Maxim DS1685EN-3 3v/5v real-time clock Datasheet

DS1685/DS1687
3V/5V Real-Time Clocks
FEATURES
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Incorporates Industry-Standard DS1287 PC
Clock plus Enhanced Features Such as Y2K
Compliant
+3V or +5V Operation
64-Bit Silicon Serial Number
Power-Control Circuitry Supports System
Power-On from Date/Time Alarm or Key
Closure
32kHz Output for Power Management
Crystal-Select Bit Allows RTC to Operate with
6pF or 12.5pF Crystal
SMI Recovery Stack
242 Bytes Battery-Backed NV RAM
Auxiliary Battery Input
RAM Clear Input
Century Register
Date Alarm Register
Compatible with Existing BIOS for Original
DS1287 Functions
Available as Chip (DS1685) or Stand-Alone
Encapsulated DIP (EDIP) with Embedded
Battery and Crystal (DS1687)
Timekeeping Algorithm Includes Leap-Year
Compensation Valid Through 2099
Underwriters Laboratory (UL) Recognized
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APPLICATIONS
Embedded Systems
Utility Meters
Security Systems
Network Hubs, Bridges, and Routers
PIN CONFIGURATIONS
24
VCC
X1
2
23
SQW
X2
3
AD0
DS1685/
DS1685S/ 22
4 DS1685E 21
AD1
5
20
VBAT
AD2
6
19
IRQ
AD3
7
18
KS
X2
X1
PWR
N.C.
VCC
SQW
1
VBAUX
RCLR
AD4
8
17
RD
AD5
9
16
GND
AD6
10
15
WR
AD7
11
14
ALE
GND
12
13
CS
AD0
AD1
AD2
AD3
AD4
AD5
N.C.
5
4
6
3
2
1
28
27
DS1685Q
7
26
25
24
23
8
22
9
21
10
20
11
12
19
13 14
15 16
17
18
AD6
N.C.
AD7
GND
CS
ALE
N.C.
PWR
VBAUX
TOP VIEW
PLCC
PDIP (0.600″)/
SO (0.300″)/
TSSOP (0.173″)
RCLR
VBAT
IRQ
KS
RD
GND
WR
PWR
1
24
VCC
N.C.
2
23
SQW
N.C.
3
22
VBAUX
AD0
4
21
RCLR
AD1
5
20
N.C.
DS1687
AD2
6
19
IRQ
AD3
7
18
KS
AD4
8
17
RD
AD5
9
16
N.C.
AD6
10
15
WR
AD7
11
14
ALE
GND
12
13
CS
EDIP (0.740″)
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19-5215; Rev 10/12
DS1685/DS1687 3V/5V Real-Time Clocks
ORDERING INFORMATION
TEMP RANGE
VOLTAGE
(V)
PIN-PACKAGE
DS1685-3+
0°C to +70°C
3
24 PDIP (0.600”)
DS1685-3
DS1685-5+
0°C to +70°C
5
24 PDIP (0.600”)
DS1685-5
-40°C to +85°C
5
24 PDIP (0.600”)
DS1685-5
DS1685E-3+
0°C to +70°C
3
24 TSSOP (0.173”)
DS1685E-3
DS1685E-5+
0°C to +70°C
5
24 TSSOP (0.173”)
DS1685E
DS1685EN-3+
-40°C to +85°C
3
24 TSSOP (0.173”)
DS1685E-3
DS1685EN-5+
-40°C to +85°C
5
24 TSSOP (0.173”)
DS1685E
DS1685E-3+T&R
0°C to +70°C
3
24 TSSOP (0.173”)/Tape & Reel
DS1685E-3
DS1685E-5+T&R
0°C to +70°C
5
24 TSSOP (0.173”)/Tape & Reel
DS1685E
DS1685EN-3+T&R
-40°C to +85°C
3
24 TSSOP (0.173”)/Tape & Reel
DS1685E-3
DS1685EN-5+T&R
-40°C to +85°C
5
24 TSSOP (0.173”) /T&R
DS1685E
DS1685Q-3+
0°C to +70°C
3
28 PLCC
DS1685Q-3
DS1685Q-5+
0°C to +70°C
5
28 PLCC
DS1685Q-5
DS1685QN-3+
-40°C to +85°C
3
28 PLCC
DS1685Q-3
DS1685QN-5+
-40°C to +85°C
5
28 PLCC
DS1685Q-5
DS1685Q-3+T&R
0°C to +70°C
3
28 PLCC/Tape & Reel
DS1685Q-3
DS1685Q-5+T&R
0°C to +70°C
5
28 PLCC/Tape & Reel
DS1685Q-5
DS1685S-3+
0°C to +70°C
3
24 SO (0.300”)
DS1685S-3
DS1685S-5+
0°C to +70°C
5
24 SO (0.300”)
DS1685S-5
DS1685S-3+T&R
0°C to +70°C
3
24 SO (0.300”)/Tape & Reel
DS1685S-3
DS1685S-5+T&R
0°C to +70°C
5
24 SO (0.300”)/Tape & Reel
DS1685S-5
DS1687-3+
0°C to +70°C
3
24 EDIP (0.740”)
DS1687-3
DS1687-5+
0°C to +70°C
5
24 EDIP (0.740”)
DS1687-5
DS1687-3IND+
-40°C to +85°C
3
24 EDIP (0.740”)
DS1687-3
DS1687-5IND+
-40°C to +85°C
5
24 EDIP (0.740”)
DS1687-5
PART*
DS1685-5IND+
0B
1B
TOP MARK*
+Denotes a lead(Pb)-free/RoHS-compliant device. A “+” anywhere on the top mark indicates a lead-free/RoHS-compliant device.
