Intersil ISL1208IU8 I2c real time clock/calendar Datasheet

ISL1208
®
I2C® Real Time Clock/Calendar
Data Sheet
July 29, 2005
Low Power RTC with Battery Backed
SRAM
Features
The ISL1208 device is a low power real time clock with
timing and crystal compensation, clock/calendar, power fail
indicator, periodic or polled alarm, intelligent battery backup
switching and battery-backed user SRAM.
The oscillator uses an external, low-cost 32.768kHz crystal.
The real time clock tracks time with separate registers for
hours, minutes, and seconds. The device has calendar
registers for date, month, year and day of the week. The
calendar is accurate through 2099, with automatic leap year
correction.
• Real Time Clock/Calendar
- Tracks Time in Hours, Minutes, and Seconds
- Day of the Week, Day, Month, and Year
• 15 Selectable Frequency Outputs
• Single Alarm
- Settable to the Second, Minute, Hour, Day of the Week,
Day, or Month
- Single Event or Pulse Interrupt Mode
• Automatic Backup to Battery or Super Cap
• Power Failure Detection
• On-Chip Oscillator Compensation
Ordering Information
PART NUMBER MARKING
FN8085.3
VDD
RANGE
TEMP.
RANGE
(°C)
• 2 Bytes Battery-Backed User SRAM
PACKAGE
• I2C Interface
- 400kHz Data Transfer Rate
ISL1208IU8
AGS
YWW
2.7V to
5.5V
-40 to
+85
8 Ld MSOP
ISL1208IU8-TK
AGS
YWW
2.7V to
5.5V
-40 to
+85
8 Ld MSOP
Tape and Reel
ISL1208IU8Z
(See Note)
ANW
YWW
2.7V to
5.5V
-40 to
+85
8 Ld MSOP
(Pb-free)
ISL1208IU8Z-TK
(See Note)
ANW
YWW
2.7V to
5.5V
-40 to
+85
8 Ld MSOP
Tape and Reel
(Pb-free)
ISL1208IB8
1208
YWW
2.7V to
5.5V
-40 to
+85
8 Ld SOIC
ISL1208IB8-TK
1208
YWW
2.7V to
5.5V
-40 to
+85
8 Ld SOIC
Tape and Reel
ISL1208IB8Z
(See Note)
1208
YWWZ
2.7V to
5.5V
-40 to
+85
8 Ld SOIC
(Pb-free)
• Set Top Box/Television
ISL1208IB8Z-TK
(See Note)
1208
YWWZ
2.7V to
5.5V
-40 to
+85
8 Ld SOIC
Tape and Reel
(Pb-free)
• Network Routers, Hubs, Switches, Bridges
NOTE: Intersil Pb-free plus anneal products employ special Pb-free material
sets; molding compounds/die attach materials and 100% matte tin plate
termination finish, which are RoHS compliant and compatible with both SnPb
and Pb-free soldering operations. Intersil Pb-free products are MSL classified
at Pb-free peak reflow temperatures that meet or exceed the Pb-free
requirements of IPC/JEDEC J STD-020.
* Contact Factory for availability.
• 400nA Battery Supply Current
• Same Pin Out as ST M41Txx and Maxim DS13xx Devices
• Small Package Options
- 8 Ld MSOP and SOIC Packages
• Pb-Free Plus Anneal Available (RoHS Compliant)
Applications
• Utility Meters
• HVAC Equipment
• Audio/Video Components
• Modems
• Cellular Infrastructure Equipment
• Fixed Broadband Wireless Equipment
• Pagers/PDA
• POS Equipment
• Test Meters/Fixtures
• Office Automation (Copiers, Fax)
Pinout
• Home Appliances
ISL1208
(8-PIN MSOP, SOIC)
TOP VIEW
• Computer Products
• Other Industrial/Medical/Automotive
X1
1
8
VDD
X2
2
7
IRQ/FOUT
VBAT
3
6
SCL
GND
4
5
SDA
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2004, 2005. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
ISL1208
Block Diagram
SDA
BUFFER
SDA
I2C
INTERFACE
SCL
BUFFER
SCL
Seconds
RTC
CONTROL
LOGIC
Minutes
Hours
Day of Week
X1
CRYSTAL
OSCILLATOR
X2
RTC
DIVIDER
Date
Month
VDD
POR
FREQUENCY
OUT
Year
ALARM
VTRIP
CONTROL
REGISTERS
USER
SRAM
SWITCH
IRQ/
FOUT
INTERNAL
SUPPLY
VBAT
Pin Descriptions
PIN
NUMBER
SYMBOL
DESCRIPTION
1
X1
The X1 pin is the input of an inverting amplifier and is intended to be connected to one pin of an external 32.768kHz
quartz crystal. X1 can also be driven directly from a 32.768kHz source.
2
X2
The X2 pin is the output of an inverting amplifier and is intended to be connected to one pin of an external 32.768kHz
quartz crystal.
3
VBAT
This input provides a backup supply voltage to the device. VBAT supplies power to the device in the event that the
VDD supply fails. This pin should be tied to ground if not used.
4
GND
Ground.
5
SDA
Serial Data (SDA) is a bidirectional pin used to transfer serial data into and out of the device. It has an open drain
output and may be wire OR’ed with other open drain or open collector outputs.
6
SCL
The Serial Clock (SCL) input is used to clock all serial data into and out of the device.
7
IRQ/FOUT
8
VDD
Interrupt Output/Frequency Output is a multi-functional pin that can be used as interrupt or frequency output pin. The
function is set via the configuration register.
Power supply.
2
FN8085.3
July 29, 2005
ISL1208
Absolute Maximum Ratings
Voltage on VDD, VBAT, SCL, SDA, and IRQ pins
(respect to ground) . . . . . . . . . . . . . . . . . . . . . . . . . . -0.5V to 7.0V
Voltage on X1 and X2 pins
(respect to ground) . . . . . . . . . . . .-0.5V to VDD + 0.5 (VDD Mode)
-0.5V to VBAT + 0.5 (VBAT Mode)
Storage Temperature . . . . . . . . . . . . . . . . . . . . . . . .-65°C to +150°C
Lead Temperature (Soldering, 10s) . . . . . . . . . . . . . . . . . . . . . 300°C
CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
DC Operating Characteristics – RTC Temperature = -40°C to +85°C, unless otherwise stated.
SYMBOL
PARAMETER
CONDITIONS
TYP
(Note 4)
MIN
MAX
UNITS
VDD
Main Power Supply
2.7
5.5
V
VBAT
Battery Supply Voltage
1.8
5.5
V
IDD1
Supply Current
IDD2
Supply Current With I2C Active
IDD3
IBAT
VDD = 5V
2
6
µA
VDD = 3V
1.2
4
µA
VDD = 5V
40
120
µA
1, 2
Supply Current (Low Power Mode)
VDD = 5V, LPMODE = 1
1.4
5
µA
1
Battery Supply Current
VBAT = 3V
400
950
nA
1
ILI
Input Leakage Current on SCL
100
nA
ILO
I/O Leakage Current on SDA
100
nA
VTRIP
NOTES
1, 2
VBAT Mode Threshold
1.6
2.2
2.6
V
VTRIPHYS
VTRIP Hysteresis
10
30
75
mV
VBATHYS
VBAT Hysteresis
15
50
100
mV
VDD = 5V
IOL = 3mA
0.4
V
VDD = 2.7V
IOL = 1mA
0.4
V
MAX
UNITS
NOTES
10
V/ms
3
IRQ/FOUT
VOL
Output Low Voltage
Power-Down Timing Temperature = -40°C to +85°C, unless otherwise stated.
SYMBOL
VDD SR-
PARAMETER
CONDITIONS
TYP
(Note 4)
MIN
VDD Negative Slewrate
Serial Interface Specifications Over the recommended operating conditions unless otherwise specified.
