Dallas DS17885E-5 3v/5v real-time clock Datasheet

DS17885/DS17887
3V/5V Real-Time Clock
www.maxim-ic.com
FEATURES
PIN ASSIGNMENT
Incorporates industry standard DS1287 PC clock
plus enhanced features:
§ Y2K compliant
§ +3V or +5V operation
§ SMI recovery stack
§ 64-bit silicon serial number
§ Power-control circuitry supports system
power-on from date/time alarm or key
closure
§ 32kHz output on power-up
§ Crystal select bit allows RTC to operate with
6pF or 12.5pF crystal
§ 114 bytes user NV RAM
§ Auxiliary battery input
§ 8kB additional NV RAM
§ RAM clear input
§ Century register
§ Date alarm register
§ Compatible with existing BIOS for original
DS1287 functions
§ Available as chip (DS17885) or standalone
module with embedded battery and crystal
(DS17887)
§ Timekeeping algorithm includes leap-year
compensation valid up to 2100
§ Underwriters Laboratory (UL) recognized
TYPICAL OPERATING CIRCUIT
1
2
3
4
5
6
7
8
9
10
11
12
PWR
X1
X2
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
GND
VCC
SQW
VBAUX
24
23
22
21
20
19
18
17
16
15
14
13
RCLR
VBAT
IRQ
KS
RD
GND
WR
ALE
CS
DS17885 24-Pin DIP
DS17885S 24-Pin SO
PWR
NC
NC
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
GND
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
20
19
18
17
16
15
14
13
VCC
SQW
VBAUX
RCLR
NC
IRQ
KS
RD
NC
WR
ALE
CS
DS17887 24-Pin
Encapsulated Package
IRQ
VBAT
RCLR
VBAUX
SQW
VCC
VCC
PWR
X1
X2
NC
ADO
AD1
AD2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
KS
RD
GND
WR
ALE
CS
GND
GND
AD7
AD6
NC
AD5
AD4
AD3
DS17885E 28-Pin TSOP
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050802
DS17885/DS17887
ORDERING INFORMATION
PART #
DESCRIPTION
DS17885XX-X RTC Chip
3
5
+3V operating range
+5V operating range
blank commercial temp range
N
industrial temp range
blank 24-pin DIP
E
28- pin TSOP
S
24- pin SO
DS17887X-X RTC Module; 24-pin DIP
3
5
+3V operating rang
+5V operating range
blank commercial temp range
N
industrial
PIN DESCRIPTION
X1
X2
RCLR
AD0–AD7 PWR
KS
CS
ALE
WR
RD
IRQ
SQW
VCC
GND
VBAT
VBAUX NC
-
Crystal Input
Crystal Output
RAM Clear Input
Multiplexed Address/Data Bus
Power-On Interrupt Output (Open Drain)
Kickstart Input
RTC Chip-Select Input
RTC Address Strobe
RTC Write Data Strobe
RTC Read Data Strobe
Interrupt Request Output (Open Drain)
Square-Wave Output
+3V or +5V Main Supply
Ground
Battery + Supply
Auxiliary Battery Supply
No Connect
DESCRIPTION
The DS17885/DS17887 are real-time clocks (RTCs) designed as successors to the industry standard
DS1285, DS1385, DS1485, DS1585, and DS1685 PC real-time clocks. These devices provide the
industry standard DS1285 clock function with either +3V or +5V operation. The DS17885 also
incorporates a number of enhanced features including a silicon serial number, power-on/off control
circuitry, 114 bytes of user NV SRAM plus 8kB of additional NV RAM, and 32.768kHz output for
sustaining power management activities.
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DS17885/DS17887
The DS17885/DS17887 power-control circuitry allows the system to be powered on by an external
stimulus such as a keyboard or by a time-and-date (wake-up) alarm. The PWR output pin is triggered by
one or either of these events, and is used to turn on an external power supply. The PWR pin is under
software control, so that when a task is complete, the system power can then be shut down.
The DS17885 is a clock/calendar chip with the features described above. An external crystal and battery
are the only components required to maintain time-of-day and memory status in the absence of power.
The DS17887 incorporates the DS17885 chip, a 32.768kHz crystal, and a lithium battery in a complete,
self-contained timekeeping module. The entire unit is fully tested at Dallas Semiconductor such that a
minimum of 10 years of timekeeping and data retention in the absence of VCC is guaranteed.
OPERATION
The block diagram in Figure 1 shows the pin connections with the major internal functions of the
DS17885/DS17887. The following paragraphs describe the function of each pin.
SIGNAL DESCRIPTIONS
GND, VCC – DC power is provided to the device on these pins. VCC is the +3V or +5V input.
SQW – Square-Wave Output. The SQW pin provides a 32kHz square-wave output, tREC, after a powerup condition has been detected. This condition sets the following bits, enabling the 32kHz output;
DV1 = 1, and E32k = 1. A square wave is output on this pin if either SQWE = 1 or E32k = 1. If E32k = 1,
then 32kHz is output regardless of the other control bits. If E32k = 0, then the output frequency is
dependent on the control bits in register A. 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 2. The SQW signal can be turned on and off using the
SQWE bit in register B or the E32k bit in extended register 4Bh. A 32kHz SQW signal is output when the
enable 32kHz (E32k) bit in extended register 4Bh is a logic 1 and VCC is above VPF. A 32kHz square
wave is also available when VCC is less than VPF if E32k = 1, ABE = 1, and voltage is applied to the
VBAUX pin.
AD0 to AD7 – Multiplexed Bidirectional Address/Data Bus. Multiplexed buses save pins because
address information time 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 DS17885
since the bus change from address to data occurs during the internal RAM access time. Addresses must
be valid prior to the latter portion of ALE, at which time the DS17885/DS17887 latches the address.
Valid write data must be present and held stable during the latter portion of the WR pulse. In a read cycle
the DS17885/DS17887 outputs 8 bits of data during the latter portion of the RD pulse. The read cycle is
terminated and the bus returns to a high impedance state as RD transitions high. The address/data bus
also serves as a bidirectional data path for the external extended RAM.
ALE – RTC Address Strobe Input; Active High. A pulse on the address strobe pin serves to
demultiplex the bus. The falling edge of ALE causes the RTC address to be latched within the
DS17885/DS17887.
–RTC Read Input; Active Low. RD identifies the time period when the DS17885/DS17887 drives
the bus with RTC read data. The RD signal is an enable signal for the output buffers of the clock.
RD
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DS17885/DS17887
– RTC Write Input; Active Low. The WR signal is an active low signal. The WR signal defines
the time period during which data is written to the addressed register.
WR
– RTC Chip-Select Input; Active Low. The chip select signal must be asserted low during a bus
cycle for DS17885/DS17887 to be accessed. CS must be kept in the active state during RD and WR
timing. Bus cycles that take place with ALE asserted but without asserting CS latches addresses.
However, no data transfer occurs.
CS
– Interrupt Request Output; Open Drain, Active Low. The IRQ pin is an active low output of
the DS17885/DS17887 that can be tied to the interrupt input of 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 application software must clear all enabled flag bits contributing to IRQ ’s active
state.
