SC
5
Low-Cost I2C Battery Monitor
DS2745
SDA
PIO
SNS
L
2
3
4
VDD
7
VIN
6
CTG
VSS
mMAX
www.maxim-ic.com
FEATURES
16-Bit Bidirectional Current Measurement
1.56mV LSB, ±51.2mV Dynamic Range
104mA LSB, ±3.4A Dynamic Range (RSNS =
15mW)
Current Accumulation Register Resolution
6.25mVhr LSB, ±204.8mVh Range
0.417mAhr LSB, ±13.65Ah Range
(R
= 15mW)
SNS
11-Bit Voltage Measurement
4.88mV LSB, 0V to 5V Input Range
11-Bit Temperature Measurement
0.125ºC Resolution, -20ºC to +70ºC
Industry Standard I
2
C* Interface
Low Power Consumption:
Active Current:
70mA typical, 100mA max
Sleep Current:
1mA typical, 3mA max
BLOCK DIAGRAM
PACK+
VDD
PIN CONFIGURATION
See Table 1 for Ordering Information.
DESCRIPTION
The DS2745 provides current-flow, voltage, and
temperature measurement data to support batterycapacity monitoring in cost-sensitive applications. The
DS2745 can be mounted on either the host side or
pack side of the application. Current measurement
and coulomb counting is accomplished by monitoring
the voltage drop across an external sense resistor,
voltage measurement is accomplished through a
separate voltage-sense input, and temperature
measurement takes place on-chip. A standard I
interface with software programmable address gives
the controlling microprocessor access to all data and
status registers inside the DS2745. A low-power sleep
mode state conserves energy when the cell pack is in
storage.
COMM
Fuel Gauge
Algorithm
µP
DS2745
SDA
SCL
SNS VSS
VIN
APPLICATIONS
Cellular
GPS
PDAs
Handheld Products
PACK-
Sense
Li+
Protector
Table 1.
DS2745U+
DS2745U+/T&R
+Denotes lead-free package.
2
*I
C is a trademark of Philips Corp. Purchase of I2C components from Maxim Integrated Products, Inc., or one of its sublicensed Associated
Companies, conveys a license under the Philips I
conforms to the I
ORDERING INFORMATION
PART MARKING PIN-PACKAGE
2745
2745
2
2
C Standard Specification as defined by Philips.
C Patent Rights to use these components in an I2C system, provided that the system
mMAX package
DS2745U+ in Tape-and-Reel
2
C
1 of 14 091405
DS2745 Low-Cost I2C Battery Monitor
ABSOLUTE MAXIMUM RATINGS*
Voltage on All Pins Relative to VSS -0.3V to +6V
Operating Temperature Range -40°C to +85°C
Storage Temperature Range -55°C to +125°C
Soldering Temperature See IPC/JEDECJ-STD-020A
* This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation
sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability.