SYMBOL
PARAMETER
TEST CONDITIONS
MIN
TYP
(Note 4)
MAX
UNITS
NOTES
SERIAL INTERFACE SPECS
VIL
SDA and SCL input buffer LOW
voltage
-0.3
0.3 x
VDD
V
VIH
SDA and SCL input buffer HIGH
voltage
0.7 x
VDD
VDD +
0.3
V
SDA and SCL input buffer hysteresis
0.05 x
VDD
Hysteresis
VOL
SDA output buffer LOW voltage,
sinking 3mA
3
0
V
0.4
V
FN8085.3
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ISL1208
Serial Interface Specifications Over the recommended operating conditions unless otherwise specified. (Continued)
SYMBOL
PARAMETER
Cpin
SDA and SCL pin capacitance
fSCL
SCL frequency
TEST CONDITIONS
MIN
TA = 25°C, f = 1MHz, VDD = 5V,
VIN = 0V, VOUT = 0V
TYP
(Note 4)
MAX
UNITS
10
pF
400
kHz
tIN
Pulse width suppression time at SDA
and SCL inputs
Any pulse narrower than the max spec
is suppressed.
50
ns
tAA
SCL falling edge to SDA output data
valid
SCL falling edge crossing 30% of VDD,
until SDA exits the 30% to 70% of VDD
window.
900
ns
tBUF
Time the bus must be free before the
start of a new transmission
SDA crossing 70% of VDD during a
STOP condition, to SDA crossing 70%
of VDD during the following START
condition.
1300
ns
tLOW
Clock LOW time
Measured at the 30% of VDD crossing.
1300
ns
tHIGH
Clock HIGH time
Measured at the 70% of VDD crossing.
600
ns
tSU:STA
START condition setup time
SCL rising edge to SDA falling edge.
Both crossing 70% of VDD.
600
ns
tHD:STA
START condition hold time
From SDA falling edge crossing 30% of
VDD to SCL falling edge crossing 70%
of VDD.
600
ns
tSU:DAT
Input data setup time
From SDA exiting the 30% to 70% of
VDD window, to SCL rising edge
crossing 30% of VDD
100
ns
tHD:DAT
Input data hold time
From SCL falling edge crossing 30% of
VDD to SDA entering the 30% to 70%
of VDD window.
0
tSU:STO
STOP condition setup time
From SCL rising edge crossing 70% of
VDD, to SDA rising edge crossing 30%
of VDD.
600
ns
tHD:STO
STOP condition hold time
From SDA rising edge to SCL falling
edge. Both crossing 70% of VDD.
600
ns
Output data hold time
From SCL falling edge crossing 30% of
VDD, until SDA enters the 30% to 70%
of VDD window.
0
ns
tR
SDA and SCL rise time
From 30% to 70% of VDD
20 +
0.1 x Cb
300
ns
tF
SDA and SCL fall time
From 70% to 30% of VDD
20 +
0.1 x Cb
300
ns
Cb
Capacitive loading of SDA or SCL
Total on-chip and off-chip
10
400
pF
Rpu
SDA and SCL bus pull-up resistor offchip
Maximum is determined by tR and tF.
For Cb = 400pF, max is about 2~2.5kΩ.
For Cb = 40pF, max is about 15~20kΩ
1
tDH
900
NOTES
ns
kΩ
NOTES:
1. IRQ & FOUT Inactive.
2. LPMODE = 0 (default).
3. In order to ensure proper timekeeping, the VDD SR- specification must be followed.
4. Typical values are for T = 25°C and 3.3V supply voltage.
4
FN8085.3
July 29, 2005
ISL1208
SDA vs SCL Timing
tHIGH
tF
SCL
tLOW
tR
tSU:DAT
tSU:STA
tHD:DAT
tHD:STA
SDA
(INPUT TIMING)
tSU:STO
tAA
tDH
tBUF
SDA
(OUTPUT TIMING)
Symbol Table
WAVEFORM
INPUTS
OUTPUTS
Must be steady
Will be steady
May change
from LOW
to HIGH
Will change
from LOW
to HIGH
May change
from HIGH
to LOW
Will change
from HIGH
to LOW
Don’t Care:
Changes Allowed
Changing:
State Not Known
N/A
Center Line is
High Impedance
5
FN8085.3
July 29, 2005
ISL1208
Typical Performance Curves
Temperature is 25°C unless otherwise specified
1E-6
1E-6
900E-9
800E-9
800E-9
600E-9
600E-9
IBAT(A)
IBAT (A)
700E-9
500E-9
400E-9
400E-9
300E-9
200E-9
200E-9
100E-9
000E+0
1.5
2.0
2.5
3.0 3.5 4.0
VBAT (V)
4.5
5.0
000E+0
5.5
FIGURE 1. IBAT vs VBAT
-20
0
20
40
TEMPERATURE (°C)
60
80
FIGURE 2. IBAT vs TEMPERATURE AT VBAT = 3V
2.4E-6
2.4E-06
2.2E-6
2.2E-06
2.0E-6
VCC = 5V
2.0E-06
1.8E-6
1.8E-06
1.6E-06
LPMODE = 0
1.6E-6
IDD1 (A)
IDD1 (A)
-40
1.4E-6
LPMODE = 1
1.2E-6
VCC = 3.3V
1.0E-6
1.4E-06
800.0E-9
1.2E-06
600.0E-9
40
60
80
400.0E-9
2.5
3.0
3.5
4.0
TEMPERATURE (°C)
FIGURE 5. IDD1 vs FOUT AT VDD = 3.3V
6
5.5
4096
FOUT (Hz)
32768
4096
32768
64
1024
16
32
4
8
1
2
1/2
1/4
1/8
1/16
1.3E-6
64
1.4E-6
1024
1.5E-6
1
1.6E-6
1/2
1.7E-6
1/4
IDD1 (A)
1.8E-6
1/32
IDD1 (A)
1.9E-6
3.0E-6
2.9E-6
2.8E-6
2.7E-6
2.6E-6
2.5E-6
2.4E-6
2.3E-6
2.2E-6
2.1E-6
2.0E-6
1.9E-6
1.8E-6
1/8
2.0E-6
1/16
2.1E-6
FOUT (Hz)
5.0
FIGURE 4. IDD1 vs VCC WITH LPMODE ON & OFF
1/32
FIGURE 3. IDD1 vs TEMPERATURE
1.2E-6
4.5
VCC (V)
16
20
32
0
4
-20
8
-40
2
1.0E-06
FIGURE 6. IDD1 vs FOUT AT VDD = 5V
FN8085.3
July 29, 2005
ISL1208
EQUIVALENT AC OUTPUT LOAD CIRCUIT FOR VDD = 5V
5.0V
X1
1533Ω
SDA
AND
IRQ/FOUT
FOR VOL= 0.4V
X2
AND IOL = 3mA
100pF
FIGURE 8. RECOMMENDED CRYSTAL CONNECTION
VBAT
FIGURE 7. STANDARD OUTPUT LOAD FOR TESTING THE
DEVICE WITH VDD = 5.0V
General Description
The ISL1208 device is a low power real time clock with
timing and crystal compensation, clock/calendar, power fail
indicator, periodic or polled alarm, intelligent battery backup
switching, and battery-backed user SRAM.
The oscillator uses an external, low-cost 32.768kHz crystal.
The real time clock tracks time with separate registers for
hours, minutes, and seconds. The device has calendar
registers for date, month, year and day of the week. The
calendar is accurate through 2099, with automatic leap year
correction.
The ISL1208's powerful alarm can be set to any
clock/calendar value for a match. For example, every
minute, every Tuesday or at 5:23 AM on March 21. The
alarm status is available by checking the Status Register, or
the device can be configured to provide a hardware interrupt
via the IRQ pin. There is a repeat mode for the alarm
allowing a periodic interrupt every minute, every hour, every
day, etc.
The device also offers a backup power input pin. This VBAT
pin allows the device to be backed up by battery or
SuperCap with automatic switchover from VDD to VBAT. The
entire ISL1208 device is fully operational from 2.0V to 5.5V
and the clock/calendar portion of the device remains fully
operational down to 1.8V (Standby Mode).
