ns
10 80 ns
0ns
30 ns
10 ns
20 ns
60 ns
40 ns
20 120 ns 6
100 ns
5
= 4.5V to 5.5V, 0°C to +70°C)
CC
30 ns
ms
2
2
ms
ms
NOTES:
1) All voltages are referenced to ground.
2) All outputs are open.
3) The MOT pin has an internal pulldown of 20kW.
4) Applies to the AD0–AD7 pins, the IRQ pin, and the SQW pin when each is in the high-impedance
state.
5) The IRQ pin is open drain.
6) Measured with a load as shown in Figure 4.
7) RTC modules can be successfully processed through conventional wave-soldering techniques as long
as temperature exposure to the lithium energy source contained within does not exceed +85°C.
However, post-solder cleaning with water-washing techniques is acceptable, provided that ultrasonic
vibration is not used to prevent damage to the crystal.
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Figure 4. OUTPUT LOAD
Figure 5. BUS TIMING FOR MOTOROLA INTERFACE
DS12887
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Figure 6. BUS TIMING FOR INTEL INTERFACE WRITE CYCLE
DS12887
Figure 7. BUS TIMING FOR INTEL INTERFACE READ CYCLE
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Figure 8. IRQ RELEASE DELAY TIMING
DS12887
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Figure 9. POWER-UP/DOWN TIMING
DS12887
POWER-UP/DOWN TIMING
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
CS at V
before Power-Down
IH
VCC Slew from 4.5V to 0V ( CS at VIH)
VCC Slew from 0V to 4.5V ( CS at VIH)
CS at V
after Power-Up
IH
t
t
t
REC
PD
t
F
R
0
300
100
20 200 ms
ms
ms
ms
(T
= +25°C)
A
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Expected Data Retention t
DR
10 years
Note: The RTC keeps time to an accuracy of ±1 minute per month during data retention time for the
period of t
DR
.
Warning: Under no circumstances are negative undershoots, of any amplitude, allowed when device is
in battery-backup mode.
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