*An “N” or “IND” denotes an industrial temperature grade device.
TYPICAL OPERATING CIRCUIT
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DS1685/DS1687 3V/5V Real-Time Clocks
DETAILED DESCRIPTION
The DS1685/DS1687 are real-time clocks (RTC) designed as successors to the industry-standard DS1285,
DS1385, DS1485, and DS1585 PC RTCs. These devices provide the industry-standard DS1285 clock function with
either +3.0V or +5.0V operation. The DS1685 also incorporates a number of enhanced features including a silicon
serial number, power-on/off control circuitry, 242 bytes of user NV SRAM, and 32.768kHz output for sustaining
power management activities.
The DS1685/DS1687 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 can be triggered by one or either of these
events, and can be 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 DS1685 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 DS1687
incorporates the DS1685 chip, a 32.768kHz crystal, and a lithium battery in a complete, self-contained timekeeping
EDIP. The entire unit is fully tested at Maxim Integrated 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 DS1685/DS1687.
The following paragraphs describe the function of each pin.
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DS1685/DS1687 3V/5V Real-Time Clocks
PIN DESCRIPTIONS
PIN
DS1685
DS1687
PDIP,
SO,
PLCC
NAME
FUNCTION
PWR
Active Low Power-On Output, Open Drain. The PWR pin is intended for use
as an on/off control for the system power. With VCC voltage removed from
the DS1685/DS1687, 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 Maxim
Integrated registers. The PWR pin can be connected through a pullup
resistor to a positive supply. The voltage of the pullup supply should be no
greater than 5.5V.
N.C.
No Connection. Pins missing by design.
EDIP
TSSOP
1
—
2
1
1, 11, 13,
2, 3, 16,
18
20
2
3
—
X1
3
4
—
X2
4–11
AD0–AD7
12
GND
4–11
12, 16
5–10, 12,
14
15, 20
Connections for Standard 32.768kHz Quartz Crystal. For greatest
accuracy, the DS1685 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 highimpedance 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.
Multiplexed, Bidirectional Address/Data Bus. 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 DS1685 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
DS1685/DS1687 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
DS1685/DS1687 outputs 8 bits of data during the latter portion of the RD
pulse. The read cycle is terminated and the bus returns to a highimpedance state as RD transitions high. The address/data bus also serves
as a bidirectional data path for the extended RAM.
Ground
13
16
13
CS
Chip-Select Input, Active-Low. The chip-select signal must be asserted low
during a bus cycle for the RTC portion of the DS1685/DS1687 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 will
latch addresses. However, no data transfer will occur.
14
17
14
ALE
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 DS1685/DS1687.
15
19
15
WR
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.
17
21
17
RD
Read Input, Active Low. RD identifies the time period when the
DS1685/DS1687 drives the bus with RTC read data. The RD signal is an
enable signal for the output buffers of the clock.
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DS1685/DS1687 3V/5V Real-Time Clocks
PIN DESCRIPTIONS (continued)
PIN
DS1685
DS1687
PDIP,
SO,
PLCC
FUNCTION
NAME
EDIP
TSSOP
18
19
20
21
22
22
23
24
25
26
18
19
—
21
22
KS
Kickstart Input, Active Low. When VCC is removed from the
DS1685/DS1687, the system can be powered on in response to an activelow transition on the KS pin, as might be generated from a key closure.
VBAUX must be present and the auxiliary-battery enable bit (ABE) and kickstart enable bit (KSE) 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. If not used, connect to VCC, or to
VBAUX if VBAUX is used.
IRQ
Interrupt-Request Output, Open Drain, Active Low. The IRQ pin is an
active-low output of the DS1685/DS1687 that can be connected 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 interruptenable bit is set. To clear the IRQ pin, the application software must clear
all the enabled flag bits contributing to IRQ’s active state asserted but
without asserting CS latch addresses. However, no data transfer occurs.
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.
VBAT must be grounded if not used. Diodes should not be placed between
VBAT and the battery. See “Conditions of Acceptability” at
www.maximintegrated.com/UL.
RCLR
RAM Clear Input, Active Low. If enabled by software, taking RCLR low
clears the 242 bytes of user RAM to FFh. When enabled, RCLR can be
activated whether or not VCC is present. The RCLR function is designed to
be used by a human interface (shorting to ground manually or by a switch)
and not to be driven with external buffers. This pin is internally pulled up.
Do not use an external pullup resistor on this pin.
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. Battery voltage must be held between +2.5V and +3.7V for proper
operation. If VBAUX is not going to be used it should be grounded, and
Auxiliary-Battery Enable bit bank 1, register 4BH, should be written to 0.
See “Conditions of Acceptability” at www.maximintegrated.com/UL.