Pin Description
X1, X2
The X1 and X2 pins are the input and output, respectively, of
an inverting amplifier. An external 32.768kHz quartz crystal
is used with the ISL1208 to supply a timebase for the real
time clock. Internal compensation circuitry provides high
accuracy over the operating temperature range from
-40°C to +85°C. This oscillator compensation network can
be used to calibrate the crystal timing accuracy over
temperature either during manufacturing or with an external
temperature sensor and microcontroller for active
compensation. The device can also be driven directly from a
32.768kHz source at pin X1.
7
This input provides a backup supply voltage to the device.
VBAT supplies power to the device in the event that the VDD
supply fails. This pin can be connected to a battery, a Super
Cap or tied to ground if not used.
IRQ/FOUT (Interrupt Output/Frequency Output)
This dual function pin can be used as an interrupt or
frequency output pin. The IRQ/FOUT mode is selected via
the frequency out control bits of the control/status register.
• Interrupt Mode. The pin provides an interrupt signal
output. This signal notifies a host processor that an alarm
has occurred and requests action. It is an open drain
active low output.
• Frequency Output Mode. The pin outputs a clock signal
which is related to the crystal frequency. The frequency
output is user selectable and enabled via the I2C bus. It is
an open drain active low output.
Serial Clock (SCL)
The SCL input is used to clock all serial data into and out of
the device. The input buffer on this pin is always active (not
gated). It is disabled when the backup power supply on the
VBAT pin is activated to minimize power consumption.
Serial Data (SDA)
SDA is a bidirectional pin used to transfer data into and out
of the device. It has an open drain output and may be ORed
with other open drain or open collector outputs. The input
buffer is always active (not gated) in normal mode.
An open drain output requires the use of a pull-up resistor.
The output circuitry controls the fall time of the output signal
with the use of a slope controlled pull-down. The circuit is
designed for 400kHz I2C interface speeds. It is disabled
when the backup power supply on the VBAT pin is activated.
VDD, GND
Chip power supply and ground pins. The device will operate
with a power supply from 2.0V to 5.5VDC. A 0.1µF capacitor
is recommended on the VDD pin to ground.
Functional Description
Power Control Operation
The power control circuit accepts a VDD and a VBAT input.
Many types of batteries can be used with Intersil RTC
products. For example, 3.0V or 3.6V Lithium batteries are
appropriate, and battery sizes are available that can power
FN8085.3
July 29, 2005
ISL1208
the ISL1208 for up to 10 years. Another option is to use a
Super Cap for applications where VDD is interrupted for up
to a month. See the Applications Section for more
information.
Normal Mode (VDD) to Battery Backup Mode
(VBAT)
To transition from the VDD to VBAT mode, both of the
following conditions must be met:
Condition 2:
VDD < VTRIP
where VTRIP ≈ 2.2V
Low Power Mode
Battery Backup Mode (VBAT) to Normal Mode
(VDD)
The ISL1208 device will switch from the VBAT to VDD mode
when one of the following conditions occurs:
Condition 1:
VDD > VBAT + VBATHYS
where VBATHYS ≈ 50mV
Condition 2:
VDD > VTRIP + VTRIPHYS
where VTRIPHYS ≈ 30mV
These power control situations are illustrated in Figures 9
and 10.
BATTERY BACKUP
MODE
VTRIP
2.2V
VBAT
1.8V
VBAT + VBATHYS
VBAT - VBATHYS
FIGURE 9. BATTERY SWITCHOVER WHEN VBAT < VTRIP
BATTERY BACKUP
MODE
VDD
Power Failure Detection
The ISL1208 provides a Real Time Clock Failure Bit (RTCF)
to detect total power failure. It allows users to determine if
the device has powered up after having lost all power to the
device (both VDD and VBAT).
Condition 1:
VDD < VBAT - VBATHYS
where VBATHYS ≈ 50mV
VDD
The I2C bus is deactivated in battery backup mode to provide
lower power. Aside from this, all RTC functions are
operational during battery backup mode. Except for SCL and
SDA, all the inputs and outputs of the ISL1208 are active
during battery backup mode unless disabled via the control
register. The User SRAM is operational in battery backup
mode down to 2V.
3.0V
VTRIP
2.2V
VTRIP + VTRIPHYS
FIGURE 10. BATTERY SWITCHOVER WHEN VBAT > VTRIP
8
Low Power Mode is useful in systems where VDD is normally
higher than VBAT at all times. The device will switch from
VDD to VBAT when VDD drops below VBAT, with about 50mV
of hysteresis to prevent any switchback of VDD after
switchover. In a system with a VDD = 5V and backup lithium
battery of VBAT = 3V, Low Power Mode can be used.
However, it is not recommended to use Low Power Mode in
a system with VDD = 3.3V ±10%, VBAT ≥ 3.0V, and when
there is a finite I-R voltage drop in the VDD line.
InterSeal™ Battery Saver
The ISL1208 has the InterSeal™ Battery Saver which
prevents initial battery current drain before it is first used. For
example, battery-backed RTCs are commonly packaged on
a board with a battery connected. In order to preserve
battery life, the ISL1208 will not draw any power from the
battery source until after the device is first powered up from
the VDD source. Thereafter, the device will switchover to
battery backup mode whenever VDD power is lost.
Real Time Clock Operation
VBAT
VTRIP
The normal power switching of the ISL1208 is designed to
switch into battery backup mode only if the VDD power is
lost. This will ensure that the device can accept a wide range
of backup voltages from many types of sources while reliably
switching into backup mode. Another mode, called Low
Power Mode, is available to allow direct switching from VDD
to VBAT without requiring VDD to drop below VTRIP. Since
the additional monitoring of VDD vs VTRIP is no longer
needed, that circuitry is shut down and less power is used
while operating from VDD. Power savings are typically
600nA at VDD = 5V. Low Power Mode is activated via the
LPMODE bit in the control and status registers.
The Real Time Clock (RTC) uses an external 32.768kHz
quartz crystal to maintain an accurate internal representation
of second, minute, hour, day of week, date, month, and year.
The RTC also has leap-year correction. The clock also
corrects for months having fewer than 31 days and has a bit
that controls 24 hour or AM/PM format. When the ISL1208
powers up after the loss of both VDD and VBAT, the clock will
FN8085.3
July 29, 2005
ISL1208
not begin incrementing until at least one byte is written to the
clock register.
Accuracy of the Real Time Clock
The accuracy of the Real Time Clock depends on the
frequency of the quartz crystal that is used as the time base
for the RTC. Since the resonant frequency of a crystal is
temperature dependent, the RTC performance will also be
dependent upon temperature. The frequency deviation of
the crystal is a function of the turnover temperature of the
crystal from the crystal’s nominal frequency. For example, a
~20ppm frequency deviation translates into an accuracy of
~1 minute per month. These parameters are available from
the crystal manufacturer. The ISL1208 provides on-chip
crystal compensation networks to adjust load capacitance to
tune oscillator frequency from -94ppm to +140ppm. For
more detailed information see the Application Section.
Single Event and Interrupt
The alarm mode is enabled via the ALME bit. Choosing
single event or interrupt alarm mode is selected via the IM
bit. Note that when the frequency output function is enabled,
the alarm function is disabled.
The standard alarm allows for alarms of time, date, day of
the week, month, and year. When a time alarm occurs in
single event mode, an IRQ pin will be pulled low and the
alarm status bit (ALM) will be set to “1”.
The pulsed interrupt mode allows for repetitive or recurring
alarm functionality. Hence, once the alarm is set, the device
will continue to alarm for each occurring match of the alarm
and present time. Thus, it will alarm as often as every minute
(if only the nth second is set) or as infrequently as once a
year (if at least the nth month is set). During pulsed interrupt
mode, the IRQ pin will be pulled low for 250ms and the alarm
status bit (ALM) will be set to “1”.
NOTE: The ALM bit can be reset by the user or cleared
automatically using the auto reset mode (see ARST bit).
The alarm function can be enabled/disabled during battery
backup mode using the FOBATB bit. For more information
on the alarm, please see the Alarm Registers Description.
I2C Serial Interface
The ISL1208 has an I2C serial bus interface that provides
access to the control and status registers and the user
SRAM. The I2C serial interface is compatible with other
industry I2C serial bus protocols using a bidirectional data
signal (SDA) and a clock signal (SCL).