RECOMMENDED DC OPERATING CONDITIONS
(2.5V £ VDD £ 5.5V; TA = 0°C to +70°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage VDD (Note 1) 2.5 5.5 V
Serial Data I/O Pin SDA (Note 1) -0.3 +5.5 V
Serial Clock Pin SCL (Note 1) -0.3 +5.5 V
Programmable I/O Pin PIO (Note 1) -0.3 +5.5 V
VIN Pin VIN (Note 1) -0.3 VDD +0.3 V
DC ELECTRICAL CHARACTERISTICS
(2.5V £ VDD £ 4.5V; TA = 0°C to +70°C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
70 100
Active Current I
Sleep-Mode Current I
Current Resolution I
Current Full-Scale
Magnitude
Current Offset I
Current Gain Error I
Accumulated Current
Resolution
Accumulated Current
Offset
Voltage Resolution V
ACTIVE
SLEEP
LSB
I
(Note 1) 51.2 mV/R
FS
OERR
GERR
q
CA
q
OERR
LSB
V
= 5.5V 105
DD
SCL = SDA = V
PIO = V
SS
SS
,
1 3
1.56
(Note 2) - 7.82 + 12.5
- 1.0 +1.0
6.25
V
= VSS, (Notes 4, 5) - 188 + 0
SNS
4.88 mV
mA
mA
mV/R
mV/R
% of
reading
mVh/R
µVh/R
per day
Voltage Full-Scale VFS 0 4.992 V
Voltage Error V
(Note 12) - 25 + 25 mV
GERR
Temperature Resolution T
Temperature Error T
Current Sample Clock
Frequency
Timebase Accuracy t
0.125 °C
LSB
- 3 + 3 ºC
ERR
f
18.6 kHz
SAMP
V
ERR
= 3.8V, TA = +25°C ±1
DD
2 of 14
%
±2
DS2745 Low-Cost I2C Battery Monitor
-20°C ≤ T
2.5V ≤ V
≤ +70°C,
A
≤ 5.5V
DD
±3
Input Resistance, VIN RIN 15
Input Logic High:
SCL, SDA
Input Logic Low:
SCL, SDA
Output Logic Low:
SDA, PIO
Pulldown Current: SCL,
SDA
Input Capacitance: SCL,
SDA
SLEEP Timeout t
Input Logic High:
PIO
Input Logic Low:
PIO
V
(Note 1) 1.5 V
IH
V
(Note 1) 0.6 V
IL
V
OL
I
0.25
PD
C
BUS
SLEEP
V
(Note 1) VDD x 0.7 V
IH
V
(Note 1) VDD x 0.3 V
IL
I
= 4mA (Note 1) 0.4 V
OL
50 pF
(Note 3) 2.2 S
2-WIRE INTERFACE TIMING SPECIFICATIONS
(VDD = 2.5V to 5.5V, TA = -20°C to +70°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
f
SCL Clock Frequency
Bus Free Time Between a
STOP and START Condition
Hold Time (Repeated)
START Condition
Low Period of SCL Clock
SCL
t
BUF
t
HD:STA
t
LOW
(Note 6) 0 400 KHz
1.3 µs
(Note 7) 0.6 µs
1.3 µs
MW
mA
t
High Period of SCL Clock
Setup Time for a Repeated
START Condition
Data Hold Time
Data Setup Time
Rise Time of Both SDA and
SCL Signals
Fall Time of Both SDA and
SCL Signals
Setup Time for STOP
Condition
Spike Pulse Widths
Suppressed by Input Filter
HIGH
t
SU:STA
t
HD:DAT
t
SU:DAT
t
R
t
F
t
SU:STO
t
SP
0.6 µs
(Note 8, 9) 0 0.9 µs
(Note 8) 100 ns
0.6 µs
0.6 µs
(Note 10) 0 50 ns
Capacitive Load for Each
Bus
B
(Note 11) 400 pF
C
Line
SCL, SDA Input
Capacitance
60 pF
C
BIN
3 of 14
20 + 0.1C
20 + 0.1C
B
B
300 ns
300 ns
Note 1:
Note 2:
Note 3:
Note 4:
Note 5:
Note 6:
Note 7:
Note 8:
Note 9:
Note 10:
Note 11:
Note 12:
Figure 1. I
DS2745 Low-Cost I2C Battery Monitor
All voltages are referenced to V
SS
.
Offset specified after auto-calibration cycle and Current Offset Bias register (COBR) set to 00h.
To properly enter sleep mode, SMOD=1, and the application should hold SDA and SCL low for longer
than the maximum t
SLEEP
.
NBEN = 0, Current Offset Bias Register (COBR) set to 00h, and Accumulation Bias Register (ABR)
set to 00h.
Parameters guaranteed by design.
Timing must be fast enough to prevent the DS2745 from entering sleep mode due to SDA,SCL low
for period >
t
SLEEP.
f
must meet the minimum clock low time plus the rise/fall times.
SCL
The maximum t
has only to be met if the device does not stretch the LOW period (t
HD:DAT
LOW
) of the
SCL
signal.