IRQ
When no interrupt conditions are present, the IRQ level is in the high-impedance state. Multiple
interrupting devices can be connected to an IRQ bus. The IRQ pin is an open-drain output and requires an
external pullup resistor. The voltage on the pullup supply should be no greater than VCC + 0.2V.
PWR – Power-On Output; Open-Drain, Active Low. The PWR pin is intended for use as an on/off
control for the system power. With VCC voltage removed from the DS17885/DS17887, PWR can be
automatically activated from a kickstart input by the KS pin or from a wake-up interrupt. Once the
system is powered on, the state of PWR can be controlled by bits in the Dallas registers. The PWR pin
can be connected through a pullup resistor to a positive supply. For 5V operation, the voltage of the
pullup supply should be no greater than 5.7V. For 3V operation, the voltage on the pullup supply should
be no greater than 3.9V.
KS – Kickstart Input; Active Low. When VCC is removed from the DS17885/DS17887, the system can
be powered on in response to an active low transition on the KS pin, as might be generated from a key
closure. VBAUX must be present and auxiliary-battery-enable bit (ABE) must be set to 1 if the kickstart
function is used, and the KS pin must be pulled up to the VBAUX supply. While VCC is applied, the KS pin
can be used as an interrupt input.
RCLR – RAM Clear Input; Active Low. If enabled by software, taking RCLR low results in the
clearing of the 114 bytes of user RAM. When enabled, RCLR can be activated whether or not VCC is
present. RCLR has an internal pullup and should not be connected to an external pullup resistor.
VBAUX – Auxiliary battery input required for kickstart and wake-up features. This input also supports
clock/calendar and user RAM if VBAT is at lower voltage or is not present. A standard +3V lithium cell or
other energy source can be used. For 3V operation, VBAUX must be held between +2.5V and +3.7V. For
5V operation, VBAUX must be held between +2.5V and +5.2V. If VBAUX is not going to be used it should
be grounded and the auxiliary-battery-enable bit bank 1, register 4BH, should = 0.
UL recognized to ensure against reverse charging current when used with a lithium battery. See
“Conditions of Acceptability” at www.maxim-ic.com/TechSupport/QA/ntrl.htm.
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DS17885/DS17887
Figure 1. BLOCK DIAGRAM
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DS17885/DS17887
DS17885 ONLY
X1, X2 – Connections for a standard 32.768kHz quartz crystal. For greatest accuracy, the DS17885 must
be used with a crystal that has a specified load capacitance of either 6pF or 12.5pF. The crystal select
(CS) bit in extended-control register 4B is used to select operation with a 6pF or 12.5pF crystal. The
crystal is attached 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 guardringed 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.” The DS17885 can also be driven by an
external 32.768kHz oscillator. In this configuration, the X1 pin is connected to the external oscillator
signal and the X2 pin is floated.
VBAT – Battery input for any standard 3V lithium cell or other energy source. Battery voltage must be
held between 2.5V and 3.7V for proper operation. UL recognized to ensure against reverse charging
current when used in conjunction with a lithium battery. See “Conditions of Acceptability” at
www.maxim-ic.com/TechSupport/AQ/ntrl.htm.
POWER-DOWN/POWER-UP 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 VCC is applied to the
DS17885/DS17887 and reaches a level of greater than VPF (power-fail trip point), the device becomes
accessible after tREC, 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.
The DS17885/DS17887 is available in either a 3V or a 5V device.
The 5V device is fully accessible and data can be written and read only when VCC is greater than 4.5V.
When VCC is below 4.5V, read and writes are inhibited. However, the timekeeping function continues
unaffected by the lower input voltage. As VCC falls below the greater of VBAT and VBAUX, the RAM and
timekeeper are switched over to a lithium battery connected either to the VBAT pin or VBAUX pin.
The 3V device is fully accessible and data can be written or read only when VCC is greater than 2.7V.
When VCC falls below VPF, access to the device is inhibited. If VPF is less than VBAT and VBAUX, the
power supply is switched from VCC to the backup supply (the greater of VBAT and VBAUX) when VCC drops
below VPF. If VPF is greater than VBAT and VBAUX, the power supply is switched from VCC to the backup
supply when VCC drops below the larger of VBAT and VBAUX.
When VCC falls below VPF, the chip is write-protected. With the possible exception of the KS , PWR ,
RCLR , and SQW pins, all inputs are ignored and all outputs are in a high-impedance state.
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DS17885/DS17887
RTC ADDRESS MAP
The address map for the RTC registers of the DS17885/DS17887 is shown in Figure 2. The address map
consists of the 14 clock/calendar registers. Ten registers contain the time, calendar, and alarm data, and
4 bytes are used for control and status. All registers 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 second byte is read-only.
Figure 2. DS17885 REAL-TIME CLOCK ADDRESS MAP
TIME, CALENDAR, AND ALARM LOCATIONS
The time and calendar information is obtained by reading the appropriate register bytes shown in Table 1.
The time, calendar, and alarm are set or initialized by writing the appropriate register bytes. The contents
of the time, calendar, and alarm registers can be either binary or binary-coded decimal (BCD) format.
Table 1 shows the binary and BCD formats of the 10 time, calendar, and alarm locations that reside in
both bank 0 and in bank 1, plus the two extended registers that reside in bank 1 only (bank 0 and
bank 1 switching is explained later in this text).
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. Also at this time, the data
format (binary or BCD) should be set by the data mode bit (DM) of Register B. All time, calendar, and
alarm registers 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 real-time clock to update the time and calendar bytes.
Once initialized, the real time clock makes all updates in the selected mode. The data mode cannot be
changed without reinitializing the 10 data bytes. 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. Once per second the 10 bytes are advanced 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., may 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.
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DS17885/DS17887
The three time 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 time 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 time alarm bytes create an interrupt every second. The three time-alarm bytes can be used with the
date alarm as described in Wake-Up/Kickstart. The century counter is discussed later in this text.
Table 1. TIME, CALENDAR, AND ALARM DATA MODES
ADDRESS
LOCATION
FUNCTION
RANGE
DECIMAL
RANGE
BINARY DATA MODE
BCD DATA MODE
OOH
Seconds
0 to 59
00 to B
00 to 59
01H
Seconds Alarm
0 to 59
00 to 3B
00 to 59
02H
Minutes
0 to 59
00 to 3B
00 to 59
03H
Minutes Alarm
0 to 59
00 to 3B
00 to 59
Hours 12-hour Mode
1 to 12
01 to 0C AM, 81 to 8C PM
01 to 12 AM, 81 to 92 PM
Hours 24-hour Mode
0 to 23
00 to 17
00 to 23
Hours Alarm 12-hour Mode
1 to 12
01 to 0C AM, 81 to 8C PM
01 to 12 AM, 81 to 92 PM
Hours Alarm 24-hour Mode
0 to 23
00 to 17
00 to 23
06H
Days of the Week Sunday = 1
1 to 7
01 to 07
01 to 07
07H
Date of Month
1 to 31
01 to 1F
01 to 31
08H
Month
1 to 12
01 to 0C
01 to 12
09H
Year
0 to 99
00 to 63
00 to 99
BANK 1, 48H
Century
0 to 99
00 to 63
00 to 99
BANK 1, 49H
Date Alarm
1 to 31
01 to 1F
01 to 31
04H
05H
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DS17885/DS17887
CONTROL REGISTERS
The four control registers; A, B, C, and D reside in both bank 0 and bank 1. These registers are accessible
at all times, even during the update cycle.