Oscillator Compensation
The ISL1208 provides the option of timing correction due to
temperature variation of the crystal oscillator for either
manufacturing calibration or active calibration. The total
possible compensation is typically -94ppm to +140ppm. Two
compensation mechanisms that are available are as follows:
1. An analog trimming (ATR) register that can be used to
adjust individual on-chip digital capacitors for oscillator
capacitance trimming. The individual digital capacitor is
selectable from a range of 9pF to 40.5pF (based upon
32.758kHz). This translates to a calculated
compensation of approximately -34ppm to +80ppm. (See
ATR description.)
2. A digital trimming register (DTR) that can be used to
adjust the timing counter by ±60ppm. (See DTR
description.)
Also provided is the ability to adjust the crystal capacitance
when the ISL1208 switches from VDD to battery backup
mode. (See Battery Mode ATR Selection for more details.)
Register Descriptions
The battery-backed registers are accessible following a
slave byte of “1101111x” and reads or writes to addresses
[00h:13h]. The defined addresses and default values are
described in the Table 1. Address 09h is not used. Reads or
writes to 09h will not affect operation of the device but should
be avoided.
REGISTER ACCESS
The contents of the registers can be modified by performing
a byte or a page write operation directly to any register
address.
The registers are divided into 4 sections. These are:
Frequency Output Mode
1. Real Time Clock (7 bytes): Address 00h to 06h.
The ISL1208 has the option to provide a frequency output
signal using the IRQ/FOUT pin. The frequency output mode
is set by using the FO bits to select 15 possible output
frequency values from 0 to 32kHz. The frequency output can
be enabled/disabled during battery backup mode using the
FOBATB bit.
2. Control and Status (5 bytes): Address 07h to 0Bh.
3. Alarm (6 bytes): Address 0Ch to 11h.
4. User SRAM (2 bytes): Address 12h to 13h.
There are no addresses above 13h.
General Purpose User SRAM
The ISL1208 provides 2 bytes of user SRAM. The SRAM will
continue to operate in battery backup mode. However, it
should be noted that the I2C bus is disabled in battery
backup mode.
9
FN8085.3
July 29, 2005
ISL1208
instruction latches all clock registers into a buffer, so an
update of the clock does not change the time being read. A
sequential read will not result in the output of data from the
memory array. At the end of a read, the master supplies a
stop condition to end the operation and free the bus. After a
read, the address remains at the previous address +1 so the
user can execute a current address read and continue
reading the next register.
Write capability is allowable into the RTC registers (00h to
06h) only when the WRTC bit (bit 4 of address 07h) is set to
“1”. A multi-byte read or write operation is limited to one
section per operation. Access to another section requires a
new operation. A read or write can begin at any address
within the section.
A register can be read by performing a random read at any
address at any time. This returns the contents of that register
location. Additional registers are read by performing a
sequential read. For the RTC and Alarm registers, the read
It is not necessary to set the WRTC bit prior to writing into
the control and status, alarm, and user SRAM registers.
TABLE 1. REGISTER MEMORY MAP
BIT
REG
ADDR. SECTION NAME
7
6
5
4
3
2
1
0
RANGE
DEFAULT
00h
SC
0
SC22
SC21
SC20
SC13
SC12
SC11
SC10
0-59
00h
01h
MN
0
MN22
MN21
MN20
MN13
MN12
MN11
MN10
0-59
00h
02h
HR
MIL
0
HR21
HR20
HR13
HR12
HR11
HR10
0-23
00h
DT
0
0
DT21
DT20
DT13
DT12
DT11
DT10
1-31
00h
04h
MO
0
0
0
MO20
MO13
MO12
MO11
MO10
1-12
00h
05h
YR
YR23
YR22
YR21
YR20
YR13
YR12
YR11
YR10
0-99
00h
06h
DW
0
0
0
0
0
DW2
DW1
DW0
0-6
00h
07h
SR
ARST
WRTC
Reserved
ALM
BAT
RTCF
N/A
01h
INT
IM
FOBATB
FO3
FO2
FO1
FO0
N/A
00h
N/A
00h
03h
08h
09h
0Ah
RTC
Control
and
Status
XTOSCB Reserved
ALME
LPMODE
Reserved
ATR
BMATR1
0Bh
DTR
Reserved
0Ch
SCA
ESCA
ASC22
ASC21
ASC20
0Dh
MNA
EMNA
AMN22
AMN21
HRA
EHRA
0
DTA
EDTA
10h
MOA
11h
0Eh
0Fh
12h
13h
Alarm
User
ATR2
ATR1
ATR0
N/A
00h
DTR2
DTR1
DTR0
N/A
00h
ASC13
ASC12
ASC11
ASC10
00-59
00h
AMN20
AMN13
AMN12
AMN11
AMN10
00-59
00h
AHR21
AHR20
AHR13
AHR12
AHR11
AHR10
0-23
00h
0
ADT21
ADT20
ADT13
ADT12
ADT11
ADT10
1-31
00h
EMOA
0
0
AMO20
AMO13
AMO12
AMO11
AMO10
1-12
00h
DWA
EDWA
0
0
0
0
ADW12
ADW11
ADW10
0-6
00h
USR1
USR17
USR16
USR15
USR14
USR13
USR12
USR11
USR10
N/A
00h
USR2
USR27
USR26
USR25
USR24
USR23
USR22
USR21
USR20
N/A
00h
10
BMATR0
ATR5
ATR4
ATR3
FN8085.3
July 29, 2005
ISL1208
Real Time Clock Registers
REAL TIME CLOCK FAIL BIT (RTCF)
Addresses [00h to 06h]
RTC REGISTERS (SC, MN, HR, DT, MO, YR, DW)
These registers depict BCD representations of the time. As
such, SC (Seconds) and MN (Minutes) range from 0 to 59,
HR (Hour) can either be a 12-hour or 24-hour mode, DT
(Date) is 1 to 31, MO (Month) is 1 to 12, YR (Year) is 0 to 99,
and DW (Day of the Week) is 0 to 6.
This bit is set to a “1” after a total power failure. This is a read
only bit that is set by hardware (ISL1208 internally) when the
device powers up after having lost all power to the device.
The bit is set regardless of whether VDD or VBAT is applied
first. The loss of only one of the supplies does not set the
RTCF bit to “1”. The first valid write to the RTC section after
a complete power failure resets the RTCF bit to “0” (writing
one byte is sufficient).
The DW register provides a Day of the Week status and uses
three bits DW2 to DW0 to represent the seven days of the
week. The counter advances in the cycle 0-1-2-3-4-5-6-0-12-… The assignment of a numerical value to a specific day
of the week is arbitrary and may be decided by the system
software designer. The default value is defined as “0”.
BATTERY BIT (BAT)
24 HOUR TIME
These bits announce if the alarm matches the real time
clock. If there is a match, the respective bit is set to “1”. This
bit can be manually reset to “0” by the user or automatically
reset by enabling the auto-reset bit (see ARST bit). A write to
this bit in the SR can only set it to “0”, not “1”.
If the MIL bit of the HR register is “1”, the RTC uses a 24hour format. If the MIL bit is “0”, the RTC uses a 12-hour
format and HR21 bit functions as an AM/PM indicator with a
“1” representing PM. The clock defaults to 12-hour format
time with HR21 = “0”.
LEAP YEARS
Leap years add the day February 29 and are defined as those
years that are divisible by 4. Years divisible by 100 are not leap
years, unless they are also divisible by 400. This means that
the year 2000 is a leap year, the year 2100 is not. The ISL1208
does not correct for the leap year in the year 2100.
Control and Status Registers
Addresses [07h to 0Bh]
The Control and Status Registers consist of the Status
Register, Interrupt and Alarm Register, Analog Trimming and
Digital Trimming Registers.
Status Register (SR)
The Status Register is located in the memory map at
address 07h. This is a volatile register that provides either
control or status of RTC failure, battery mode, alarm trigger,
write protection of clock counter, crystal oscillator enable and
auto reset of status bits.