This device internally provides a hold time of at least 300 ns for the SDA signal (referred to the
VIHmin of
the SCL signal) to bridge the undefined region of the falling edge of SCL.
Filters on SDA and SCL suppress noise spikes at the input buffers and delay the sampling instant.
C
¾total capacitance of one bus line in pF.
B
The first voltage measurement after writing the ACR or after device POR is not valid.
2
C Bus Timing Diagram
SDA
t
SCL
F
t
LOW
t
HD;S TA
S
t
R
t
HD;DAT
t
SU;DAT
t
F
t
SU;STA
t
HD;S TA
Sr P S
t
SP
t
SU;STO
t
R
t
BUF
4 of 14
PIN DESCRIPTION
PIN SYMBOL FUNCTION
1
2
3
4
5 V
6
SCL
SDA
PIO
SNS
SS
CTG
DS2745 Low-Cost I2C Battery Monitor
Serial Clock Input. 2-Wire clock line. Input only. Connect this pin to the CLOCK
terminal of the battery pack. Pin has an internal pulldown (I
disconnection.
Serial Data Input/Output. 2-Wire data line. Open-drain output driver. Connect this pin
to the DATA terminal of the battery pack. Pin has an internal pulldown (I
disconnection.
General Purpose Input/Output. Open-drain output driver with input sense. Connect
to a pull up resistor for bidirectional operation.
Sense Resistor Connection. Connect to the negative terminal of the battery pack.
Connect the sense resistor between V
Device Ground. Connect to the negative terminal of the Li+ cell outside the cell
protection FETs. Connect the sense resistor between VSS and SNS.
Connect to Ground. Connect to the negative terminal of the Li+ cell outside the cell
protection FETs.
and SNS.
SS
) for sensing
PD
) for sensing
PD
7
8 V
VIN
DD
Voltage Sense Input. The voltage of the Li+ cell is monitored through this input pin.
Power-Supply Input. Connect to the positive terminal of the Li+ cell through a
decoupling network.
Figure 2. BLOCK DIAGRAM
THERMAL
SENSE
M
U
VIN
X
BIAS
VOLTAGE
REFERENCE
ADC1
ADC2
TIMEBASE
TEMPERATURE
VOLTAGE
CURRENT
ACR
2-WIRE
INTERFACE
VDD
SCL
SDA
STATUS
1kW 1kW
SNS VSS
+-
S
chip
ground
5 of 14
PIO
DS2745 Low-Cost I2C Battery Monitor
DETAILED DESCRIPTION
Current is measured bidirectionally over a dynamic range of ±51.2mV with a resolution of 1.56mV. Assuming a
15mW sense resistor, the current sense range is ±3.4A, with a 1 LSB resolution of 104mA. Current measurements
are performed at regular intervals and accumulated with each measurement to support accurate “coulomb
counting”. Each current measurement is reported with sign and magnitude in the two-byte Current register. The
Accumulated Current register (ACR) reports the coulomb count and supports a wide range of battery sizes. Battery
voltage measurements are reported in the two-byte Voltage register with 11-bit (4.88mV) resolution, and
Temperature is reported in the two-byte Temperature register with 0.125C resolution.
The DS2745 measurements can be used directly to provide accurate fuel gauging in typical use conditions, or
along with FuelPack™ algorithms to form a complete and accurate solution for estimating remaining capacity over
wide temperature and operating conditions.
Through its two wire I
and Measurement registers. Additionally, the I
2
C interface, the DS2745 allows a host system read/write access to the Status, Configuration
2
C slave address can be changed from the default after power up.
Figure 3. APPLICATION EXAMPLE
PACK+
500W
1kW
DS2745
1 Cell Li+
Protection IC
(Li+/Polymer)
102 102
103
5. 6V
PIO CTG
VSS
VDD
2.5V
SNS
SCLVIN
SDA
R
SNS
(1)
5.6V 5.6V
(1)
(1)(1)
(1) Optional for 8kV/15kV ESD
150W
150W
CLOCK
DATA
PACK -
FuelPack is a trademark of Dallas Semiconductor.