REGISTER A
MSB
BIT 7
UIP
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
DV2
DV1
DV0
RS3
RS2
RS1
LSB
BIT 0
RS0
UIP – Update-in-Progress. The 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 244µs.
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. Writing the SET bit in Register B to a 1 inhibits any update transfer and clears the
UIP status bit.
DV2, DV1, DV0 – These bits are defined as follows:
DV2 = Countdown Chain
1 – Resets countdown chain only if DV1 = 1
0 – Countdown chain enabled
DV1
= Oscillator Enable
0 – Oscillator off
1 – Oscillator on, VCC power-up state
DV0
= Bank Select
0 – Original bank
1 – Extended registers
A pattern of 01x is the only combination of bits that turns the oscillator on and allows 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 01x is written to DV2, DV1, and DV0.
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:
§
Enable the interrupt with the PIE bit;
§
Enable the SQW output pin with the SQWE or E32k bits;
§
Enable both at the same time and the same rate; or
§
Enable neither.
Table 2 lists the periodic interrupt rates and the square-wave frequencies that can be chosen with the RS
bits.
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DS17885/DS17887
REGISTER B
MSB
BIT 7
SET
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
LSB
BIT 0
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 internal functions of the
DS17885/DS17887.
PIE – Periodic Interrupt Enable. The PIE bit is a read/write bit, which 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 PF bit is still set at the
periodic rate. PIE is not modified by any internal DS17885/DS17887 functions.
AIE – Alarm Interrupt Enable. The AIE bit is a read/write bit which, 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 internal functions of the
DS17885/DS17887 do not affect the AIE bit.
UIE – Update-Ended Interrupt Enable. The UIE bit is a read/write bit that enables the update-end flag
(UF) bit in Register C to assert IRQ . The SET bit going high clears the UIE bit.
SQWE – Square-Wave Enable. When the SQWE bit is set to a 1 and E32k = 0, a square-wave signal at
the frequency set by the rate-selection bits RS3 through RS0 is driven out on the SQW pin. When the
SQWE bit is set to 0and E32k = 0, the SQW pin is held low. SQWE is a read/write bit. SQWE is set to a
1 when VCC is powered up.
DM – Data Mode. The 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. A 1 in DM signifies binary data while a 0 in DM specifies BCD data.
24/12 – 24/12-Control Bit. This 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.
DSE – Daylight Savings Enable. The 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.
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DS17885/DS17887
REGISTER C
MSB
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
PF
AF
UF
0
0
0
IRQF
LSB
BIT 0
0
IRQF – Interrupt Request Flag. This bit is set to a 1 when one or more of the following are true:
PF = PIE = 1
AF = AIE = 1
UF = UIE = 1
WF = WIE= 1
KF = KSE= 1
RF = RIE = 1
i.e., IRQF = (PF x PIE) + (AF x AIE) + (UF x UIE) + (WF x WIE) + (KF x KSE) + (RF x RIE)
Any time the IRQF bit is a 1, the IRQ pin is driven low. Flag bits PF, AF, and UF are cleared after
Register C is read by the program.
PF – Periodic Interrupt Flag. This 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 software read of Register C.
AF – Alarm Interrupt Flag. A 1 in the 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 read of Register
C clears AF.
UF – Update Ended Interrupt Flag. This bit is set after each update cycle. When the UIE bit is set to 1,
the 1 in UF causes the IRQF bit to be a 1, which asserts the IRQ pin. UF is cleared by reading Register C.
BIT 3 to BIT 0 – These are unused bits of the status Register C. These bits always read 0 and cannot be
written.
REGISTER D
MSB
BIT 7
VRT
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
0
0
0
0
0
0
LSB
BIT 0
0
VRT – Valid RAM and Time. This bit is a read-only status bit. When VRT = 0, the RTC and RAM data
are questionable and indicates that the lithium energy source has been exhausted and should be replaced.
This bit indicates that status of the VBAT and VBAUX inputs.
BIT 6 to 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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DS17885/DS17887
NV RAM–RTC
The general-purpose NV RAM bytes are not dedicated to any special function within the
DS17885/DS17887. They can be used by the application program as nonvolatile memory and are fully
available during the update cycle.
The user RAM is divided into two separate memory banks. When the bank 0 is selected, the 14 real-time
clock registers and 114 bytes of user RAM are accessible. When bank 1 is selected, an additional 4kB of
user RAM are accessible through the extended RAM address and data registers.
INTERRUPT CONTROL
The DS17885/DS17887 includes six separate, fully automatic sources of interrupt for a processor:
1)
2)
3)
4)
5)
6)
Alarm Interrupt
Periodic Interrupt
Update-Ended Interrupt
Wake-Up Interrupt
Kickstart Interrupt
RAM Clear Interrupt
The conditions that generate each of these independent interrupt conditions are described in greater detail
elsewhere in this data sheet. This section describes the overall control of the interrupts.
The application software can select which interrupts, if any, should be used. There are 6 bits including 3
bits in Register B and 3 bits in Extended Register 4B that enable the interrupts. The extended register
locations are described later. Writing a logic 1 to an interrupt-enable bit permits that interrupt to be
initiated when the event occurs. A logic 0 in the 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 be set at an active level, even though the event initiating the interrupt condition may have
occurred much earlier. As a result, there are cases where the software should clear these earlier generated
interrupts before first enabling new interrupts.
When an interrupt event occurs, the relating flag bit is set to a logic 1 in Register C or in Extended
Register 4A. These flag bits are set regardless of the setting of the corresponding enable bit located either
in Register B or in Extended Register 4B. The flag bits can be used in a polling mode without enabling
the corresponding enable bits.
However, care should be taken when using the flag bits of Register C because they are automatically
cleared to 0 immediately after they are read. Double latching is implemented on these bits so that bits that
are set remain stable throughout the read cycle. All bits that were set are cleared when read and new
interrupts that are pending during the read cycle are held until after the cycle is completed. 1 bit, 2 bits, or
3 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 flag bits in Extended Register 4A are not automatically cleared following a read. Instead, each flag
bit can be cleared to 0 only by writing 0 to that bit.
When using the flag bits with fully enabled interrupts, the IRQ line is driven low when an interrupt flag
bit is set and its corresponding enable bit is also set. IRQ is held low as long as at least one of the six
12 of 38
DS17885/DS17887
possible 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 as a result of one of the six possible active sources. Therefore,
determination that the DS17885/DS17887 initiated an interrupt is accomplished by reading Register C
and finding IRQF = 1. IRQF remains set until all enabled interrupt flag bits are cleared to 0.