TABLE 2. STATUS REGISTER (SR)
ADDR
07h
Default
7
6
5
4
3
2
1
0
ARST XTOSCB reserved WRTC reserved ALM BAT RTCF
0
0
0
0
0
0
0
0
This bit is set to a “1” when the device enters battery backup
mode. This bit can be reset either manually by the user or
automatically reset by enabling the auto-reset bit (see ARST
bit). A write to this bit in the SR can only set it to “0”, not “1”.
ALARM BIT (ALM)
NOTE: An alarm bit that is set by an alarm occurring during an SR
read operation will remain set after the read operation is complete.
WRITE RTC ENABLE BIT (WRTC)
The WRTC bit enables or disables write capability into the
RTC Timing Registers. The factory default setting of this bit
is “0”. Upon initialization or power up, the WRTC must be set
to “1” to enable the RTC. Upon the completion of a valid
write (STOP), the RTC starts counting. The RTC internal
1Hz signal is synchronized to the STOP condition during a
valid write cycle.
CRYSTAL OSCILLATOR ENABLE BIT (XTOSCB)
This bit enables/disables the internal crystal oscillator. When
the XTOSCB is set to “1”, the oscillator is disabled, and the
X1 pin allows for an external 32kHz signal to drive the RTC.
The XTOSCB bit is set to “0” on powerup.
AUTO RESET ENABLE BIT (ARST)
This bit enables/disables the automatic reset of the BAT and
ALM status bits only. When ARST bit is set to “1”, these
status bits are reset to “0” after a valid read of the respective
status register (with a valid STOP condition). When the
ARST is cleared to “0”, the user must manually reset the
BAT and ALM bits.
Interrupt Control Register (INT)
TABLE 3. INTERRUPT CONTROL REGISTER (INT)
ADDR
11
7
08h
IM
Default
0
6
5
4
3
2
1
0
ALME LPMODE FOBATB FO3 FO2 FO1 FO0
0
0
0
0
0
0
0
FN8085.3
July 29, 2005
ISL1208
FREQUENCY OUT CONTROL BITS (FO <3:0>)
These bits enable/disable the frequency output function and
select the output frequency at the IRQ/FOUT pin. See
Table 4 for frequency selection. When the frequency mode is
enabled, it will override the alarm mode at the IRQ/FOUT pin.
NOTE: When the frequency output mode is enabled, the alarm function
is disabled.
INTERRUPT/ALARM MODE BIT (IM)
TABLE 4. FREQUENCY SELECTION OF FOUT PIN
FREQUENCY,
FOUT
UNITS
is cleared to “0”, the alarm function is disabled. The alarm
function can operate in either a single event alarm or a periodic
interrupt alarm (see IM bit).
FO3
FO2
FO1
FO0
0
Hz
0
0
0
0
32768
Hz
0
0
0
1
4096
Hz
0
0
1
0
1024
Hz
0
0
1
1
64
Hz
0
1
0
0
32
Hz
0
1
0
1
16
Hz
0
1
1
0
8
Hz
0
1
1
1
4
Hz
1
0
0
0
2
Hz
1
0
0
1
1
Hz
1
0
1
0
1/2
Hz
1
0
1
1
1/4
Hz
1
1
0
0
1/8
Hz
1
1
0
1
1/16
Hz
1
1
1
0
1/32
Hz
1
1
1
1
FREQUENCY OUTPUT AND INTERRUPT BIT (FOBATB)
This bit enables/disables the FOUT/IRQ pin during battery
backup mode (i.e. VBAT power source active). When the
FOBATB is set to “1” the FOUT/IRQ pin is disabled during
battery backup mode. This means that both the frequency
output and alarm output functions are disabled. When the
FOBATB is cleared to “0”, the FOUT/IRQ pin is enabled
during battery backup mode.
LOW POWER MODE BIT (LPMODE)
This bit enables/disables low power mode. With
LPMODE = “0”, the device will be in normal mode and the
VBAT supply will be used when VDD < VBAT - VBATHYS and
VDD < VTRIP. With LPMODE = “1”, the device will be in low
power mode and the VBAT supply will be used when
VDD < VBAT - VBATHYS. There is a supply current saving of
about 600nA when using LPMODE = “1” with VDD = 5V.
(See Typical Performance Curves: IDD vs VCC with
LPMODE ON & OFF.)
ALARM ENABLE BIT (ALME)
This bit enables/disables the interrupt mode of the alarm
function. When the IM bit is set to “1”, the alarm will operate
in the interrupt mode, where an active low pulse width of
250ms will appear at the IRQ/FOUT pin when the RTC is
triggered by the alarm as defined by the alarm registers (0Ch
to 11h). When the IM bit is cleared to “0”, the alarm will
operate in standard mode, where the IRQ/FOUT pin will be
tied low until the ALM status bit is cleared to “0”.
IM BIT
INTERRUPT/ALARM FREQUENCY
0
Single Time Event Set By Alarm
1
Repetitive/Recurring Time Event Set By Alarm
Analog Trimming Register
ANALOG TRIMMING REGISTER (ATR<5:0>)
X1
CX1
Crystal
Oscillator
X2
CX2
FIGURE 11. DIAGRAM OF ATR
Six analog trimming bits, ATR0 to ATR5, are provided in
order to adjust the on-chip load capacitance value for
frequency compensation of the RTC. Each bit has a different
weight for capacitance adjustment. For example, using a
Citizen CFS-206 crystal with different ATR bit combinations
provides an estimated ppm adjustment range from -34 to
+80ppm to the nominal frequency compensation. The
combination of analog and digital trimming can give up to -94
to +140ppm of total adjustment.
The effective on-chip series load capacitance, CLOAD,
ranges from 4.5pF to 20.25pF with a mid-scale value of
12.5pF (default). CLOAD is changed via two digitally
controlled capacitors, CX1 and CX2, connected from the X1
This bit enables/disables the alarm function. When the ALME
bit is set to “1”, the alarm function is enabled. When the ALME
12
FN8085.3
July 29, 2005
ISL1208
and X2 pins to ground (see Figure 11). The value of CX1 and
CX2 is given by the following formula:
C
X
= ( 16 ⋅ b5 + 8 ⋅ b4 + 4 ⋅ b3 + 2 ⋅ b2 + 1 ⋅ b1 + 0.5 ⋅ b0 + 9 )pF
The effective series load capacitance is the combination of
CX1 and CX2:
1
C
= ----------------------------------LOAD
1
1
 ---------- + -----------
C

X1 C X2
C
LOAD
16 ⋅ b5 + 8 ⋅ b4 + 4 ⋅ b3 + 2 ⋅ b2 + 1 ⋅ b1 + 0.5 ⋅ b0 + 9
=  ----------------------------------------------------------------------------------------------------------------------------- pF


2
For example, CLOAD(ATR=00000) = 12.5pF,
CLOAD(ATR=100000) = 4.5pF, and CLOAD(ATR=011111) =
20.25pF. The entire range for the series combination of load
capacitance goes from 4.5pF to 20.25pF in 0.25pF steps.
Note that these are typical values.
BATTERY MODE ATR SELECTION (BMATR <1:0>)
Since the accuracy of the crystal oscillator is dependent on
the VDD/VBAT operation, the ISL1208 provides the capability
to adjust the capacitance between VDD and VBAT when the
device switches between power sources.
DELTA
CAPACITANCE
(CBAT TO CVDD)
BMATR1
BMATR0
0
0
0pF
0
1
-0.5pF (≈ +2ppm)
1
0
+0.5pF (≈ -2ppm)
1
1
+1pF (≈ -4ppm)
DIGITAL TRIMMING REGISTER (DTR <2:0>)
The digital trimming bits DTR0, DTR1, and DTR2 adjust the
average number of counts per second and average the ppm
error to achieve better accuracy.
• DTR2 is a sign bit. DTR2 = “0” means frequency
compensation is >0. DTR2 = “1” means frequency
compensation is <0.
• DTR1 and DTR0 are both scale bits. DTR1 gives 40ppm
adjustment and DTR0 gives 20ppm adjustment.