6 of 14
DS2745 Low-Cost I2C Battery Monitor
POWER MODES
The DS2745 operates in one of two power modes: active and sleep. While in active mode, the DS2745 operates as
a high-precision battery monitor with voltage, temperature, current and accumulated current measurements
acquired continuously and the resulting values updated in the measurement registers. Read and write access is
allowed to all registers.
In sleep mode, the DS2745 operates in a low-power mode with no measurement activity. Serial access to current,
accumulated current, and status/control registers is allowed in sleep mode if V
The DS2745 operating mode transitions from SLEEP to ACTIVE when:
> 2V.
DD
SDA > V
The DS2745 operating mode transitions from ACTIVE to SLEEP when:
SMOD = 1 AND (SDA < VIL AND SCL < VIL) for t
CAUTION: If SMOD = 1, pull-up resistors are required on SCL and/or SDA in order to ensure that the DS2745
transitions from SLEEP to ACTIVE mode when the battery is charged. If the bus is not pulled up, the DS2745
remains in SLEEP and cannot accumulate the charge current. This caution statement applies particularly to on a
battery that is charged on a standalone charger.
OR SCL > VIH
IH
.
SLEEP
VOLTAGE MEASUREMENT
Battery voltage is measured at the VIN input with respect to VSS over a range of 0V to 4.992V and with a
resolution of 4.88mV. The result is updated every 440ms and placed in the VOLTAGE register in two’s compliment
form. Voltages above the maximum register value are reported as 7FFFh.
Figure 4. VOLTAGE REGISTER FORMAT
MSB—Address 0C LSB—Address 0D
9
S 2
MSb LSb MSb LSb
28 27 26 25 24 23 22 21 20 X X X X X
“S”: sign bit(s), “X”: reserved
The input impedance of VIN is sufficiently large (>15MW) to be connected to a high impedance voltage divider in
order to support multiple cell applications. The pack voltage should be divided by the number of series cells to
present a single cell average voltage to the VIN input. Note that the first voltage measurement made after the
DS2745 is powered or after the ACR register is written will not be valid. The host should wait one measurement
cycle after either of these two conditions occur before reading voltage.
Units: 4.88mV
TEMPERATURE MEASUREMENT
The DS2745 uses an integrated temperature sensor to measure battery temperature with a resolution of 0.125°C.
Temperature measurements are updated every 440ms and placed in the Temperature Register in two’s
complement form.
7 of 14
Figure 5. TEMPERATURE REGISTER FORMAT
MSB—Address 0A LSB—Address 0B
9
S 2
MSb LSb MSb LSb
28 27 26 25 24 23 22 21 20 X X X X X
DS2745 Low-Cost I2C Battery Monitor
“S”: sign bit, “X”: reserved
Units: 0.125°C
CURRENT MEASUREMENT
In the active mode of operation, the DS2745 continually measures the current flow into and out of the battery by
measuring the voltage drop across a low-value current-sense resistor, R
, connected between the SNS and VSS
SNS
pins. The voltage sense range between SNS and VSS is ±51.2mV. Note that positive current values occur when
V
is less than VSS, and negative current values occur when V
SNS
is greater than VSS. Peak signal amplitudes up
SNS
to 102mV are allowed at the input as long as the continuous or average signal level does not exceed ±51.2mV over
the conversion cycle period. The ADC samples the input differentially at with an 18.6kHz sample clock and updates
the current register at the completion of each conversion cycle. Figure 6 describes the current measurement
register format and resolution. Charge currents above the maximum register value are reported at the maximum
value (7FFFh = +51.2mV). Discharge currents below the minimum register value are reported at the minimum
value (8000h = -51.2mV).