OSCILLATOR CONTROL BITS
When the DS17887 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 01X in bits 4 through
6 of Register A turns the oscillator on and enable the countdown chain. Not that this is different than the
dS1287, which required a pattern of 010 in these bits. DV0 is now a “don’t care” because it is used for
selection between register banks 0 and 1. 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
The SQW pin can be programmed to output a variety of frequencies divided down from the 32.768kHz
crystal tied to X1 and X2. The square-wave output is enabled and disabled through the SQWE bit in
Register B or the E32k bit in extended register 4Bh. If the square wave is enabled (SQWE = 1 or
E32k = 1), then the output frequency is determined by the settings of the E32k bit in Extended Register
4Bh and by the RS3–0 bits in Register A. If E32k = 1, then a 32.768kHz square wave is output on the
SQW pin regardless of the settings of RS3–0 and SQWE.
If E32k = 0, then the square-wave output frequency is determined by the RS3–0 bits. These bits control a
1-of-15 decoder that selects one of 13 taps that divide the 32.768kHz frequency. The RS3–0 bits establish
the SQW output frequency as shown in Table 2. In addition, RS3–0 bits control the periodic interrupt
selection as described below.
If E32k = 1, and the auxiliary-battery-enable bit (ABE, bank 1; register 04BH) is enabled, and voltage is
applied to VBAUX, then the 32kHz square-wave output signal is output on the SQW pin in the absence of
VCC. This facility is provided to clock external power-management circuitry. If any of the above
requirements are not met, no square-wave output signal is generated on the SQW pin in the absence of
VCC.
PERIODIC INTERRUPT SELECTION
The periodic interrupt causes 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 RS3–0 bits in Register A, which select
the square-wave frequency (Table 2). Changing the 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 and
E32k bits control the square-wave output. Similarly, the PIE bit in Register B enables the periodic
interrupt. The periodic interrupt can be used with software counters to measure inputs, create output
intervals, or await the next needed software function.
13 of 38
DS17885/DS17887
Table 2. PERIODIC INTERRUPT RATE AND SQUARE-WAVE OUTPUT
FREQUENCY
EXTENDED
REGISTER B
SELECT BITS REGISTER A
tPI PERIODIC
INTERRUPT RATE
SQW OUTPUT
FREQUENCY
E32k
RS3
RS2
RS1
RS0
0
0
0
0
0
None
None
0
0
0
0
1
3.90625ms
256Hz
0
0
0
1
0
7.8125ms
128Hz
0
0
0
1
1
122.070µs
8.192kHz
0
0
1
0
0
244.141µs
4.096kHz
0
0
1
0
1
488.281µs
2.048kHz
0
0
1
1
0
976.5625µs
1.024kHz
0
0
1
1
1
1.953125ms
512Hz
0
1
0
0
0
3.90625ms
256Hz
0
1
0
0
1
7.8125ms
128Hz
0
1
0
1
0
15.625ms
64Hz
0
1
0
1
1
31.25ms
32Hz
0
1
1
0
0
62.5ms
16Hz
0
1
1
0
1
125ms
8Hz
0
1
1
1
0
250ms
4Hz
0
1
1
1
1
500ms
2Hz
1
X
X
X
X
*
32.768kHz
*RS3 to RS0 determine periodic interrupt rates as listed for E32k = 0.
14 of 38
DS17885/DS17887
UPDATE CYCLE
The serialized RTC 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, alarm,
and elapsed time byte is frozen and does not update as the time increments. However, the time countdown
chain continues to update the internal copy of the buffer. This feature allows the 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 alarm
locations.
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 is 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
244µs later. If a low is read on the UIP bit, the user has at least 244µs before the time/calendar data is
changed. Therefore, the user should avoid interrupt service routines that would cause the time needed to
read valid time/calendar data to exceed 244µs.
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 tBUC allow valid time and date information to be reached at each occurrence
of the periodic interrupt. The reads should be complete within (tPI / 2 + tBUC) to ensure that data is not
read during the update cycle.
Figure 3. UPDATE-ENDED AND PERIODIC INTERRUPT RELATIONSHIP
15 of 38
DS17885/DS17887
EXTENDED FUNCTIONS
The extended functions provided by the DS17885/DS17887 that are new to the RAMified RTC family
are accessed by a software-controlled bank-switching scheme, as illustrated in Figure 4. In bank 0, the
clock/calendar registers and 50 bytes of user RAM are in the same locations as for the DS1287. As a
result, existing routines implemented within BIOS, DOS, or application software packages can gain
access to the DS17885/DS17887 clock registers with no changes. Also in bank 0, an extra 64 bytes of
RAM are provided at addresses just above the original locations for a total of 114 directly addressable
bytes of user RAM.
When bank 1 is selected, the clock/calendar registers and the original 50 bytes of user RAM still appear
as bank 0. However, the Dallas registers that provide control and status for the extended functions are
accessed in place of the additional 64 bytes of user RAM. The major extended functions controlled by the
Dallas registers are listed below:
§
§
§
§
§
§
§
§
§
64-bit Silicon Serial Number
Century Counter
RTC Write Counter
Date Alarm
Auxiliary Battery Control/Status
Wake-Up
Kickstart
RAM Clear Control/Status
4kB Extended RAM Access
The bank selection is controlled by the state of the DV0 bit in register A. To access bank 0 the DV0 bit
should be written to a 0. To access bank 1, DV0 should be written to a 1. Register locations designated as
reserved in the bank 1 map are reserved for future use by Dallas Semiconductor. Bits in these locations
cannot be written and return a 0 if read.
Silicon Serial Number
A unique 64-bit lasered serial number is located in bank 1, registers 40h–47h. This serial number is
divided into three parts. The first byte in register 40h contains a model number to identify the device type
of the DS17885/DS17887. Registers 41h–46h contain a unique binary number. Register 47h contains a
CRC byte used to validate the data in registers 40h–46h. All 8 bytes of the serial number are read-only
registers.
The DS17885/DS17887 is manufactured such that no two devices contain an identical number in
locations 41h–47h.
Century Counter
A register has been added in bank 1, location 48H, to keep track of centuries. The value is read in either
binary or BCD according to the setting of the DM bit.
RTC Write Counter
An 8-bit counter located in extended register bank 1, 5Eh, counts the number of times the RTC is written
to. This counter is incremented on the rising edge of the WR signal every time that the CS signal
qualifies it. This counter is a read-only register and rolls over after 256 RTC write pulses. This counter
16 of 38
DS17885/DS17887
can be used to determine if and how many RTC writes have occurred since the last time this register was
read.
Auxiliary Battery
The VBAUX input is provided to supply power from an auxiliary battery for the DS17885/DS17887
kickstart, wake-up, and SQW output features in the absence of VCC. This power source must be available
in order to use these auxiliary features when no VCC is applied to the device.
The auxiliary-battery-enable (ABE; bank 1, register 04BH) bit in extended control register 4B is used to
turn the auxiliary battery on and off for the above functions in the absence of VCC. When set to a 1, VBAUX
battery power is enabled; when cleared to 0, VBAUX battery power is disabled to these functions.
In the DS17885/DS17887, this auxiliary battery can be used as the primary backup power source for
maintaining the clock/calendar, user RAM, and extended external RAM functions. This occurs if the
VBAT pin is at a lower voltage than VBAUX. If the DS17885 is to be backed-up using a single battery with
the auxiliary features enabled, then VBAUX should be used and VBAT should be grounded. If VBAUX is not
to be used, it should be grounded and ABE should be cleared to 0.