A range from -60ppm to +60ppm can be represented by
using these three bits (see Table 5).
TABLE 5. DIGITAL TRIMMING REGISTERS
DTR REGISTER
DTR2
DTR1
DTR0
ESTIMATED
FREQUENCY
PPM
0
0
0
0 (default)
0
0
1
+20
0
1
0
+40
0
1
1
+60
1
0
0
0
1
0
1
-20
1
1
0
-40
1
1
1
-60
Alarm Registers
Addresses [0Ch to 11h]
The alarm register bytes are set up identical to the RTC
register bytes, except that the MSB of each byte functions as
an enable bit (enable = “1”). These enable bits specify which
alarm registers (seconds, minutes, etc) are used to make the
comparison. Note that there is no alarm byte for year.
The alarm function works as a comparison between the
alarm registers and the RTC registers. As the RTC
advances, the alarm will be triggered once a match occurs
between the alarm registers and the RTC registers. Any one
alarm register, multiple registers, or all registers can be
enabled for a match.
There are two alarm operation modes: Single Event and
periodic Interrupt Mode:
• Single Event Mode is enabled by setting the ALME bit to
“1”, the IM bit to “0”, and disabling the frequency output.
This mode permits a one-time match between the alarm
registers and the RTC registers. Once this match occurs,
the ALM bit is set to “1” and the IRQ output will be pulled
low and will remain low until the ALM bit is reset. This can
be done manually or by using the auto-reset feature.
• Interrupt Mode is enabled by setting the ALME bit to “1”,
the IM bit to “1”, and disabling the frequency output. The
IRQ output will now be pulsed each time an alarm occurs.
This means that once the interrupt mode alarm is set, it
will continue to alarm for each occurring match of the
alarm and present time. This mode is convenient for
hourly or daily hardware interrupts in microcontroller
applications such as security cameras or utility meter
reading.
To clear an alarm, the ALM bit in the status register must be
set to “0” with a write. Note that if the ARST bit is set to 1
(address 07h, bit 7), the ALM bit will automatically be cleared
when the status register is read.
13
FN8085.3
July 29, 2005
ISL1208
Below are examples of both Single Event and periodic
Interrupt Mode alarms.
Example 1 – Alarm set with single interrupt (IM=”0”)
Once the registers are set, the following waveform will be
seen at IRQ-:
RTC and alarm registers are both “30” sec
A single alarm will occur on January 1 at 11:30am.
A. Set Alarm registers as follows:
ALARM
REGISTER 7
BIT
6
5
4
3
2
1
0
HEX
SCA
0
0
0
0
0
0
0
0
00h Seconds disabled
DESCRIPTION
MNA
1
0
1
1
0
0
0
0
B0h Minutes set to 30,
enabled
Note that the status register ALM bit will be set each time the
alarm is triggered, but does not need to be read or cleared.
HRA
1
0
0
1
0
0
0
1
91h Hours set to 11,
enabled
User Registers
DTA
1
0
0
0
0
0
0
1
81h Date set to 1,
enabled
MOA
1
0
0
0
0
0
0
1
81h Month set to 1,
enabled
DWA
0
0
0
0
0
0
0
0
00h Day of week
disabled
B. Also the ALME bit must be set as follows:
CONTROL
REGISTER 7
INT
0
BIT
6
5
4
3
2
1
0
HEX
1
x
x
0
0
0
0
x0h
DESCRIPTION
Enable Alarm
xx indicate other control bits
After these registers are set, an alarm will be generated when
the RTC advances to exactly 11:30am on January 1 (after
seconds changes from 59 to 00) by setting the ALM bit in the
status register to “1” and also bringing the IRQ output low.
Example 2 – Pulsed interrupt once per minute (IM=”1”)
Interrupts at one minute intervals when the seconds register
is at 30 seconds.
A. Set Alarm registers as follows:
BIT
ALARM
REGISTER 7 6 5 4 3 2 1 0 HEX
SCA
DESCRIPTION
1 0 1 1 0 0 0 0 B0h Seconds set to 30,
enabled
MNA
0 0 0 0 0 0 0 0 00h Minutes disabled
HRA
0 0 0 0 0 0 0 0 00h Hours disabled
DTA
0 0 0 0 0 0 0 0 00h Date disabled
MOA
0 0 0 0 0 0 0 0 00h Month disabled
DWA
0 0 0 0 0 0 0 0 00h Day of week disabled
B. Set the Interrupt register as follows:
BIT
CONTROL
REGISTER 7 6 5 4 3 2 1 0 HEX
INT
DESCRIPTION
1 1 x x 0 0 0 0 x0h Enable Alarm and Int
Mode
60 sec
Addresses [12h to 13h]
These registers are 2 bytes of battery-backed user memory
storage.
I2C Serial Interface
The ISL1208 supports a bidirectional bus oriented protocol.
The protocol defines any device that sends data onto the
bus as a transmitter and the receiving device as the receiver.
The device controlling the transfer is the master and the
device being controlled is the slave. The master always
initiates data transfers and provides the clock for both
transmit and receive operations. Therefore, the ISL1208
operates as a slave device in all applications.
All communication over the I2C interface is conducted by
sending the MSB of each byte of data first.
Protocol Conventions
Data states on the SDA line can change only during SCL
LOW periods. SDA state changes during SCL HIGH are
reserved for indicating START and STOP conditions (See
Figure 12). On power up of the ISL1208, the SDA pin is in
the input mode.
All I2C interface operations must begin with a START
condition, which is a HIGH to LOW transition of SDA while
SCL is HIGH. The ISL1208 continuously monitors the SDA
and SCL lines for the START condition and does not
respond to any command until this condition is met (See
Figure 12). A START condition is ignored during the powerup sequence.
All I2C interface operations must be terminated by a STOP
condition, which is a LOW to HIGH transition of SDA while
SCL is HIGH (See Figure 12). A STOP condition at the end
of a read operation or at the end of a write operation to
memory only places the device in its standby mode.
An acknowledge (ACK) is a software convention used to
indicate a successful data transfer. The transmitting device,
either master or slave, releases the SDA bus after
transmitting eight bits. During the ninth clock cycle, the
xx indicate other control bits
14
FN8085.3
July 29, 2005
ISL1208
Byte of a write operation. The master must respond with an
ACK after receiving a Data Byte of a read operation.
receiver pulls the SDA line LOW to acknowledge the
reception of the eight bits of data (See Figure 13).
The ISL1208 responds with an ACK after recognition of a
START condition followed by a valid Identification Byte, and
once again after successful receipt of an Address Byte. The
ISL1208 also responds with an ACK after receiving a Data
SCL
SDA
DATA
STABLE
START
DATA
CHANGE
DATA
STABLE
STOP
FIGURE 12. VALID DATA CHANGES, START, AND STOP CONDITIONS
SCL FROM
MASTER
1
8
9
SDA OUTPUT FROM
TRANSMITTER
HIGH IMPEDANCE
HIGH IMPEDANCE
SDA OUTPUT FROM
RECEIVER
START
ACK
FIGURE 13. ACKNOWLEDGE RESPONSE FROM RECEIVER
WRITE
SIGNALS FROM
THE MASTER
SIGNAL AT SDA
SIGNALS FROM
THE ISL1208
S
T
A
R
T
ADDRESS
BYTE
IDENTIFICATION
BYTE
1 1 0 1 1 1 1 0
S
T
O
P
DATA
BYTE
0 0 0 0
A
C
K
A
C
K
A
C
K
FIGURE 14. BYTE WRITE SEQUENCE
15
FN8085.3
July 29, 2005
ISL1208
Device Addressing
Write Operation
Following a start condition, the master must output a Slave
Address Byte. The 7 MSBs are the device identifier. These
bits are “1101111”. Slave bits “1101” access the register.
Slave bits “111” specify the device select bits.
A Write operation requires a START condition, followed by a
valid Identification Byte, a valid Address Byte, a Data Byte,
and a STOP condition. After each of the three bytes, the
ISL1208 responds with an ACK. At this time, the I2C
interface enters a standby state.