Figure 6. CURRENT REGISTER FORMATS
MSB—Address 0E LSB—Address 0F
14
S 2
MSb LSb MSb LSb
“S”: sign bit
213 212 211 210 29 28 27 26 25 24 23 22 21 20
Units: 2
0
= 1.5625mV/Rsns
Table 2. CURRENT RESOLUTION FOR VARIOUS RSNS VALUES
CURRENT RESOLUTION (1 LSB)
CONVERSION
TIME
3.5s
|V
- V
SNS
|
20mW 15mW 10mW 5mW
SS
1.5625mV 78.13mA 104.2mA 156.3mA 312.5mA
R
SNS
Table 3. CURRENT RANGE FOR VARIOUS RSNS VALUES
CURRENT INPUT RANGE
R
V
- V
SNS
20mW 15mW 10mW 5mW
SS
±51.2mV ±2.56A ±3.41A ±5.12A ±10.24A
Every 1024th conversion, the ADC measures its input offset to facilitate offset correction. Offset correction occurs
approximately once per hour. The resulting correction factor is applied to the subsequent 1023 measurements.
During the offset correction conversion, the ADC does not measure the SNS to VSS signal. A maximum error of
1/1024 in the accumulated current register (ACR) is possible, however, to reduce the error, the current
measurement just prior to the offset conversion is displayed in the current register and is substituted for the
dropped current measurement in the current accumulation process. The error due to offset correction is typically
much less than 1/1024.
SNS
8 of 14
DS2745 Low-Cost I2C Battery Monitor
CURRENT ACCUMULATION
The Accumulated Current register (ACR) serves as an up/down counter holding a running count of charge stored in
the battery. Current measurement results, plus a programmable bias value are internally summed, or accumulated,
at the completion of each current measurement conversion period with the results displayed in the ACR. The ACR
has a range of 0mVh to +409.6mVh with an LSb of 6.25µVhAdditional registers hold fractional results of each
accumulation, however, these bits are not user accessible. The ACR count clamps at FFFFh when accumulating
charge values and at 0000h when accumulating discharge values.
Read and write access is allowed to the ACR. Whenever the ACR is written, fractional accumulation results are
cleared. A write to the ACR also forces the ADC to measure its offset and update the offset correction factor.
Current measurement and accumulation resume (using the new offset correction) with the second conversion
following the write to the ACR. Figure 7 describes the ACR address, format, and resolution.
Figure 7. ACCUMULATED CURRENT REGISTER FORMAT
MSB—Address 10 LSB—Address 11
14
S 2
MSb LSb MSb LSb
“S”: sign bit
213 212 211 210 29 28 27 26 25 24 23 22 21 20
Units: 6.25mVh/Rsns
Table 4. ACCUMULATED CURRENT RANGE FOR VARIOUS RSNS VALUES
ACR RANGE
R
V
- V
SNS
20mm 15mm 10mW 5mW
SS
409.6mVh 20.48Ah 27.31Ah 40.96Ah 81.92Ah
SNS
CURRENT OFFSET BIAS
The Current Offset Bias register (COBR) allows a programmable offset value to be added to raw current
measurements. The result of the raw current measurement plus the COBR value is displayed as the current
measurement result in the CURRENT register, and is used for current accumulation. The COBR value can be used
to correct for a static offset error, or can be used to intentionally skew the current results and therefore the current
accumulation.
Read and write access is allowed to COBR. Whenever the COBR is written, the new value is applied to all
subsequent current measurements. COBR can be programmed in 1.56mV steps to any value between +198.1mV
and -199.7mV. The COBR value is stored as a two’s complement value in volatile memory, and must be initialized
via the interface on power-up. Figure 8 describes the COBR address, format, and resolution.
Figure 8. CURRENT OFFSET BIAS REGISTER FORMAT
Address 61
S 2
MSb LSb
“S”: sign bit Units: 1.56mV/Rsns
6
25 24 23 22 21 20
9 of 14
DS2745 Low-Cost I2C Battery Monitor
CURRENT BLANKING
The Current Blanking feature modifies current measurement result prior to being accumulated in the ACR. Current
Blanking occurs conditionally when a current measurement (raw current + COBR) falls in one of two defined
ranges. The first range prevents charge currents less than 100mV from being accumulated. The second range
prevents discharge currents less than 25mV in magnitude from being accumulated. Charge current blanking is
always performed, however, discharge current blanking must be enabled by setting the NBEN bit in the
Status/Config register. See the register description for additional information.