Wake-Up/Kickstart
The DS17885/DS17887 incorporates a wake-up feature that powers on the system at a predetermined
date and time through activation of the PWR output pin. In addition, the kickstart feature allows the
system to be powered up in response to a low-going transition on the KS pin, without operating voltage
applied to the VCC pin. As a result, system power can be applied upon such events as a key closure or
modem-ring-detect signal. In order to use either the wake-up or the kickstart features, the
DS17885/DS17887 must have an auxiliary battery connected to the VBAUX pin, the oscillator must be
running, and the countdown chain must not be in reset (Register A DV2, DV1, DV0 = 01X). If DV2,
DV1, and DV0 are not in this required state, the PWR pin is not driven low in response to a kickstart or
wake-up condition, while in battery-backed mode.
The wake-up feature is controlled through the wake-up-interrupt-enable bit in extended control register
4B (WIE, bank 1, 04BH). Setting WIE to 1 enables the wake-up feature, clearing WIE to 0 disables it.
Similarly, the kickstart feature is controlled by the kickstart-interrupt-enable bit in extended control
register 4B (KSE, bank 1, 04BH).
A wake-up sequence occurs as follows: When wake-up is enabled through WIE = 1 while the system is
powered down (no VCC voltage), the clock/calendar monitors the current date for a match condition with
the date alarm register (bank 1, register 049H). With the date alarm register, the hours, minutes, and
seconds alarm bytes in the clock/calendar register map (bank 0, registers 05H, 03H, and 01H) are also
monitored. As a result, a wake-up occurs at the date and time specified by the date, hours, minutes, and
seconds alarm register values. This additional alarm occurs regardless of the programming of the AIE bit
(bank 0, register B, 0BH). When the match condition occurs, the PWR pin is automatically driven low.
This output can be used to turn on the main system power supply that provides VCC voltage to the
DS17885/DS17887 as well as the other major components in the system. Also at this time, the wake-up
flag (WF, bank 1, register 04AH) is set, indicating that a wake-up condition has occurred.
17 of 38
DS17885/DS17887
A kickstart sequence occurs when kickstarting is enabled through KSE = 1. While the system is powered
down, the KS input pin is monitored for a low-going transition of minimum pulse width tKSPW. When
such a transition is detected, the PWR line is pulled low, as it is for a wake-up condition. Also at this
time, the kickstart Flag (KF, bank 1, register 04AH) is set, indicating that a kickstart condition has
occurred.
The timing associated with both the wake-up and kickstarting sequences is illustrated in the
Wake-Up/Kickstart Timing Diagram in the Electrical Specifications section of this data sheet. The timing
associated with these functions is divided into five intervals, labeled 1 to 5 on the diagram.
The occurrence of either a kickstart or wake-up condition causes the PWR pin to be driven low, as
described above. During interval 1, if the supply voltage on the DS17885/DS17887 VCC pin rises above
the greater of VBAT or VPF before the power-on timeout period (tPOTO) expires, then PWR remains at the
active low level. If VCC does not rise above the greater of VBAT or VPF in this time, then the PWR output
pin is turned off and returns to its high-impedance level. In this event, the IRQ pin also remains tri-stated.
The interrupt flag bit (either WF or KF) associated with the attempted power-on sequence remains set
until cleared by software during a subsequent system power-on.
If VCC is applied within the timeout period, then the system power-on sequence continue as shown in
intervals 2 to 5 in the timing diagram. During interval 2, PWR remains active and IRQ is driven to its
active low level, indicating that either WF or KF was set in initiating the power-on. In the diagram KS is
assumed to be pulled up to the VBAUX supply. Also at this time, the PAB bit is automatically cleared to 0
in response to a successful power-on. The PWR line remains active as long as the PAB remains cleared to
0.
At the beginning of interval 3, the system processor has begun code execution and clears the interrupt
condition of WF and/or KF by writing 0’s to both of these control bits. As long as no other interrupt
within the DS17885/DS17887 is pending, the IRQ line is taken inactive once these bits are reset.
Execution of the application software can proceed. During this time, both the wake-up and kickstart
functions can be used to generate status and interrupts. WF is set in response to a date, hours, minutes,
and seconds match condition. KF is set in response to a low-going transition on KS . If the associated
interrupt-enable bit is set (WIE and/or KSE), then the IRQ line is driven active low in response to enabled
event. In addition, the other possible interrupt sources within the DS17885/DS17887 can cause IRQ to be
driven low. While system power is applied, the on-chip logic always attempts to drive the PWR pin active
in response to the enabled kickstart or wake-up condition. This is true even if PWR was previously
inactive as the result of power being applied by some means other than wake-up or kickstart.
The system can be powered down under software control by setting the PAB bit to a logic 1. This causes
the open-drain PWR pin to be placed in a high-impedance state, as shown at the beginning of interval 4 in
the timing diagram. As VCC voltage decays, the IRQ output pin is placed in a high-impedance state when
VCC goes below VPF. If the system is to be again powered on in response to a wake-up or kickstart, then
the both the WF and KF flags should be cleared and WIE and/or KSE should be enabled prior to setting
the PAB bit.
18 of 38
DS17885/DS17887
During interval 5, the system is fully powered down. Battery backup of the clock calendar and NV RAM
is in effect and IRQ is tri-stated, and monitoring of wake-up and kickstart takes place. If PRS = 1, PWR
stays active; otherwise, if PRS = 0 PWR is tri-stated.
RAM Clear
The DS17885/DS17887 provides a RAM clear function for the 114 bytes of user RAM. When enabled,
this function can be performed regardless of the condition of the VCC pin.
The RAM clear function is enabled or disabled through the RAM clear-enable bit (RCE; bank 1, register
04BH). When this bit is set to a logic 1, the 114 bytes of user RAM is cleared (all bits set to 1) when an
active low transition is sensed on the RCLR pin. This action has no affect on either the clock/calendar
settings or upon the contents of the extended RAM. The RAM clear flag (RF, bank 1, register 04AH) is
set when the RAM clear operation has been completed. If VCC is present at the time of the RAM clear and
RIE = 1, the IRQ line is also be driven low upon completion. The interrupt condition can be cleared by
writing a 0 to the RF bit. The IRQ line then returns to its inactive high level provided there are no other
pending interrupts. Once the RCLR pin is activated, all read/write accesses are locked out for a minimum
recover time, specified as tREC in Electrical Characteristics.
When RCE is cleared to 0, the RAM clear function is disabled. The state of the RCLR pin has no affect
on the contents of the user RAM, and transitions on the RCLR pin have no affect on RF.
4k x 8 Extended RAM
The DS17885/DS17887 provides 4k x 8 of on-chip SRAM that is controlled as nonvolatile storage
sustained from a lithium battery. On power-up, the RAM is taken out of write-protect status by the
internal power-OK signal (POK) generated from the write-protect circuitry. The on-chip 4k x 8 NV
SRAM is accessed through the eight multiplexed address/data lines AD7 to AD0. Access to the SRAM is
controlled by three on-chip latch registers. Two registers are used to hold the SRAM address, and the
other register is used to hold read/write data. The SRAM address space is from 00h to 0FFFh.