Square-Wave Output. The SQW pin provides a 32kHz square-wave output,
tREC, after a power-up 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 3. 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 enable32kHz (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.
23
27
23
SQW
24
28
24
VCC
DC Power for Primary Power Supply. When VCC is applied within the
normal limits, the device sis fully accessible and data can be written and
read. When VCC is below VPF reads and writes are inhibited.
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DS1685/DS1687 3V/5V Real-Time Clocks
Figure 1. Block Diagram
X1
OSC
X2
Divide by
64
Divide
by 8
Divide by
64
DS1687 only
16:1 Mux
VCC
GND
VBAUX
VBAT
Power
Control
Square
Wave
Generator
SQW
IRQ
Generator
IRQ
PWR
KS
DS1687 only
CS
RD
WR
ALE
Registers A, B,C,D
BUS
Interface
Clock/Calender
Update Logic
Clock/Calendar and
Alarm Registers
Buffered Clock/
Calendar and Alarm
Registers
RAM
User Ram
Clear
114 Bytes
Logic
AD0 - AD7
RLCR
Select
DS1685/DS1687
OSCILLATOR CIRCUIT
Extended RAM Addr/
Data Registers
Extended Control/
Status Registers
64-Bit Serial Number
Century Counter
Date Alarm
RTC Address -2
RTC Address -3
Extended
User RAM
128 Bytes
The DS1685 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or
capacitors to operate. Table 1 specifies several crystal parameters for the external crystal, and Figure 2 shows a
functional schematic of the oscillator circuit. The oscillator is controlled by an enable bit in the control register.
Oscillator startup times are highly dependent upon crystal characteristics, PC board leakage, and layout. High ESR
and excessive capacitive loads are the major contributors to long startup times. A circuit using a crystal with the
recommended characteristics and proper layout usually starts within one second.
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DS1685/DS1687 3V/5V Real-Time Clocks
Table 1. Crystal Specifications*
PARAMETER
SYMBOL
8B
9B
Nominal Frequency
Series Resistance
Load Capacitance
MIN
fO
ESR
CL
TYP
32.768
6, 12.5
MAX
UNITS
50
kHz
kΩ
pF
*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 specifications.
CLOCK ACCURACY
The accuracy of the clock is dependent on the accuracy of the crystal and the accuracy of the match between the
capacitive load of the oscillator circuit and the capacitive load for which the crystal was trimmed. Additional error is
added by crystal frequency drift caused by temperature shifts. External circuit noise coupled into the oscillator
circuit can result in the clock running fast. Figure 3 shows a typical PC board layout for isolation of the crystal and
oscillator from noise. Refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks for
detailed information.
The DS1685 can also be driven by an external 32.768 kHz oscillator. In this configuration, the X1 pin is connected
to the external oscillator signal and the X2 pin is floated. Refer to Application Note 58: Crystal Considerations with
Dallas Real-Time Clocks for detailed information about crystal selection and crystal layout.
Figure 2. Oscillator Circuit Showing Internal Bias Network
RTC
Countdown
Chain
CL2
CL1
RTC
Registers
X2
X1
Crystal
Figure 3. Typical Crystal Layout
Local ground plane (Layer 2)
X1
crystal
X2
NOTE: AVOID ROUTING SIGNAL LINES IN
THE CROSSHATCHED AREA (UPPER
LEFT QUADRANT) OF THE PACKAGE
UNLESS THERE IS A GROUND PLANE
BETWEEN THE SIGNAL LINE AND THE
DEVICE PACKAGE.
GND
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DS1685/DS1687 3V/5V Real-Time Clocks
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. . At least one back up supply must remain within the minimum
and maximum limits whenever VCC is not at a valid level. When VCC is applied and exceeds 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. If the
oscillator is not enabled, the oscillator enable bit will be enabled on power up, and the device becomes immediately
accessible.
The DS1685/DS1687 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
falls below VPF, 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, reads and writes are 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 device inhibits access by internally disabling the CS input. With the possible
exception of the KS, PWR, and SQW pins, all inputs are ignored and all outputs are in a high-impedance state.
TIME, CALENDAR, AND ALARM LOCATIONS
The time and calendar information is obtained by reading the appropriate register bytes shown in Table 2. 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 2 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 are 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. Invalid time and date entries will result in undefined operation. 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. If the
oscillator is running, the time and date registers will update 500ms after the countdown chain is enabled.
Once initialized, the RTC 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 one 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.
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
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DS1685/DS1687 3V/5V Real-Time Clocks
“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 the Wake-Up/Kickstart section. The century counter is discussed later in
this text.
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) Bit 7 of the seconds byte is read-only.