The last bit of the Slave Address Byte defines a read or write
operation to be performed. When this R/W bit is a “1”, then a
read operation is selected. A “0” selects a write operation
(Refer to Figure 15).
After loading the entire Slave Address Byte from the SDA
bus, the ISL1208 compares the device identifier and device
select bits with “1101111”. Upon a correct compare, the
device outputs an acknowledge on the SDA line.
Following the Slave Byte is a one byte word address. The
word address is either supplied by the master device or
obtained from an internal counter. On power up the internal
address counter is set to address 0h, so a current address
read of the CCR array starts at address 0h. When required,
as part of a random read, the master must supply the 1 Word
Address Bytes as shown in Figure 16.
In a random read operation, the slave byte in the “dummy
write” portion must match the slave byte in the “read”
section. For a random read of the Clock/Control Registers,
the slave byte must be “1101111x” in both places.
R/W
SLAVE
ADDRESS BYTE
A1
A0
WORD ADDRESS
D1
D0
DATA BYTE
1
1
0
1
1
1
1
A7
A6
A5
A4
A3
A2
D7
D6
D5
D4
D3
D2
Read Operation
A Read operation consists of a three byte instruction
followed by one or more Data Bytes (See Figure 16). The
master initiates the operation issuing the following
sequence: a START, the Identification byte with the R/W bit
set to “0”, an Address Byte, a second START, and a second
Identification byte with the R/W bit set to “1”. After each of
the three bytes, the ISL1208 responds with an ACK. Then
the ISL1208 transmits Data Bytes as long as the master
responds with an ACK during the SCL cycle following the
eighth bit of each byte. The master terminates the read
operation (issuing a STOP condition) following the last bit of
the last Data Byte (See Figure 16).
The Data Bytes are from the memory location indicated by
an internal pointer. This pointer initial value is determined by
the Address Byte in the Read operation instruction, and
increments by one during transmission of each Data Byte.
After reaching the memory location 13h the pointer “rolls
over” to 00h, and the device continues to output data for
each ACK received.
FIGURE 15. SLAVE ADDRESS, WORD ADDRESS, AND DATA
BYTES
SIGNALS
FROM THE
MASTER
S
T
A
R
T
SIGNAL AT
SDA
IDENTIFICATION
BYTE WITH
R/W=0
S
T IDENTIFICATION
A
BYTE WITH
R
R/W = 1
T
ADDRESS
BYTE
S
T
O
P
A
C
K
1 1 0 1 1 1 1 1
1 1 0 1 1 1 1 0
A
C
K
SIGNALS FROM
THE SLAVE
A
C
K
A
C
K
A
C
K
FIRST READ
DATA BYTE
LAST READ
DATA BYTE
FIGURE 16. READ SEQUENCE
16
FN8085.3
July 29, 2005
ISL1208
Application Section
Oscillator Crystal Requirements
The ISL1208 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table 6
lists some recommended surface mount crystals and the
parameters of each. This list is not exhaustive and other
surface mount devices can be used with the ISL1208 if their
specifications are very similar to the devices listed. The
crystal should have a required parallel load capacitance of
12.5pF and an equivalent series resistance of less than 50k.
The crystal’s temperature range specification should match
the application. Many crystals are rated for -10°C to +60°C
(especially through hole and tuning fork types), so an
appropriate crystal should be selected if extended
temperature range is required.
TABLE 6. SUGGESTED SURFACE MOUNT CRYSTALS
In addition to the analog compensation afforded by the
adjustable load capacitance, a digital compensation feature
is available for the ISL1208. There are 3 bits known as the
Digital Trimming Register (DTR). The range provided is
±60ppm in increments of 20ppm. DTR operates by adding or
skipping pulses in the clock counter. It is very useful for
coarse adjustments of frequency drift over temperature or
extending the adjustment range available with the ATR
register.
Initial accuracy is best adjusted by enabling the frequency
output (using the INT register, address 08h), and monitoring
the ~IRQ/FOUT pin with a calibrated frequency counter.
The frequency used is unimportant, although 1Hz is the
easiest to monitor. The gating time should be set long
enough to ensure accuracy to at least 1ppm. The ATR
should be set to the center position, or 100000Bh, to begin
with. Once the initial measurement is made, then the ATR
register can be changed to adjust the frequency. Note that
increasing the ATR register for increased capacitance will
lower the frequency, and vice-versa. If the initial
measurement shows the frequency is far off, it will be
necessary to use the DTR register to do a coarse
adjustment. Note that most all crystals will have tight enough
initial accuracy at room temperature so that a small ATR
register adjustment should be all that is needed.
MANUFACTURER
PART NUMBER
Citizen
CM200S
Epson
MC-405, MC-406
Raltron
RSM-200S
SaRonix
32S12
Ecliptek
ECPSM29T-32.768K
ECS
ECX-306
Temperature Compensation
Fox
FSM-327
The ATR and DTR controls can be combined to provide
crystal drift temperature compensation. The typical
32.768kHz crystal has a drift characteristic that is similar to
that shown in Figure 17. There is a turnover temperature
(T0) where the drift is very near zero. The shape is parabolic
as it varies with the square of the difference between the
actual temperature and the turnover temperature.
The ISL1208 device contains circuitry for adjusting the
frequency of the crystal oscillator. This circuitry can be used
to trim oscillator initial accuracy as well as adjust the
frequency to compensate for temperature changes.
The Analog Trimming Register (ATR) is used to adjust the
load capacitance seen by the crystal. There are six bits of
ATR control, with linear capacitance increments available for
adjustment. Since the ATR adjustment is essentially “pulling”
the frequency of the oscillator, the resulting frequency
changes will not be linear with incremental capacitance
changes. The equations which govern pulling show that
lower capacitor values of ATR adjustment will provide larger
increments. Also, the higher values of ATR adjustment will
produce smaller incremental frequency changes. These
values typically vary from 6-10 ppm/bit at the low end to
<1ppm/bit at the highest capacitance settings. The range
afforded by the ATR adjustment with a typical surface mount
crystal is typically -34 to +80ppm around the ATR=0 default
setting because of this property. The user should note this
when using the ATR for calibration. The temperature drift of
the capacitance used in the ATR control is extremely low, so
this feature can be used for temperature compensation with
good accuracy.
17
0.0
-20.0
-40.0
-60.0
PPM
Crystal Oscillator Frequency Adjustment
-80.0
-100.0
-120.0
-140.0
-160.0
-40 -30 -20 -10 0
10 20 30 40 50 60 70 80
TEMPERATURE (°C)
FIGURE 17. RTC CRYSTAL TEMPERATURE DRIFT
If full industrial temperature compensation is desired in an
ISL1208 circuit, then both the DTR and ATR registers will
need to be utilized (total correction range = -94 to +140ppm).
FN8085.3
July 29, 2005
ISL1208
A system to implement temperature compensation would
consist of the ISL1208, a temperature sensor, and a
microcontroller. These devices may already be in the system
so the function will just be a matter of implementing software
and performing some calculations. Fairly accurate
temperature compensation can be implemented just by
using the crystal manufacturer’s specifications for the
turnover temperature T0 and the drift coefficient (β). The
formula for calculating the oscillator adjustment necessary
is:
Do not run the serial bus lines or any high speed logic lines
in the vicinity of the crystal. These logic level lines can
induce noise in the oscillator circuit to cause misclocking.
Add a ground trace around the crystal with one end
terminated at the chip ground. This will provide termination
for emitted noise in the vicinity of the RTC device.
Adjustment (ppm) = (T – T0)2 * β
Once the temperature curve for a crystal is established, then
the designer should decide at what discrete temperatures
the compensation will change. Since drift is higher at
extreme temperatures, the compensation may not be
needed until the temperature is greater than 20°C from T0.
PPM ADJUSTMENT
A sample curve of the ATR setting vs. Frequency Adjustment
for the ISL1208 and a typical RTC crystal is given in Figure
18. This curve may vary with different crystals, so it is good
practice to evaluate a given crystal in an ISL1208 circuit
before establishing the adjustment values.