ACCUMULATION BIAS
The Accumulation Bias register (ABR) allows a programmable offset value to be added to the current accumulation
process. The new ACR value results from the addition of the Current register value plus ABR plus the previous
ACR value. ABR can be used to intentionally skew the current accumulation to estimate system stand-by currents
that are too small to measure. ABR value is not subject to the Current Blanking thresholds.
Read and write access is allowed to the ABR. Whenever the ABR is written, the new value is applied to all
subsequent current measurements. ABR can be set to any value between +198.1mV and -199.7mV in 1.56mV
steps. The ABR value is stored as a two’s complement value in volatile memory, and must be initialized via the
interface on power-up. Figure 9 describes the ABR address, format, and resolution.
Figure 9. ACCUMULATION BIAS REGISTER FORMAT
Address 62
S 2
MSb LSb
“S”: sign bit Units: 1.56mV/Rsns
6
25 24 23 22 21 20
MEMORY
The DS2745 has memory space with registers for instrumentation, status, and control. When the MSB of a twobyte register is read, both the MSB and LSB are latched and held for the duration of the read data command to
prevent updates during the read and ensure synchronization between the two register bytes. For consistent results,
always read the MSB and the LSB of a two-byte register during the same read data command sequence.
10 of 14
DS2745 Low-Cost I2C Battery Monitor
Table 5. MEMORY MAP
ADDRESS (HEX) DESCRIPTION READ/WRITE POR DEFAULT
00 Reserved —
01 Status/Config Register R/W
02 to 08 Reserved —
09 to 0D Reserved —
0A Temperature Register MSB R
0B Temperature Register LSB R
0C Voltage Register MSB R
0D Voltage Register LSB R
0E Current Register MSB R
0F Current Register LSB R
10 Accumulated Current Register MSB R/W
11 Accumulated Current Register LSB R/W
12 to 61 Reserved —
61 Offset Bias Register R/W
62 Accumulation Bias Register R/W
63 to FF Reserved —
11000000b
No Change
No Change
00h
00h
STATUS/CONFIG REGISTER
The Status/Config register is read/write with individual bits designated as read only. Bit values indicate status as
well as program or select device functionality.
Figure 10. STATUS/CONFIG REGISTER FORMAT
ADDRESS 01
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
X PORF SMOD NBEN PIO A2 A1 A0
X — Reserved.
PORF — The Power-On-Reset Flag is set to indicate initial power-up. PORF is not cleared internally. The user
must write this flag value to a 0 in order to use it to indicate subsequent power-up events. If PORF indicates a
power-on-reset, the ACR could be misaligned with the actual battery state of charge. The system can request a
charge to full in order to synchronize the ACR with the battery charge state. PORF is read/write-to-zero.
SMOD — SLEEP Mode Enable. A value of 1 allows the DS2745 to enter sleep mode when both SDA and SCL
pins is low for 2s. A value of 0 disables the transition to sleep mode. The power-up default of SMOD = 0.
NBEN — Negative Blanking Enable. A value of 1 enables blanking of negative current values up to 25mV. A value
of 0 disables blanking of negative currents. The power-up default of NBEN = 0.
PIO — Programmable Input/Output. PIO provides both control of the PIO open-drain output driver and readback of
the PIO pin logic level. Writing a 0 to PIO drives PIO pin low. Writing a 1 deactivates the PIO output and allows
readback of an external signal. Reading PIO returns the logic state on the pin. PIO is RESET on POR.
A2:A0 — I
power-up default slave address of 1001000b, accessing the DS2745 requires the modified slave address following
a start or repeated start.