Table 2A. Time, Calendar, and Alarm Data Modes—BCD Mode (DM = 0)
ADDRESS
BIT 7
00H
0
10 Seconds
Seconds
01H
0
10 Seconds
Seconds
02H
0
10 Minutes
Minutes
03H
0
10 Minutes
Minutes
04H
05H
AM/PM
0
AM/PM
0
BIT 6
BIT 5
0
0
BIT 4
BIT 3
BIT 2
10 Hour
10 Hour
0
0
10 Hour
10 Hr
0
FUNCTION
RANGE
Seconds
Seconds
Alarm
Minutes
Minutes
Alarm
00-59
Hours
Hours
Hours
Hours
Alarm
0
0
0
0
0
0
0AH
UIP
DV2
DV1
DV0
RS3
RS2
RS1
RS0
Control
0BH
SET
PIE
AIE
UIE
SQWE
DM
24/12
DSE
Control
0CH
IRQF
PF
AF
UF
0
0
0
0
Control
0DH
VRT
0
0
0
0
0
0
0
Control
48H
49H
0
BIT 0
06H
07H
08H
09H
10 Year
0
10 Date
0
10 Month
BIT 1
Date
Month
Year
Century
10 Date
Date
Day
Day
Date
Month
Year
Bank 1
Century
Bank 1
Date Alarm
00-59
00-59
00-59
1-12
+AM/PM
00-23
1-12
+AM/PM
00-23
01-07
01-31
01-12
00-99
00-99
01-31
X = Read/Write Bit.
Note 1: Unless otherwise specified, the state of the registers is not defined when power is first applied.
Note 2: Except for the seconds register, 0 bits in the time and date registers can be written to a 1, but may be modified when the clock updates.
0 bits should always be written to 0 except for alarm mask bits.
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DS1685/DS1687 3V/5V Real-Time Clocks
Table 2. Time, Calendar, and Alarm Data Modes—Binary Mode (DM = 1)
ADDRESS
BIT 7
BIT 6
00H
0
0
01H
0
02H
03H
BIT 5
BIT 4
FUNCTION
RANGE
Seconds
Seconds
00-3B
0
Seconds
Seconds
Alarm
00-3B
0
0
Minutes
Minutes
00-3B
0
0
Minutes
Minutes
Alarm
00-3B
AM/PM
04H
0
0
BIT 2
0
0
BIT 0
Hours
Hours
0
0
BIT 1
Hours
0
AM/PM
05H
BIT 3
00-17
Hours
0
Hours
Alarm
Hours
0
06H
0
0
0
07H
0
0
0
08H
0
0
0
09H
0
0AH
UIP
DV2
DV1
DV0
RS3
RS2
RS1
RS0
Control
0BH
SET
PIE
AIE
UIE
SQWE
DM
24/12
DSE
Control
0CH
IRQF
PF
AF
UF
0
0
0
0
Control
0DH
VRT
0
0
0
0
0
0
0
Control
Day
Date
Month
Year
48H
49H
0
0
Century
10 Date
Date
10 of 34
1-0C
+AM/PM
1-0C
+AM/PM
00-17
Day
01-07
Date
01-1F
Month
01-0C
Year
00-63
Bank 1
Century
00-63
Bank 1
Date Alarm
01-1F
DS1685/DS1687 3V/5V Real-Time Clocks
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 (0Ah)
MSB
BIT 7
UIP
BIT 6
DV2
BIT 5
DV1
BIT 4
DV0
BIT 3
RS3
BIT 2
RS2
BIT 1
RS1
LSB
BIT 0
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 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 three bits are used to turn the oscillator on or off and to reset the countdown chain. A
pattern of 01X is the only combination of bits that will turn the oscillator on and allow the RTC to keep time. A
pattern of 11X will enable the oscillator but holds the countdown chain in reset. The next update will occur at
500ms after a pattern of 01X is written to DV0, DV1, and DV2. The oscillator enable bit, DV1, will be set to a 1
when VCC is applied.
DV2 = Countdown Chain
1 – resets countdown chain only if DV1=1
0 – countdown chain enabled
DV1 = Oscillator Enable
1 – oscillator on
0 – oscillator off
DV0 = Bank Select
1 – extended registers
0 – original bank
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)
2)
3)
4)
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 3 lists the periodic interrupt rates and the square-wave frequencies that can be chosen with the RS bits.
11 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
Register B (0Bh)
MSB
BIT 7
SET
BIT 6
PIE
BIT 5
AIE
BIT 4
UIE
BIT 3
SQWE
BIT 2
DM
BIT 1
24/12
LSB
BIT 0
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 DS1685/DS1687.
PIE – The periodic-interrupt enable 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
DS1685/DS1687 functions.
AIE – The alarm-interrupt enable (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 DS1685/DS1687 do not affect the AIE bit.
UIE – The update-ended interrupt-enable (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 – When the square-wave enable (SQWE) bit is set to a 1 and E32K = 0, a square-wave signal at the
frequency set by the rate-selection bits RS3–RS0 is driven out on the SQW pin. When the SQWE bit is set to 0 and
E32K = 0, the SQW pin is held low. 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. 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.
DSE - The Daylight Savings Enable (DSE) bit is a read/write bit that enables two daylight savings adjustments
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. When DSE is enabled, the
internal logic tests for the first/last Sunday condition at 1:59:59 AM. If the DSE bit is not set when the test occurs,
the daylight savings function will not operate correctly. These adjustments do not occur when the DSE bit is a
zero. This bit is not affected by internal functions.
12 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
Register C (0Ch)
MSB
BIT 7
IQRF
LSB
BIT 0
0
10B
BIT 6
PF
BIT 5
AF
BIT 4
UF
BIT 3
0
BIT 2
0
BIT 1
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
WF = WIE = 1
AF = AIE = 1
KF = KSE = 1
UF = UIE = 1
RF = RIE = 1
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 – 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–RS0 bits establish the periodic rate. PF is set to a 1 independently of the state of
the PIE bit. When both PF and PIE are 1’s, the IRQ signal is active and sets the IRQF bit. This bit may be cleared
by reading 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. This bit may be cleared by reading Register C.