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
-10.0
-20.0
-30.0
-40.0
FIGURE 19. SUGGESTED LAYOUT FOR ISL1208 AND
CRYSTAL
In addition, it is a good idea to avoid a ground plane under
the X1 and X2 pins and the crystal, as this will affect the load
capacitance and therefore the oscillator accuracy of the
circuit. If the ~IRQ/FOUT pin is used as a clock, it should be
routed away from the RTC device as well. The traces for the
VBAT and VCC pins can be treated as a ground, and should
be routed around the crystal.
Super Capacitor Backup
0
5
10 15 20 25 30 35 40 45 50 55 60
ATR SETTING
FIGURE 18. ATR SETTING vs OSCILLATOR FREQUENCY
ADJUSTMENT
This curve is then used to figure what ATR and DTR settings
are used for compensation. The results would be placed in a
lookup table for the microcontroller to access.
Layout Considerations
The crystal input at X1 has a very high impedance, and
oscillator circuits operating at low frequencies such as
32.768kHz are known to pick up noise very easily if layout
precautions are not followed. Most instances of erratic
clocking or large accuracy errors can be traced to the
susceptibility of the oscillator circuit to interference from
adjacent high speed clock or data lines. Careful layout of the
RTC circuit will avoid noise pickup and insure accurate
clocking.
Figure 19 shows a suggested layout for the ISL1208 device
using a surface mount crystal. Two main precautions should
be followed:
18
The ISL1208 device provides a VBAT pin which is used for a
battery backup input. A Super Capacitor can be used as an
alternative to a battery in cases where shorter backup times
are required. Since the battery backup supply current
required by the ISL1208 is extremely low, it is possible to get
months of backup operation using a Super Capacitor.
Typical capacitor values are a few µF to 1 Farad or more
depending on the application.
If backup is only needed for a few minutes, then a small
inexpensive electrolytic capacitor can be used. For extended
periods, a low leakage, high capacity Super Capacitor is the
best choice. These devices are available from such vendors
as Panasonic and Murata. The main specifications include
working voltage and leakage current. If the application is for
charging the capacitor from a +5V ±5% supply with a signal
diode, then the voltage on the capacitor can vary from ~4.5V
to slightly over 5.0V. A capacitor with a rated WV of 5.0V
may have a reduced lifetime if the supply voltage is slightly
high. The leakage current should be as small as possible.
For example, a Super Capacitor should be specified with
leakage of well below 1µA. A standard electrolytic capacitor
with DC leakage current in the microamps will have a
severely shortened backup time.
Below are some examples with equations to assist with
calculating backup times and required capacitance for the
ISL1208 device. The backup supply current plays a major
FN8085.3
July 29, 2005
ISL1208
part in these equations, and a typical value was chosen for
example purposes. For a robust design, a margin of 30%
should be included to cover supply current and capacitance
tolerances over the results of the calculations. Even more
margin should be included if periods of very warm
temperature operation are expected.
Example 1. Calculating backup time given
voltages and capacitor value
Combining with Equation 2 gives the equation for backup
time:
TBACKUP = CBAT * (VBAT2 - VBAT1) / (IBATAVG + ILKG)
seconds
(EQ. 5)
where
CBAT = 0.47F
VBAT2 = 4.7V
1N4148
VBAT1 = 1.8V
ILKG = 0 (assumed minimal)
2.7V to 5.5V
VBAT
VCC
Solving equation 4 for this example, IBATAVG = 4.387E-7 A
CBAT
TBACKUP = 0.47 * (2.9) / 4.38E-7 = 3.107E6 sec
GND
FIGURE 20. SUPERCAPACITOR CHARGING CIRCUIT
In Figure 20, use CBAT = 0.47F and VCC = 5.0V. With VCC =
5.0V, the voltage at VBAT will approach 4.7V as the diode
turns off completely. The ISL1208 is specified to operate
down to VBAT = 1.8V. The capacitance charge/discharge
equation is used to estimate the total backup time:
I = CBAT * dV/dT
Example 2. Calculating a capacitor value for a
given backup time
(EQ. 1)
(EQ. 2)
CBAT = dT*I/dV
CBAT is the backup capacitance and dV is the change in
voltage from fully charged to loss of operation. Note that
ITOT is the total of the supply current of the ISL1208 (IBAT)
plus the leakage current of the capacitor and the diode, ILKG.
In these calculations, ILKG is assumed to be extremely small
and will be ignored. If an application requires extended
operation at temperatures over 50°C, these leakages will
increase and hence reduce backup time.
Note that IBAT changes with VBAT almost linearly (see
Typical Performance Curves). This allows us to make an
approximation of IBAT, using a value midway between the
two endpoints. The typical linear equation for IBAT vs VBAT
is:
IBAT = 1.031E-7*(VBAT) + 1.036E-7 Amps
CBAT = 0.70 * 35.96 = 25.2 days
Referring to Figure 20 again, the capacitor value needs to be
calculated to give 2 months (60 days) of backup time, given
VCC = 5.0V. As in Example 1, the VBAT voltage will vary from
4.7V down to 1.8V. We will need to rearrange Equation 2 to
solve for capacitance:
Rearranging gives
dT = CBAT * dV/ITOT to solve for backup time.
Since there are 86,400 seconds in a day, this corresponds to
35.96 days. If the 30% tolerance is included for capacitor
and supply current tolerances, then worst case backup time
would be:
(EQ. 3)
Using this equation to solve for the average current given 2
voltage points gives:
(EQ. 6)
Using the terms described above, this equation becomes:
CBAT = TBACKUP * (IBATAVG + ILKG)/(VBAT2 – VBAT1)
(EQ. 7)
where
TBACKUP = 60 days * 86,400 sec/day = 5.18 E6 sec
IBATAVG = 4.387 E-7 A (same as Example 1)
ILKG = 0 (assumed)
VBAT2 = 4.7V
VBAT1 = 1.8V
Solving gives
CBAT = 5.18 E6 * (4.387 E-7)/(2.9) = 0.784F
If the 30% tolerance is included for tolerances, then worst
case cap value would be
CBAT = 1.3 *.784 = 1.02F
IBATAVG = 5.155E-8*(VBAT2 + VBAT1) + 1.036E-7 Amps
(EQ. 4)
19
FN8085.3
July 29, 2005
ISL1208
Packaging Information
8-Lead Miniature Small Outline Gull Wing Package Type M
0.118 ± 0.002
(3.00 ± 0.05)
0.012 + 0.006 / -0.002
(0.30 + 0.15 / -0.05)
0.0256 (0.65) Typ.
R 0.014 (0.36)
0.118 ± 0.002
(3.00 ± 0.05)
0.030 (0.76)
0.0216 (0.55)
0.036 (0.91)
0.032 (0.81)
0.040 ± 0.002
(1.02 ± 0.05)
7° Typ.
0.008 (0.20)
0.004 (0.10)
0.0256" Typical
0.150 (3.81)
Ref.
0.193 (4.90)
Ref.
0.007 (0.18)
0.005 (0.13)
0.025"
Typical
0.220"
FOOTPRINT
0.020"
Typical
8 Places
NOTE:
1. All dimensions in inches and (millimeters).
20
FN8085.3
July 29, 2005
ISL1208
Packaging information
8-Lead Plastic, SOIC, Package Code S8
0.150 (3.80) 0.228 (5.80)
0.158 (4.00) 0.244 (6.20)
Pin 1 Index
Pin 1
0.014 (0.35)
0.019 (0.49)
0.188 (4.78)
0.197 (5.00)
(4X) 7°
0.053 (1.35)
0.069 (1.75)
0.004 (0.19)
0.010 (0.25)
0.050 (1.27)
0.010 (0.25)
X 45°
0.020 (0.50)
0.050"Typical
0.050"
Typical
0° - 8°
0.0075 (0.19)
0.010 (0.25)
0.250"
0.016 (0.410)
0.037 (0.937)
FOOTPRINT
0.030"
Typical
8 Places
NOTE: All dimensions in inches (in parentheses in millimeters).
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com
21
FN8085.3
July 29, 2005