11 of 14
2
C Slave Address bits. A2:A0 set the lower 3 bits of the I2C slave address. When modified from the
DS2745 Low-Cost I2C Battery Monitor
2-WIRE BUS SYSTEM
The 2-Wire bus system supports operation as a slave only device in a single or multi-slave, and single or multimaster system. Up to 128 slave devices may share the bus by uniquely setting the 7-bit slave address. The 2-wire
interface consists of a serial data line (SDA) and serial clock line (SCL). SDA and SCL provide bidirectional
communication between the DS2745 slave device and a master device at speeds up to 400kHz. The DS2745’s
SDA pin operates bidirectionally, that is, when the DS2745 receives data, SDA operates as an input, and when the
DS2745 returns data, SDA operates as an open-drain output, with the host system providing a resistive pull-up.
The DS2745 always operates as a slave device, receiving and transmitting data under the control of a master
device. The master initiates all transactions on the bus and generates the SCL signal as well as the START and
STOP bits which begin and end each transaction.
Bit Transfer
One data bit is transferred during each SCL clock cycle, with the cycle defined by SCL transitioning low-to-high and
then high-to-low. The SDA logic level must remain stable during the high period of the SCL clock pulse. Any
change in SDA when SCL is high is interpreted as a START or STOP control signal.
Bus Idle
The bus is defined to be idle, or not busy, when no master device has control. Both SDA and SCL remain high
when the bus is idle. The STOP condition is the proper method to return the bus to the idle state.
START and STOP Conditions
The master initiates transactions with a START condition (S), by forcing a high-to-low transition on SDA while SCL
is high. The master terminates a transaction with a STOP condition (P), a low-to-high transition on SDA while SCL
is high. A Repeated START condition (Sr) can be used in place of a STOP then START sequence to terminate one
transaction and begin another without returning the bus to the idle state. In multimaster systems, a Repeated
START allows the master to retain control of the bus. The START and STOP conditions are the only bus activities
in which the SDA transitions when SCL is high.
Acknowledge Bits
Each byte of a data transfer is acknowledged with an Acknowledge bit (A) or a No Acknowledge bit (N). Both the
master and the DS2745 slave generate acknowledge bits. To generate an Acknowledge, the receiving device must
pull SDA low before the rising edge of the acknowledge-related clock pulse (ninth pulse) and keep it low until SCL
returns low. To generate a No Acknowledge (also called NAK), the receiver releases SDA before the rising edge of
the acknowledge-related clock pulse and leaves SDA high until SCL returns low. Monitoring the acknowledge bits
allows for detection of unsuccessful data transfers. An unsuccessful data transfer can occur if a receiving device is
busy or if a system fault has occurred. In the event of an unsuccessful data transfer, the bus master should reattempt communication.
Data Order
A byte of data consists of 8 bits ordered most significant bit (msb) first. The least significant bit (lsb) of each byte is
followed by the Acknowledge bit. DS2745 registers composed of multi-byte values are ordered most significant
byte (MSB) first. The MSB of multi-byte registers is stored on even data memory addresses.
Slave Address
A bus master initiates communication with a slave device by issuing a START condition followed by a Slave
Address (SAddr) and the read/write (R/W) bit. When the bus is idle, the DS2745 continuously monitors for a
START condition followed by its slave address. When the DS2745 receives a slave address that matches the value
in its Status/Config register, it responds with an Acknowledge bit during the clock period following the R/W bit. The
default Slave Address at power-up is 1001000. The lower three bits of the slave address can be re-programmed,
refer to the Status/Config register description for details.
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DS2745 Low-Cost I2C Battery Monitor
Read/Write Bit
The R/W bit following the slave address determines the data direction of subsequent bytes in the transfer. R/W = 0
selects a write transaction, with the following bytes being written by the master to the slave. R/W = 1 selects a read
transaction, with the following bytes being read from the stave by the master.
Bus Timing
The DS2745 is compatible with any bus timing up to 400kHz. No special configuration is required to operate at any
speed.