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. This bit may be cleared by reading Register C.
BIT 3, BIT2, BIT 1, BIT 0 - These are unused bits of the status Register C. These bits always read 0 and cannot be
written.
Register D (0Dh)
MSB
BIT 7
VRT
BIT 6
0
BIT 5
0
BIT 4
0
BIT 3
0
BIT 2
0
BIT 1
0
LSB
BIT 0
0
VRT – The valid RAM and time (VRT) bit indicates the condition of the battery connected to the VBAT pin or the
battery connected to VBAUX, whichever is at a higher voltage. This bit is not writable and should always be a 1 when
read. If a 0 is ever present, an exhausted lithium energy source is indicated and both the contents of the RTC data
and RAM data are questionable.
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 will always read 0.
13 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
NV RAM—RTC
The 242 general-purpose NV RAM bytes are not dedicated to any special function within the DS1685/DS1687.
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 RTC registers and
114 bytes of user RAM are accessible. When bank 1 is selected, an additional 128 bytes of user RAM are
accessible through the extended RAM address and data registers.
INTERRUPT CONTROL
The DS1685/DS1687 includes six separate, fully automatic sources of interrupt for a processor:
1) Alarm Interrupt
2) Periodic Interrupt
3) Update-Ended Interrupt
4) Wake-Up Interrupt
5) Kickstart Interrupt
6) RAM Clear Interrupt
The conditions that generate each of these independent interrupt conditions are described in detail in other
sections of this text. This section describes the overall control of the interrupts.
The application software can select which interrupts, if any, are to be used. There are a total of 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 set at an active level, even though
the event initiating the interrupt condition might 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 as they are automatically cleared to 0
immediately after they are read. Double latching is implemented on these bits so that set bits 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. 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 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 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 DS1685/DS1687 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.
14 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
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 by 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-16
decoder, which selects one of 13 taps that divide the 32.768kHz frequency. The RS3–0 bits establish the SQW
output frequency as shown in Table 3. 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 squarewave output signal is generated on the SQW pin in the absence of VCC.
A pattern of 01X in the DV2, DV1, and DV0 bits respectively turns the oscillator on and enables the countdown
chain. Note 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 the oscillator’s countdown chain is held in reset, as it was in the
DS1287. Any other bit combination for DV2 and DV1 keeps the oscillator off.
OSCILLATOR CONTROL BITS
When the DS1687 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 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.
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 3). 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 squarewave 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.
15 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
Table 3. Periodic Interrupt Rate and Square-Wave Output Frequency
EXT.
REG 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–RS0 determine periodic interrupt rates as listed for E32K = 0.
UPDATE CYCLE
The 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 UIP bit 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 4). 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.
16 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
Figure 4. Update-Ended And Periodic-Interrupt Relationship
1 second
UIP
tBUC
UF
tPI/2
tPI/2
PF
t PI
tPI = PERIODIC INTERRUPT TIME INTERNAL PER TABLE 3.
tBUC = DELAY BEFORE UPDATE CYCLE = 244µs.
EXTENDED FUNCTIONS
The extended functions provided by the DS1685/DS1687 that are new to the RAMified RTC family are accessed by
a software-controlled bank-switching scheme, as illustrated in Figure 5. 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 DS1685/DS1687 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.
17 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
Figure 5. DS1685/DS1687 Register Map and Extended Register Bank Definition
MSB
00
0D
0E
BANK 0 DV0
=0
TIMEKEEPING AND CONTROL
LSB
MSB
00
0D
0E
BANK 1
DV0 =
LSB
TIMEKEEPING AND CONTROL
50 BYTES-USER RAM
3F
1
50 BYTES-USER RAM
3F
40
BYTE SERIAL NUMBER
BYTE SERIAL NUMBER
41
1
42
2
ND
43
3
RD
BYTE SERIAL NUMBER
44
4
TH
BYTE SERIAL NUMBER
45
46
47
48
49
64 BYTES-USER RAM
MODEL NUMBER BYTE
ST
5
TH
6
BYTE SERIAL NUMBBER
TH
BYTE SERIAL NUMBER
CRC BYTE
CENTURY BYTE
DATE ALARM
4A
EXTENDED CONTROL REG 4A
4B
4C
4D
4E
EXTENDED CONTROL REG 4B
RESERVED
RESERVED
RTC ADDRESS-2
4F
50
51
RTC ADDRESS-3
EXTENDED RAM ADDRESS
RESERVED
52
53
54
RESERVED
EXTENDED RAM DATA PORT
EXTENDED
RAM
128 x 8
RESERVED
7F
7F
When bank 1 is selected, the clock/calendar registers and the original 50 bytes of user RAM still appear as bank 0.
However, the registers that provide control and status for the extended functions are accessed in place of the
additional 64 bytes of user RAM. The major functions controlled by the extended registers are listed below:
1)
2)
3)
4)
64-Bit Silicon Serial Number
Century counter
Date Alarm
Auxiliary Battery Control/Status
5)
6)
7)
8)
Wake Up
Kickstart
RAM Clear Control/Status
128 Bytes 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 Maxim Integrated. Bits in these locations cannot be written and return a
0 if read.