Access to the extended 4k x 8 RAM is controlled by three of the Dallas registers shown in Figure 4. The
Dallas registers in bank 1 must first be selected by setting the DV0 bit in register A to a logic 1. The 12bit address of the RAM location to be accessed must be loaded into the extended RAM address registers
located at 50h and 51h. The least significant address byte should be written to location 50h, and the most
significant 4-bits (right-justified) should be loaded in location 51h. Data in the addressed location can be
read by performing a read operation from location 53h, or written to by performing a write operation to
location 53h. Data in any addressed location can be read or written repeatedly without changing the
address in location 50h and 51h.
To read or write consecutive extended RAM locations, a burst mode feature can be enabled to increment
the extended RAM address. To enable the burst mode feature, set the BME bit in the extended control
register 4Ah, to a logic 1. With burst mode enabled, write the extended RAM starting address location to
registers 50h and 51h. Then read or write the extended RAM data from/to register 53h. The extended
RAM address locations are automatically incremented on the rising edge of RD or WR only when
register 53h is being accessed. Refer to the Burst Mode Timing Waveform.
19 of 38
DS17885/DS17887
Figure 4. DS17885/DS17887 EXTENDED REGISTER BANK DEFINITION
MSB
BANK 0
DV0 = 0
LSB
00
BANK 1
DV0 = 1
MSB
TIMEKEEPING AND CONTROL
0D
TIMEKEEPING AND CONTROL
0D
0E
0E
50 BYTES - USER RAM
50 BYTES - USER RAM
3F
3F
40
40
MODEL NUMBER BYTE
41
1ST BYTE SERIAL NUMBER
42
2ND BYTE SERIAL NUMBER
43
3RD BYTE SERIAL NUMBER
44
4TH BYTE SERIAL NUMBER
45
5TH BYTE SERIAL NUMBER
46
6TH BYTE SERIAL NUMBER
64 BYTES - USER RAM
47
CRC BYTE
48
CENTURY BYTE
49
DATE ALARM
4A
EXTENDED CONTROL REG 4A
4B
EXTENDED CONTROL REG 4B
4C
RESERVED
4D
RESERVED
4E
RTC ADDRESS - 2
4F
RTC ADDRESS - 3
50
EXTENDED RAM ADDR - LSB
EXTENDED
51
EXTENDED RAM ADDR - MSB
RAM
52
RESERVED
8k X 8
53
EXTENDED RAM DATA PORT
54
RESERVED
55
RESERVED
56
RESERVED
57
RESERVED
58
RESERVED
59
RESERVED
5A
RESERVED
5B
RESERVED
5C
RESERVED
5D
RESERVED
5E
RTC WRITE COUNTER
5F
RESERVED
7F
LSB
00
7F
20 of 38
DS17885/DS17887
EXTENDED CONTROL REGISTERS
Two extended control registers are provided to supply controls and status information for the extended
features offered by the DS17885/DS17887. These are designated as extended control registers 4A and 4B
and are located in register bank 1, locations 04AH and 04BH, respectively. The functions of the bits
within these registers are described as follows.
EXTENDED CONTROL REGISTER 4A
MSB
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
LSB
BIT 0
VRT2
INCR
BME
*
PAB
RF
WF
KF
VRT2 – Valid RAM and Time 2. This status bit gives the condition of the auxiliary batter. It is set to a
logic 1 condition when the external lithium battery is connected to the VBAUX. If this bit is read as a logivc
0, the external battery should be replaced.
INCR – Increment in Progress Status. This bit is set to a 1 when an increment to the time/date registers
is in progress and the alarm checks are being made. INCR is set to a 1 at 122µs before the update cycle
starts and is cleared to 0 at the end of each update cycle.
BME – Burst Mode Enable. The burst mode enable bit allows the extended user RAM address registers
to automatically increment for consecutive reads and writes. When BME is set to a logic 1, the automatic
incrementing is enabled and when BME is set to a logic 0, the automatic incrementing is disabled.
PAB – Power-Active Bar-Control. When this bit is 0, the PWR pin is in the active low state. When this
bit is 1, the PWR pin is in the high-impedance state. This bit can be written to a logic 1 or 0 by the user. If
either WF and WIE = 1 or KF and KSE = 1, the PAB bit is cleared to 0.
RF – Ram Clear Flag. This bit is set to a logic 1 when a high-to-low transition occurs on the RCLR
input if RCE = 1. The RF bit is cleared by writing it to a logic 0. This bit can also be written to a logic 1
to force an interrupt condition.
WF – Wake-Up Alarm Flag. This bit is set to 1 when a wake-up alarm condition occurs or when the
user writes it to a 1. WF is cleared by writing it to a 0.
KF – Kickstart Flag. This bit is set to a 1 when a kickstart condition occurs or when the user writes it to
a 1. This bit is cleared by writing it to a logic 0.
*Reserved bits. These bits are reserved for future use by Dallas Semiconductor. They can be read and
written, but have no affect on operation.
21 of 38
DS17885/DS17887
EXTENDED CONTROL REGISTER 4B
MSB
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
LSB
BIT 0
ABE
E32k
CS
RCE
PRS
RIE
WIE
KSE
ABE – Auxiliary Battery Enable. This bit when written to a logic 1 enables the VBAUX pin for extended
functions.
E32k – Enable 32.768kHz Output. This bit when written to a logic 1 enables the 32.768kHz oscillator
frequency to be output on the SQW pin. E32k is set to a 1 when VCC is powered up.
CS – Crystal Select. When CS is set to a 0, the oscillator is configured for operation with a crystal that
has a 6pF specified load capacitance. When CS = 1, the oscillator is configured for a 12.5pF crystal. CS is
disabled in the DS17887 module and should be set to CS = 0.
RCE – RAM Clear Enable. When set to a 1, this bit enables a low level on RCLR to clear all 114 bytes
of user RAM. When RCE = 0, RCLR and the RAM clear function are disabled.
PRS – PAB Reset Select. When set to a 0, the PWR pin is set high-z when the DS17885 goes into power
fail. When set to a 1, the PWR pin remains active upon entering power fail.
RIE – RAM Clear Interrupt Enable. When RIE is set to a 1, the IRQ pin is driven low when a RAM
clear function is completed.
WIE – Wake-up Alarm Interrupt Enable. When VCC voltage is absent and WIE is set to a 1, the PWR
pin is driven active low when a wake-up condition occurs, causing the WF bit to be set to 1. When VCC is
then applied, the IRQ pin is also driven low. If WIE is set while system power is applied, both IRQ and
PWR are driven low in response to WF being set to 1. When WIE is cleared to a 0, the WF bit has no
affect on the PWR or IRQ pins.
KSE – Kickstart Interrupt Enable. When VCC voltage is absent and KSE is set to a 1, the PWR pin is
driven active low when a kickstart condition occurs ( KS pulsed low), causing the KF bit to be set to 1.
When VCC is then applied, the IRQ pin is also driven low. If KSE is set to 1 while system power is
applied, both IRQ and PWR are driven low in response to KF being set to 1. When KSE is cleared to a 0,
the KF bit has no affect on the PWR or IRQ pins.
