LE
AVAILAB
DS2438
Smart Battery Monitor
FEATURES
§
§
§
§
§
§
§
§
§
§
®
Unique 1-Wire interface requires only one
port pin for communication
Provides unique 64-bit serial number
Eliminates thermistors by sensing battery
temperature on-chip
On-board A/D converter allows monitoring
of battery voltage for end-of-charge and endof-discharge determination
On-board integrated current accumulator
facilitates fuel gauging
Elapsed time meter in binary format
40-byte nonvolatile user memory available
for storage of battery-specific data
Reverts to low-power sleep mode on battery
pack disconnect (feature disabled on
DS2438AZ)
Operating range -40ºC to +85ºC
Applications include portable computers,
portable/cellular phones, consumer
electronics, and handheld instrumentation
PIN ASSIGNMENT
GND
1
8
DQ
VSENS+
2
7
NC
VSENS-
3
6
NC
VAD
4
5
VDD
DS2438Z, DS2438AZ
8-Pin SOIC (150-mil)
PIN DESCRIPTION
DQ
VAD
VSENS+
VSENSVDD
GND
NC
-
Data In/Out
General A/D input
Battery current monitor input (+)
Battery current monitor input (-)
Power Supply (2.4V to 10.0V)
Ground
No connect
DESCRIPTION
The DS2438 Smart Battery
Monitor Diagrams
provides several functions that are desirable to carry in a battery
Functional
pack: a means of tagging a battery pack with a unique serial number, a direct-to-digital temperature
sensor which eliminates the need for thermistors in the battery pack, an A/D converter which measures
the battery voltage and current, an integrated current accumulator which keeps a running total of all
current going into and out of the battery, an elapsed time meter, and 40 bytes of nonvolatile EEPROM
memory for storage of important parameters such as battery chemistry, battery capacity, charging
methodology and assembly date. Information is sent to/from the DS2438 over a 1-Wire interface, so that
only one wire (and ground) needs to be connected from a central microprocessor to a DS2438. This
means that battery packs need only have three output connectors: battery power, ground, and the 1-Wire
interface.
Because each DS2438 contains a unique silicon serial number, multiple DS2438s can exist on the same
1-Wire bus. This allows multiple battery packs to be charged or used in the system simultaneously.
Applications for the smart battery monitor include portable computers, cellular telephones, and handheld
instrumentation battery packs in which it is critical to monitor real-time battery performance. Used in
Pin Configurations appear at end of data sheet.
conjunction
with a continued
microcontroller
in sheet.
the host system, the DS2438 provides a complete smart battery
Functional Diagrams
at end of data
pack
solution
that
is
fully
chemistry-independent.
The customization for a particular battery chemistry
UCSP is a trademark of Maxim Integrated Products, Inc.
and capacity is realized in the code programmed into the microcontroller and DS2438 EEPROM, and
only a software revision is necessary should a designer wish to change battery pack chemistry.
of 29 contact Maxim Direct
For pricing, delivery, and ordering information,1please
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
070605
DS2438
ORDERING INFORMATION
PART
DS2438Z+
DS2438Z+T&R
DS2438AZ+
DS2438AZ+T&R
DS2438Z
DS2438Z/T&R
DS2438AZ
DS2438AZ/T&R
MARKING
DS2438
DS2438
DS2438A
DS2438A
DS2438
DS2438
DS2438A
DS2438A
PACKAGE INFORMATION
8-Pin SOIC
DS2438Z+ on Tape-and-Reel
8-Pin SOIC
DS2438AZ+ on Tape-and-Reel
8-Pin SOIC
DS2438Z on Tape-and-Reel
8-Pin SOIC
DS2438AZ on Tape-and-Reel
+ Denotes lead-free package.
DETAILED PIN DESCRIPTION
PIN
SYMBOL
DESCRIPTION
1
GND
2
VSENS+
Ground
Battery Input: connection for battery current to be monitored (see text)
3
VSENS-
Battery Input: connection for battery current to be monitored (see text)
4
VAD
ADC Input: input for general purpose A/D
5
VDD
VDD Pin: input supply voltage
6, 7
NC
No Connect
8
DQ
Data Input/Out: for 1-Wire operation: Open drain
OVERVIEW
The block diagram of Figure 1 shows the seven major components of the DS2438:
1.