18 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
SILICON SERIAL NUMBER
A unique 64-bit lasered serial number is located in bank 1, registers 40h to 47h. This serial number is divided into
three parts. The first byte in register 40h contains a model number, 71h, to identify the device type. Registers 41h
to 46h contain a unique binary number. Register 47h contains a CRC byte used to validate the data in registers
40h to 46h. All 8 bytes of the serial number are read-only registers.
The DS1685/DS1687 is manufactured such that no two devices contain an identical number in locations 41h to
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.
AUXILIARY BATTERY
The VBAUX input is provided to supply power from an auxiliary battery for the DS1685/DS1687 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 on and
off the auxiliary battery for the above functions in the absence of VCC. When set to a 1, VBAUX battery power is
enabled, and when cleared to 0, VBAUX battery power is disabled to these functions.
In the DS1685/DS1687, 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 DS1685 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 DS1685/DS1687 incorporates a wake-up feature that can power the system on 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 DS1685/DS1687 must have an auxiliary battery connected to the
VBAUX pin and 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 does not drive 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 kick-start
feature is controlled through 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 by 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). In conjunction 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 automatically drives low. This output can be used to turn on the main system power
supply, which provides VCC voltage to the DS1685/DS1687 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.
A kickstart sequence occurs when kickstarting is enabled by 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 pulls low, as it does 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.
19 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
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 DS1685/DS1687 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 highimpedance level. In this event, the IRQ pin also remains three-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 continues 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 DS1685/DS1687 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 DS1685/DS1687 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 opendrain 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.
During Interval 5, the system is fully powered down. Battery backup of the clock calendar and NV RAM is in effect
and IRQ is three-stated, and monitoring of wake-up and kickstart takes place. If PRS = 1, PWR stays active;
otherwise, if PRS = 0, PWR is three-stated.
RAM CLEAR
The DS1685/DS1687 provides a RAM clear function for the 242 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 by the RAM clear-enable bit (RCE; bank 1, register 04Bh). When
RCE is set to a logic 1 and RF=0, the 242 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
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 is 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 the Electrical Characteristics
section.
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.
20 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
128 x 8 EXTENDED RAM
The DS1685/DS1687 provides 128 x 8 of on-chip SRAM, which is controlled as nonvolatile storage sustained by
VBAT and/or VBAUX. On power-up, the RAM is accessible after TREC.
The on-chip 128 x 8 NV SRAM is accessed by the eight multiplexed address/data lines AD7–AD0. Access to the
SRAM is controlled by two on-chip latch registers. One register is 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 7Fh.
Access to the extended 128 x 8 RAM is controlled by two of the registers shown in Figure 5. The registers in bank
1 must first be selected by setting the DV0 bit in register A to a logic 1. The 7-bit address of the RAM location to be
accessed must be loaded into the extended RAM address register located at 50h. Data in the addressed location
may 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 may be read or written repeatedly without changing the address in
location 50h.
EXTENDED CONTROL REGISTERS
Two extended control registers are provided to supply controls and status information for the extended features
offered by the DS1685/DS1687. 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
VRT2
BIT 6
INCR
BIT 5
*
BIT 4
*
BIT 3
PAB
BIT 2
RF
BIT 1
WF
LSB
BIT 0
KF
VRT2 – This status bit gives the condition of the auxiliary battery. 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 logic 0, the external battery should be replaced.
INCR – Increment-in-Progress status bit. 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.
PAB – Power-Active Bar control bit. 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 Maxim Integrated. They can be read and written, but have no affect on operation.
21 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
Extended Control Register 4B
MSB
BIT 7
ABE
BIT 6
E32K
BIT 5
CS
BIT 4
RCE
BIT 3
PRS
BIT 2
RIE
BIT 1
WIE
LSB
BIT 0
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. This bit is set to a logic 1 when VCC is applied.
CS – Crystal Select Bit. 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
DS1687 EDIP and should be set to CS = 0.
RCE – RAM Clear-Enable bit. When set to a 1, this bit enables a low level on RCLR to clear all 242 bytes of user
RAM. When RCE = 0, RCLR and the RAM clear function are disabled.
PRS – PAB Reset-Select Bit. When set to a 0, the PWR pin is set high-Z when the DS1685 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 activelow 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.
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.
RTC ADDRESS
RTC ADDRESS-1
4Eh
RTC ADDRESS-2
4Fh
RTC ADDRESS-3
SMI RECOVERY STACK
7
6
5
4
3
2
1
0
DV0
AD6
AD5
AD4
AD3
AD2
AD1
AD0
REGISTER BIT DEFINITION
22 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
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.
ALE
1
2
3
4
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 34
DS1685/DS1687 3V/5V Real-Time Clocks
ABSOLUTE MAXIMUM RATINGS
Voltage Range on Any Pin Relative to Ground………………………………………………………………….0.3V to +6V
Operating Temperature Range, Commercial ...……………………………………………………………….0°C to +70°C
Operating Temperature Range, Industrial…………………………………………………………………...-40°C to +85°C
Storage Temperature Range
EDIP………………………………....……………………………………………………………………….-40°C to +85°C
PDIP, SO, TSSOP……………………………….…………………………………....………………….-55°C to +125°C
Lead Temperature (soldering, 10s)……….....………………………..........................……………………………..+260°C
(Note: EDIP is hand or wave-soldered only.)