22 of 38
DS17885/DS17887
SYSTEM MAINTENANCE INTERRUPT (SMI) RECOVERY STACK
An SMI recovery register stack is located in the extended register bank, locations 4Eh and 4Fh. This
register stack, shown below, can be used by the BIOS to recover from an SMI occurring during an RTC
read or write.
The RTC address is latched on the falling edge of the ALE signal. Each time an RTC address is latched,
the register address stack is pushed. The stack is only four registers deep, holding the three previous RTC
addresses in addition to the current RTC address being accessed. The following waveform illustrates how
the BIOS could recover the RTC address when an SMI occurs.
1) The RTC address is latched.
2) An SMI is generated before an RTC read or write occurs.
3) RTC address 0Ah is latched and the address from 1 is pushed to the “RTC Address–1” stack location.
This step is necessary to change the bank select bit, DV0 = 1.
4) RTC address 4Eh is latched and the address from 1 is pushed to location 4Eh, “RTC Address–2”
while 0Ah is pushed to the “RTC Address–1” location. The data in this register, 4Eh, is the RTC
address lost due to the SMI.
23 of 38
DS17885/DS17887
ABSOLUTE MAXIMUM RATINGS*
Voltage Range on Any Pin Relative to Ground
Storage Temperature Range
Soldering Temperature Range
-0.3V to +7.0V
-40°C to +85°C
+260°C for 10 seconds (DIP) (Note 13)
See IPC/JEDEC Standard J-STD-020A for
Surface Mount Devices
* 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.
OPERATING RANGE
RANGE
Commercial
Industrial
TEMP RANGE (°C)
0 to +70
-40 to +85
RECOMMENDED DC OPERATING CONDITIONS
PARAMETER
VCC
3V ±10% or 5V ±10%
3V ±10% or 5V ±10%
Over the operating range
SYMBOL
MIN
TYP
MAX
UNITS
Power Supply Voltage 5V
Operation
VCC
4.5
5.0
5.5
V
Power Supply Voltage 3V
Operation
VCC
2.7
3.0
3.7
V
VIH
2.2
VCC + 0.3
V
Input Logic 1
5V ±10%
3V ±10%
Input Logic 0
2.0
VIL
-0.3
0.6
V
Battery Voltage
VBAT
2.5
3.7
V
Auxiliary Battery Voltage;
VCC = 5.0V
VBAUX
2.5
5.2
V
Auxiliary Battery Voltage;
VCC = 3.0V
VBAUX
2.5
3.7
V
24 of 38
NOTES
DS17885/DS17887
DC ELECTRICAL CHARACTERISTICS
PARAMETER
SYMBOL
Over the operating range (5V)
MIN
TYP
MAX
UNITS
NOTES
Average VCC Power Supply
Current
ICC1
25
50
mA
1, 2
CMOS Standby Current
( CS = VCC - 0.2V)
ICC2
1
3
mA
1, 2
Input Leakage Current
(Any Input)
IIL
-1
+1
mA
Output Leakage Current
IOL
-1
+1
mA
Output Logic 1 Voltage
(IOUT = -1.0mA)
VOH
2.4
Output Logic 0 Voltage
(IOUT = +2.1mA)
VOL
Power-Fail Trip Point
VPF
Battery-Switch Voltage
VSW
VBAT,
VBAUX
Battery Leakage OSC ON
IBAT1
0.50
Battery Leakage OSC OFF
IBAT2
0.050
I/O Leakage
PWR
IRQ
Output at 0.4V
Output at 0.4V
5
V
0.4
V
4.5
V
3
V
8
0.7
mA
11
0.4
mA
11
+1
mA
4
IOLPWR
10.0
mA
IOLIRQ
2.1
mA
ILO
4.25
-1
25 of 38
4.37
DS17885/DS17887
DC ELECTRICAL CHARACTERISTICS
PARAMETER
SYMBOL
Over the operating range (3V)
MIN
TYP
MAX
UNITS
NOTES
Average VCC Power-Supply
Current
ICC1
15
30
mA
1, 2
CMOS Standby Current
( CS = VCC - 0.2V)
ICC2
0.5
2
mA
1, 2
Input Leakage Current
(Any Input)
IIL
-1
+1
mA
Output Leakage Current
IOL
-1
+1
mA
Output Logic 1 Voltage
at -0.4mA
VOH
2.4
Output Logic 0 Voltage
at +0.8mA
VOL
Power-Fail Trip Point
VPF
Battery Leakage OSC ON
Battery Leakage OSC OFF
I/O Leakage
PWR
IRQ
Output at 0.4V
Output at 0.4V
5
V
0.4
V
2.6
2.7
V
3
IBAT1
0.50
0.7
mA
11
IBAT2
0.050
0.4
mA
11
+1
mA
4
IOLPWR
4
mA
IOLIRQ
0.8
mA
MAX
UNITS
45
kHz
kΩ
pF
ILO
2.5
-1
CRYSTAL SPECIFICATIONS*
PARAMETER
Nominal Frequency
Series Resistance
Load Capacitance
SYMBOL
MIN
FO
ESR
CL
TYP
32.768
6 or 12.5
NOTES
*The crystal, traces, and crystal input pins should be isolated from RF generating signals. Refer to Application
Note 58, “Crystal Considerations for Dallas Real-Time Clocks” for additional specification.