2.
3.
4.
5.
6.
7.
64-bit lasered ROM
temperature sensor
battery voltage A/D
battery current A/D
current accumulators
elapsed time meter
40-byte nonvolatile user-memory
Each DS2438 contains a unique 64-bit lasered ROM serial number so that several battery packs can be
charged/monitored by the same host system. Furthermore, other Dallas products featuring the same
1-Wire bus architecture with a 64-bit ROM can reside on the same bus; refer to the Dallas Automatic
Identification Data book for the specifications of these products.
Communication to the DS2438 is via a 1-Wire port. With the 1-Wire port, the memory and control
functions will not be available until the ROM function protocol has been established. The master must
first provide one of four ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, or 4)
Skip ROM. These commands operate on the 64-bit lasered ROM portion of each device and can singulate
a specific device if many are present on the 1-Wire line as well as to indicate to the bus master how many
and what types of devices are present. After a ROM function sequence has been successfully executed,
the memory and control functions are accessible and the master may then provide any one of the six
memory and control function commands.
2 of 29
DS2438
Control function commands may be issued which instruct the DS2438 to perform a temperature
measurement or battery voltage A/D conversion. The result of these measurements will be placed in the
DS2438’s memory map, and may be read by issuing a memory function command which reads the
contents of the temperature and voltage registers. Additionally, the charging/discharging battery current is
measured without user intervention, and again, the last completed result is stored in DS2438 memory
space. The DS2438 uses these current measurements to update three current accumulators; the first stores
net charge for fuel gauge calculations, the second accumulates the total charging current over the life of
the battery, and the remaining accumulator tallies battery discharge current. The elapsed time meter data,
which can be used in calculating battery self-discharge or time-related charge termination limits, also
resides in the DS2438 memory map and can be extracted with a memory function command. The
nonvolatile user memory of the DS2438 consists of 40 bytes of EEPROM. These locations may be used
to store any data the user wishes and are written to using a memory function command. All data and
commands are read and written least significant bit first.
PARASITE POWER
The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power
whenever the DQ pin is high. DQ will provide sufficient power as long as the specified timing and
voltage requirements are met (see the section titled “1-Wire Bus System”). The advantage of parasite
power is that the ROM may be read in absence of normal power, i.e., if the battery pack is completely
discharged.
DS2438 BLOCK DIAGRAM Figure 1
3 of 29
DS2438
OPERATION-MEASURING TEMPERATURE
The DS2438 measures temperatures through the use of an on-board temperature measurement technique.
The temperature reading is provided in a 13-bit, two’s complement format, which provides 0.03125°C of
resolution. Table 1 describes the exact relationship of output data to measured temperature. The data is
transmitted serially over the 1-Wire interface. The DS2438 can measure temperature over the range of
-55°C to +125°C in 0.03125°C increments. For Fahrenheit usage, a lookup table or conversion factor
must be used.
Note that temperature is represented in the DS2438 in terms of a 0.03125°C LSb, yielding the following
13-bit format. The 3 least significant bits of the Temperature Register will always be 0. The remaining
13 bits contain the two’s complement representation of the temperature in °C, with the MSb holding the
sign (S) bit. See “Memory Map” section for the Temperature Register address location.
Temperature Register Format Table 1
2-1
2-2
2-3
2-4
MSb
S
2-5
0
0
(unit = °C)
26
25
24
23
22
0
LSB
LSb
21
20
MSB
TEMPERATURE
DIGITAL OUTPUT (Binary)
DIGITAL OUTPUT (Hex)
+125°C
01111101 00000000
7D00h
+25.0625°C
00011001 00010000
1910h
+0.5°C
00000000 10000000
0080h
0°C
00000000 00000000
0000h
-0.5°C
11111111 10000000
FF80h
-25.0625°C
11100110 11110000
E6F0h
-55°C
11001001 00000000
C900h
OPERATION-MEASURING BATTERY VOLTAGE
The on-board analog-to-digital converter (ADC) has 10 bits of resolution and will perform a conversion
when the DS2438 receives a command protocol (Convert V) instructing it to do so. The result of this
measurement is placed in the 2-byte Voltage Register. The range for the DS2438 ADC is 0 volt to
10 volt; this range is suitable for NiCd or NiMH battery packs up to six cells, and for lithium ion battery
packs of two cells. The full-scale range of the ADC is scaled to 10.23 volt, resulting in a resolution of
10 mV.