Soldering Temperature (reflow)…………………………………………………...................................………...…+260°C
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
(TA = 0°C to +70°C, TA = -40°C to +85°C.)
PARAMETER
Power-Supply
Voltage
(-5)
(-3)
Input Logic 1
Pullup Voltage
VCC
VIH
PWR
IRQ
Input Logic 0
Battery Voltage
Auxiliary Battery
Voltage
SYMBOL
(-5)
(-3)
MIN
TYP
MAX
4.5
5.0
5.5
2.7
3.0
3.7
2.2
VCC + 0.3
5.5
VPUPWR
VCC + 0.2
UNITS
NOTES
V
1
V
1
V
1
VIL
-0.3
0.6
V
1
VBAT
2.5
3.7
V
1
2.5
5.2
2.5
3.7
V
1
VBAUX
24 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
DC ELECTRICAL CHARACTERISTICS
(VCC = 5.0V±10% or VCC = 3.0V±10%, TA = 0°C to +70°C, TA = -40°C to +85°C.)
PARAMETER
SYMBOL
(-5)
Average VCC PowerSupply Current
MIN
ICC1
TYP
MAX
7
15
5
10
1
3
0.5
2
2B
(-3)
(-5)
CMOS Standby
Current
(CS = VCC - 0.2V)
ICC2
3B
(-3)
UNITS
NOTES
mA
2, 3
mA
2, 3
Input Leakage Current
(Any Input)
IIL
-1
+1
µA
Output Leakage Current
IOL
-1
+1
µA
Output Logic 1 Voltage
(IOUT = -1.0mA)
VOH
2.4
Output Logic 0 Voltage
(IOUT = +2.1mA)
VOL
Power-Fail Trip Point
Battery Switch
Voltage
(-5)
(-3)
(-5)
Battery Leakage Current
I/O Leakage
PWR Output at 0.4V
IRQ Output at 0.4V
VPF
(-5)
(-3)
(-5)
(-3)
V
0.4
4.25
4.37
4.5
2.5
2.6
2.7
VSW
VBAT,
VBAUX
IBATLKG
10
ILO
5
-1
8
100
nA
12
+1
µA
4
mA
1
mA
1
MAX
UNITS
NOTES
4
2.1
IOLIRQ
V
V
10.0
IOLPWR
V
0.8
DC ELECTRICAL CHARACTERISTICS
(VCC = 0V, TA = 0°C to +70°C, TA = -40°C to +85°C.)
PARAMETER
SYMBOL
MIN
TYP
VBAT, VBAUX Current
(OSC ON, E32K = 0)
IBAT1
500
nA
12
VBAT, VBAUX Current
(OSC OFF)
IBAT2
200
nA
12
25 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
RTC AC TIMING CHARACTERISTICS
(VCC = 3.0V±10%, TA = 0°C to +70°C, TA = -40°C to +85°C)
PARAMETER
SYMBOL
MIN
tCYC
260
PWRWL
100
ns
PWRWH
100
ns
Cycle Time
Pulse Width, RD/WR Low
Pulse Width, RD/WR High
4B
5B
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
30
ns
Muxed Address Hold Time
from ALE Fall
tAHL
15
ns
RD or WR High Setup to ALE
Rise
tASD
30
ns
PWASH
80
ns
ALE Low Setup to RD or WR
Fall
tASED
30
ns
Output-Data Delay Time from
RD
tDDR
20
Data Setup Time
tDSW
60
IRQ Release from RD
tIRD
Pulse-Width ALE High
30
50
80
AC TEST CONDITIONS
Output Load: 50pF
Input Pulse Levels: 0 to 3.0V
Timing Measurement Reference Levels
Input : 1.5V
Output: 1.5V
Input Pulse Rise and Fall Times: 5ns
26 of 34
ns
ns
ns
ns
2
NOTES
µs
6
DS1685/DS1687 3V/5V Real-Time Clocks
RTC AC TIMING CHARACTERISTICS
(VCC = 5.0V±10%, TA = 0°C to +70°C, TA = -40°C to 85°C.)