2-Wire Command Protocols
The command protocols involve several transaction formats. The simplest format consists of the master writing the
START bit, slave address, R/W bit, and then monitoring the acknowledge bit for presence of the DS2745. More
complex formats such as the Write Data, Read Data and Function command protocols write data, read data and
execute device specific operations. All bytes in each command format require the slave or host to return an
Acknowledge bit before continuing with the next byte. Each function command definition outlines the required
transaction format. The following key applies to the transaction formats.
Table 3. 2-Wire Protocol Key
KEY DESCRIPTION KEY DESCRIPTION
S START bit Sr Repeated START
SAddr Slave Address (7-bit) W R/W bit = 0
FCmd Function Command byte R R/W bit = 1
MAddr Memory Address byte P STOP bit
Data Data byte written by master Data Data byte returned by slave
A Acknowledge bit - Master A
N No Acknowledge - Master N
Acknowledge bit¾Slave
No Acknowledge¾Slave
Basic Transaction Formats
Write: S SAddr W A MAddr A Data0 A P
A write transaction transfers one or more data bytes to the DS2745. The data transfer begins at the memory
address supplied in the MAddr byte. Control of the SDA signal is retained by the master throughout the transaction,
except for the Acknowledge cycles.
Read: S SAddr W A MAddr A Sr SAddr R A Data0 N P
Write Portion Read Portion
A read transaction transfers one or more bytes from the DS2745. Read transactions are composed of two parts, a
write portion followed by a read portion, and is therefore inherently longer than a write transaction. The write portion
communicates the starting point for the read operation. The read portion follows immediately, beginning with a
Repeated START, Slave Address with R/W set to a 1. Control of SDA is assumed by the DS2745 beginning with
the Slave Address Acknowledge cycle. Control of the SDA signal is retained by the DS2745 throughout the
transaction, except for the Acknowledge cycles. The master indicates the end of a read transaction by responding
to the last byte it requires with a No Acknowledge. This signals the DS2745 that control of SDA is to remain with
the master following the Acknowledge clock.
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DS2745 Low-Cost I2C Battery Monitor
Write Data Protocol
The write data protocol is used to write to register and shadow RAM data to the DS2745 starting at memory
address MAddr. Data0 represents the data written to MAddr, Data1 represents the data written to MAddr + 1 and
DataN represents the last data byte, written to MAddr + N. The master indicates the end of a write transaction by
sending a STOP or Repeated START after receiving the last acknowledge bit.
S SAddr W A MAddr A Data0 A Data1 A … DataN A P
The msb of the data to be stored at address MAddr can be written immediately after the MAddr byte is
acknowledged. Because the address is automatically incremented after the least significant bit (lsb) of each byte is
received by the DS2745, the msb of the data at address MAddr + 1 is can be written immediately after the
acknowledgement of the data at address MAddr. If the bus master continues an auto-incremented write transaction
beyond address FFh, the DS2745 ignores the data. Data is also ignored on writes to read-only addresses and
reserved addresses. Incomplete bytes and bytes that are Not Acknowledged by the DS2745 are not written to
memory.
Read Data Protocol
The Read Data protocol is used to read register and shadow RAM data from the DS2745 starting at memory
address specified by MAddr. Data0 represents the data byte in memory location MAddr, Data1 represents the data
from MAddr + 1 and DataN represents the last byte read by the master.
S SAddr W A MAddr A Sr SAddr R A Data0 A Data1 A … DataN N P
Data is returned beginning with the most significant bit (msb) of the data in MAddr. Because the address is
automatically incremented after the least significant bit (lsb) of each byte is returned, the msb of the data at
address MAddr + 1 is available to the host immediately after the acknowledgement of the data at address MAddr. If
the bus master continues to read beyond address FFh, the DS2745 outputs data values of FFh. Addresses labeled
“Reserved” in the memory map return undefined data. The bus master terminates the read transaction at any byte
boundary by issuing a No Acknowledge followed by a STOP or Repeated START.
Package Information
For the latest package outline information, go to www.maxim-ic.com/DallasPackInfo.
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