While the ADC has a range that extends down to 0 volt, it is important to note that the battery voltage can
also be the supply voltage to the DS2438. As such, the accuracy of the ADC begins to degrade below
battery voltages of 2.4 volt, and the ability to make conversions is limited by the operating voltage range
of the DS2438.
Voltage is expressed in this register in scaled binary format, as outlined in Table 2. Note that while codes
exist for values below 2.4 volt, accuracy of the ADC and the limitation on the DS2438’s supply voltage
make it unlikely that these values would be used in actual practice. See “Memory Map” section for the
Voltage Register address location.
4 of 29
DS2438
VOLTAGE REGISTER FORMAT Table 2
27
26
25
MSb
0
24
23
22
21
(unit = 10 mV)
0
0
0
0
0
20
LSB
LSb
29
28
MSB
BATTERY
VOLTAGE
0.05V
DIGITAL OUTPUT (Binary)
DIGITAL OUTPUT (Hex)
0000 0000 0000 0101
0005h
2.7V
0000 0001 0000 1110
010Eh
3.6V
0000 0001 0110 1000
0168h
5V
0000 0001 1111 0100
01F4h
7.2V
0000 0010 1101 0000
02D0h
9.99V
0000 0011 1110 0111
03E7h
10V
0000 0011 1110 1000
03E8H
For applications requiring a general purpose voltage A/D converter, the DS2438 can be configured so that
the result of a Convert V command will place the scaled binary representation of the voltage on the VAD
input (as opposed to the VDD input) into the Voltage Register in the same format described in Table 2.
Depending upon the state of the Status/Configuration Register, either (but not both) the VDD or VAD
voltage will be stored in the Voltage Register upon receipt of the Convert V command. Refer to the
description of the Status/Configuration Register in the Memory Map section for details. If the VAD input
is used as the voltage input, the A/D will be accurate for 1.5V < VAD < 2VDD over the range 2.4V < VDD <
5.0V. This feature gives the user the ability to have a voltage A/D that meets spec accuracy for inputs
over the entire range of 1.5V < VAD < 10V for VDD = 5.0V.
OPERATION - MEASURING BATTERY CURRENT
The DS2438 features an A/D converter that effectively measures the current flow into and out of the
battery pack by measuring the voltage across an external sense resistor. It does so in the background at a
rate of 36.41 measurements/sec; thus, no command is required to initiate current flow measurements.
However, the DS2438 will only perform current A/D measurements if the IAD bit is set to “1” in the
Status/Configuration Register. The DS2438 measures current flow in and out of the battery through the
VSENS pins; the voltage from the VSENS+ pin to the VSENS- pin is considered to be the voltage across the
current sense resistor, RSENS. The VSENS+ terminal may be tied directly to the RSENS resistor, however, for
VSENS-, we recommend use of an RC low pass filter between it and the GND end of RSENS (see the block
diagram in Figure 1). Using a 100 kW (min) resistor (RF) and a 0.1 mF tantalum capacitor (CF), the filter
cutoff is approximately 15.9 Hz. The current A/D measures at a rate of 36.41 times per second, or once
every 27.46 ms. This filter will capture the effect of most current spikes, and will thus allow the current
accumulators to accurately reflect the total charge which has gone into or out of the battery.
The voltage across current sense resistor RSENS is measured by the ADC and the result is placed in the
Current Register in two’s complement format. The sign (S) of the result, indicating charge or discharge,
resides in the most significant bit of the Current Register, as shown in Table 3. See “Memory Map” in
Figure 7 for the Current Register address location.
5 of 29
DS2438
CURRENT REGISTER FORMAT Table 3
(This register actually stores the voltage measured across current sense resistor RSENS.
This value can be used to calculate battery pack current using the equation below.)