PARAMETER
1B
SYMBOL
MIN
tCYC
195
PWRWL
75
ns
PWRWH
75
ns
Cycle Time
Pulse Width, RD/WR Low
Pulse Width, RD/WR High
6B
7B
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
30
ns
Muxed Address Hold Time
from ALE Fall
tAHL
15
ns
RD or WR High Setup to ALE
Rise
tASD
25
ns
PWASH
40
ns
ALE Low Setup to RD or WR
Fall
tASED
30
ns
Output-Data Delay Time from
RD
tDDR
20
Data Setup Time
tDSW
60
IRQ Release from RD
tIRD
Pulse-Width ALE High
30
50
60
AC TEST CONDITIONS
Output Load: 50pF
Input Pulse Levels: 0 to 3.0V
Timing Measurement Reference Levels
Input : 1.5V
Output: 1.5V
Input Pulse Rise and Fall Times: 5ns
27 of 34
ns
ns
ns
ns
2
NOTES
µs
6
DS1685/DS1687 3V/5V Real-Time Clocks
DS1685/DS1667 BUS TIMING FOR READ CYCLE TO RTC AND RTC REGISTERS
DS1685/DS1687 BUS TIMING FOR WRITE CYCLE TO RTC AND RTC REGISTERS
28 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
POWER-UP/DOWN TIMING—5V
(TA = +25°C)
PARAMETER
SYMBOL
CS High to Power-Fail
tPF
Recovery at Power-Up
tREC
MIN
TYP
MAX
UNITS
0
ns
150
NOTES
ms
VCC Slew Rate Power-Down
tF
4.0 ≤ VCC ≤ 4.5V
300
µs
VCC Slew Rate Power-Down
tFB
3.0 ≤ VCC ≤ 4.0V
10
µs
VCC Slew Rate Power-Up
tR
4.5V ≥ VCC ≥ 4.0V
0
µs
Expected Data Retention
tDR
10
years
9, 10
SYMBOL
MIN
MAX
UNITS
NOTES
0
ns
POWER-UP/DOWN TIMING—3V
(TA = +25°C)
PARAMETER
CS High to Power-Fail
tPF
Recovery at Power-Up
tREC
TYP
150
ms
tF
2.5 ≤ VCC ≤ 2.7V
300
µs
VCC Slew Rate Power-Up
tR
2.7V ≥ VCC ≥ 2.5V
0
µs
Expected Data Retention
tDR
10
years
VCC Slew Rate Power-Down
9, 10
WARNING: Under no circumstances are negative undershoots, of any amplitude, allowed
when device is in battery-backup mode.
CAPACITANCE
(TA = +25°C)
PARAMETER
SYMBOL
Input Capacitance on All Inputs
Except X1 and X2
Output Capacitance on IRQ,
SQW and DQ pins
MIN
TYP
MAX
UNITS
CIN
12
pF
COUT
12
pF
MAX
UNITS
NOTES
WAKE-UP/KICKSTART TIMING
(TA = +25°C)
PARAMETER
SYMBOL
MIN
Kickstart Input Pulse Width
tKSPW
2
µs
Wake-Up/Kickstart Power On
Timeout
tPOTO
2
seconds
29 of 34
TYP
NOTES
7
DS1685/DS1687 3V/5V Real-Time Clocks
POWER-UP CONDITION—3V
VIH
CS
tREC
2.7V
2.6V
2.5V
VCC
tR
POWER FAIL
POWER-DOWN CONDITION—3V
CS
VIH
tPF
tF
VCC
2.7V
2.6V
2.5V
POWER FAIL
30 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
POWER-UP CONDITION—5V
VIH
CS
tREC
4.5V
4.25V
4.0V
VCC
tR
POWER FAIL
POWER-DOWN CONDITION—5V
CS
VIH
tPF
tF
VCC
tFB
4.5V
4.25V
4.0V
3.0V
POWER FAIL
31 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
WAKE-UP/KICKSTART TIMING
*THIS CONDITION CAN OCCUR WITH THE 3V DEVICE.
NOTE: TIME INTERVALS SHOWN ABOVE ARE REFERENCED IN WAKE-UP/KICKSTART SECTION.
Note 1:
Note 2:
Note 3:
Note 4:
Note 5:
Note 6:
Note 7:
Note 8:
Note 9:
Note 10:
All voltages are referenced to ground.
Typical values are at +25°C and nominal supplies.
Outputs are open.
Applies to the AD0–AD7 pins and the SQW pin when each is in a high-impedance state.
The IRQ and PWR pins are open-drain outputs.
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.
VSW is determined by the larger of VBAT and VBAUX.
The DS1687 keeps time to an accuracy of ±1 minute per month at 25°C during data retention time for the period of tDR.
tDR is the amount of time that the internal battery can power the internal oscillator and internal registers of the DS1687
at 25°C.
Note 11: RTC Encapsulate DIP 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. Post solder
cleaning with water washing techniques is acceptable, if ultrasonic vibration is not used.
Note 12: IBAT1 and IBAT2 are measured at VBAT or VBAUX = 3.5V and with recommended crystal type on X1 and X2.
32 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
PACKAGE INFORMATION
For the latest package outline information and land patterns, go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix
character, but the drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE
PACKAGE CODE
DOCUMENT NO.
24 PDIP
P24+3
21-0044
28 PLCC
Q28+4
21-0049
24 TSSOP
U24+1
21-0066
24 SO
W24+7
21-0042
24 EDIP
MDP24+2
21-0241
33 of 34
DS1685/DS1687 3V/5V Real-Time Clocks
REVISION HISTORY
REVISION
DATE
DESCRIPTION
4/10
Removed the leaded parts from the Ordering Information table;
updated the storage temperature ranges, added the lead temperature,
and updated the soldering temperature for all packages in the
Absolute Maximum Ratings.
10/12
Correct Silicon Model number, correct hex address formatting.
PAGES
CHANGED
2, 24
15, 19, 20, 21
34 of 34
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000
© 2012 Maxim Integrated Products, Inc.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.