26 of 38
DS17885/DS17887
RTC AC TIMING CHARACTERISTICS
PARAMETER
Over the operating range (3V)
SYMBOL
MIN
tCYC
360
Pulse-Width, RD / WR Low
PWRWL
200
ns
Pulse-Width, RD / WR High
PWRWH
150
ns
Cycle Time
Input Rise and Fall
tR , t F
TYP
MAX
UNITS
DC
ns
30
ns
Chip-Select Setup Time
Before WR or RD
tCS
20
ns
Chip-Select Hold Time
tCH
0
ns
Read Data Hold Time
tDHR
10
Write Data Hold Time
tDHW
0
ns
Muxed Address Valid Time to
ALE Fall
tASL
40
ns
Muxed Address Hold Time to
ALE Fall
tAHL
10
ns
or WR High Setup to
ALE Rise
tASD
30
ns
PWASH
40
ns
tASED
30
ns
tDDR
20
tDSW
70
RD
Pulse-Width ALE High
ALE Low Setup to RD or WR
Fall
Output Data-Delay Time from
RD
Data Setup Time
IRQ
Release from RD
tIRD
90
200
AC TEST CONDITIONS
Output Load: 50pF
Input Pulse Levels: 0V to 3.0V
Timing Measurement Reference Levels
Input: 1.5V
Output: 1.5V
Input Pulse Rise and Fall Times: 5ns
27 of 38
ns
ns
ns
2
NOTES
ms
6
DS17885/DS17887
RTC AC TIMING CHARACTERISTICS
PARAMETER
Over the operating range (5V)
SYMBOL
MIN
tCYC
240
Pulse-Width, RD / WR Low
PWRWL
120
ns
Pulse-Width, RD / WR High
PWRWH
80
ns
Cycle Time
TYP
MAX
UNITS
DC
ns
Input Rise and Fall Time
tR, tF
Chip-Select Setup Time
before WR or RD
tCS
20
ns
Chip-Select Hold Time
tCH
0
ns
Read Data Hold Time
tDHR
10
Write Data Hold Time
tDHW
0
ns
Muxed Address Valid Time to
ALE Fall
tASL
20
ns
Muxed Address Hold Time to
ALE fall
tAHL
10
ns
or WR High Setup to
ALE Rise
tASD
25
ns
PWASH
40
ns
tASED
30
ns
tDDR
20
tDSW
30
RD
Pulse-Width ALE High
ALE Low Setup to RD or WR
Fall
Output Data-Delay Time from
RD
Data Setup Time
IRQ
Release from RD
30
tIRD
50
120
AC TEST CONDITIONS
Output Load: 50pF
Input Pulse Levels: 0V to 3.0V
Timing Measurement Reference Levels
Input: 1.5V
Output: 1.5V
Input Pulse Rise and Fall Times: 5ns
28 of 38
ns
ns
ns
ns
2
NOTES
ms
6
DS17885/DS17887
BUS TIMING FOR READ CYCLE TO RTC AND RTC REGISTERS
BUS TIMING FOR WRITE CYCLE TO RTC AND RTC REGISTERS
29 of 38
DS17885/DS17887
POWER-UP/POWER-DOWN TIMING, 5V
PARAMETER
SYMBOL
High to Power-Fail
tPF
Recovery at Power-Up
tREC
CS
VCC Slew Rate Power-Down
VCC Slew Rate Power-Down
VCC Slew Rate Power-Up
Expected Data Retention
(TA = +25°C)
MIN
TYP
MAX
UNITS
0
ns
150
tF
4.0 £ VCC £ 4.5V
tFB
3.0 £ VCC £ 4.0V
ms
10
ms
0
ms
10
years
4.5 ³ VCC ³ 4.0V
tDR
ms
300
tR
POWER-UP/POWER-DOWN TIMING, 3V
PARAMETER
SYMBOL
High to Power-Fail
tPF
Recovery at Power-Up
tREC
CS
VCC Slew Rate Power-Down
VCC Slew Rate Power-Up
Expected Data Retention
tF
2.5 £ VCC £ 3.0V
9, 10
(TA = +25°C)
MIN
TYP
MAX
UNITS
0
ns
150
ms
0
ms
10
years
3.0 ³ VCC ³ 2.5V
NOTES
ms
300
tR
tDR
NOTES
9, 10
Warning: Under no circumstances are negative undershoots, of any amplitude, allowed when device is
in battery back-up mode.
CAPACITANCE
PARAMETER
Input Capacitance
Output Capacitance
(TA = +25°C)
SYMBOL
MIN
TYP
MAX
UNITS
CIN
12
pF
COUT
12
pF
WAKE-UP/KICKSTART TIMING
PARAMETER
NOTES
(TA = +25°C)
SYMBOL
MIN
Kickstart-Input Pulse-Width
tKSPW
2
ms
Wake-up/Kickstart Power-On
Timeout
tPOTO
2
s
30 of 38
TYP
MAX
UNITS
NOTES
7
DS17885/DS17887
POWER-UP CONDITION, 3V
POWER-DOWN CONDITION, 3V
31 of 38
DS17885/DS17887
POWER-UP CONDITION, 5V
POWER-DOWN CONDITION, 5V
32 of 38
DS17885/DS17887
WAKE-UP/KICKSTART TIMING
Note: Time intervals shown above are referenced in Wake-Up/Kickstart section.
*This condition can occur with the 3V device.
33 of 38
DS17885/DS17887
BURST MODE TIMING WAVEFORM
NOTES:
1)
2)
3)
4)
5)
6)
7)
Typical values are at +25°C and nominal supplies.
Outputs are open.
Write-protection trip point occurs during power-fail prior to switchover from VCC to VBAT.
Applies to the AD0 to AD7 pins, and the SQW pin when each is in a high-impedance state.
The IRQ and PWR pins are open-drain.
Measured with a load of 50pF + 1 TTL gate.
Wake-up kickstart timeout generated only when the oscillator is enabled and the countdown chain is
not reset.
8) VSW is determined by the larger of VBAT and VBAUX.
9) The DS17887 keeps time to an accuracy of ±1 minute per month during data retention time for the
period of tDR.
10) tDR is the amount of time that the internal battery can power the internal oscillator and internal
registers of the DS17887.
11) IBAT1 and IBAT2 are measured at VBAT = 3.5V.
12) 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. Postsolder cleaning with water-washing techniques is acceptable, provided that ultrasonic vibration is not
used.
34 of 38
DS17885/DS17887
DS17885 24-PIN DIP
PKG
DIM
A IN
MM
B IN
MM
C IN
MM
D IN
MM
E IN
MM
F IN
MM
G IN
MM
H IN
MM
J IN
MM
K IN
MM
24-PIN
MIN
MAX
1.245
1.270
31.62
32.25
0.530
0.550
13.46
13.97
0.140
0.160
3.56
4.06
0.600
0.625
15.24
15.88
0.015
0.050
0.380
1.27
0.120
0.145
3.05
3.68
0.090
0.110
2.29
2.79
0.625
0.675
15.88
17.15
0.008
0.012
0.20
0.30
0.015
0.022
0.38
0.56
35 of 38
DS17885/DS17887
DS17885 24-PIN SO
The chamfer on the body is optional. If it is not present, a terminal 1 identifier must be positioned so that
one-half or more of its area is contained in the hatched zone.
PKG
DIM
A IN
MM
A1 IN
MM
A2 IN
MM
b IN
MM
C IN
MM
D IN
MM
e IN
MM
E1 IN
MM
H IN
MM
L IN
MM
Θ
24-PIN
MIN
MAX
0.094
0.105
2.38
2.68
0.004
0.012
0.102
0.30
0.089
0.095
2.26
2.41
0.013
0.020
0.33
0.51
0.009
0.013
0.229
0.33
0.598
0.612
15.19
15.54
0.050 BSC
1.27 BSC
0.290
0.300
7.37
7.62
0.398
0.416
10.11
10.57
0.016
0.040
0.40
1.02
0°
8°
36 of 38
DS17885/DS17887
DS17885 28-PIN TSOP
PKG
DIM
A
A1
A2
b
c
D
D1
E
e
L
L1
28-PIN
MIN
MAX
—
1.20
0.05
—
0.91
1.02
0.18
0.27
0.15
0.20
13.20
13.60
11.70
11.90
7.90
8.10
0.55 BSC
0.30
0.70
0.80 BSC
56-G5003-000
37 of 38
DS17885/DS17887
DS17887 RTC PLUS RAM
Note: Pins 2, 3, 16, and 20 are missing by design.
PKG
DIM
A IN
MM
B IN
MM
C IN
MM
D IN
MM
E IN
MM
F IN
MM
G IN
MM
H IN
MM
J IN
MM
K IN
MM
24-PIN
MIN
MAX
1.320
1.335
33.53
33.91
0.720
0.740
18.29
18.80
0.345
0.370
8.76
9.40
0.100
0.130
2.54
3.30
0.015
0.030
0.38
0.76
0.100
0.140
2.79
3.56
0.090
0.110
2.29
2.79
0.590
0.630
14.99
16.00
0.008
0.012
0.20
0.30
0.015
0.021
0.38
0.53
38 of 38