27
26
25
MSb
S
24
23
22
21
(unit = 0.2441mV)
S
S
S
S
S
20
LSB
LSb
29
28
MSB
The battery pack current is calculated from the Current Register value using the equation:
(where RSENS is in W)
I = Current Register / (4096 * RSENS)
For example, if 1.25A is flowing into the pack, and the pack uses a 0.025W sense resistor, the DS2438
will write the value 12810 to the Current Register. From this value, battery pack current can be calculated
to be:
I = 128 / ( 4096 * 0.025) = 1.25A
Because small current ADC offset errors can have a large cumulative effect when current is integrated
over time, the DS2438 provides a method for canceling offset errors in the current ADC. After each
current measurement is completed, the measured value is added to the contents of the Offset Register and
the result is then stored in the Current Register. The Offset Register is a two-byte nonvolatile read/write
register formatted in two’s-complement format. The four MSb’s of the register contain the sign of the
offset, as shown in Table 4.
OFFSET REGISTER FORMAT Table 4
24
23
MSb
X
22
21
20
0
0
(unit = 0.2441 mV)
X
X
S
28
27
0
LSB
LSb
26
25
MSB
The following process can be used to calibrate the current ADC:
1. Write all zeroes to the Offset Register
2. Force zero current through RSENS
3. Read the Current Register value
4. Disable the current ADC by setting the IAD bit in the Status/Configuration Register to “0”
5. Change the sign of the previously-read Current Register value by performing the two’s complement
and write the result to the Offset Register
6. Enable the current ADC by setting the IAD bit in the Status/Configuration Register to “1”
NOTE:
When writing to the Offset Register, current measurement MUST be disabled (IAD bit set to “0”).
The current ADC calibration process is done for each DS2438 device prior to shipment. However, for
best results, battery pack manufacturers should calibrate the current ADC during initial battery pack
testing, and the host system should calibrate whenever possible (during battery charging, for example).
6 of 29
DS2438
OPERATION - CURRENT ACCUMULATORS
The DS2438 tracks the remaining capacity of a battery using the Integrated Current Accumulator (ICA).
The ICA maintains a net accumulated total of current flowing into and out of the battery; therefore, the
value stored in this register is an indication of the remaining capacity in a battery and may be used in
performing fuel gauge functions. In addition, the DS2438 has another register that accumulates only
charging (positive) current (CCA) and one that accumulates only discharging (negative) current (DCA).
The CCA and DCA give the host system the information needed to determine the end of life of a
rechargeable battery, based on total charge/discharge current over its lifetime.
The current measurement described above yields the voltage across sense resistor RSENS measured every
27.46 ms. This value is then used to increment or decrement the ICA register, increment the CCA (if
current is positive), or increment the DCA (if current is negative). The ICA is a scaled 8-bit volatile
binary counter that integrates the voltage across RSENS over time. The ICA is only
incremented/decremented if the IAD bit is set to 1 in the Status/Configuration Register. Table 5 illustrates
the contents of the ICA. See Memory Map section for the address location of the ICA.
ICA REGISTER FORMAT Table 5
(This register accumulates the voltage measured across current sense resistor RSENS. This
value can be used to calculate remaining battery capacity using the equation below.)
27
MSb
26
25
24
23
22
(unit = 0.4882 mVhr)
21
20
LSb
Remaining battery capacity is calculated from the ICA value using the equation:
Remaining Capacity = ICA / (2048 * RSENS)
(where RSENS is in W)
For example, if a battery pack has 0.625 Ahr of remaining capacity, and the pack uses a 0.025W sense
resistor, the ICA will contain the value 32. From this value, remaining capacity can be calculated to be:
Remaining Capacity = 32 / ( 2048 * 0.025) = 0.625 Ahr
Since the accuracy of the current ADC is +2 LSb, measurements of very small currents can be inaccurate
by a high percentage. Because these inaccuracies can turn into large ICA errors when accumulated over a
long period of time, the DS2438 provides a method for filtering out potentially erroneous small signals so
that they are not accumulated. The DS2438’s Threshold Register specifies a current measurement
magnitude (after offset cancellation) above which the measurement is accumulated in the ICA, CCA and
DCA and below which it is not accumulated. The format of the Threshold Register is shown in Table 6.
The power-on default Threshold Register value is 00h (no threshold).
NOTE:
When writing to the Threshold Register, current measurement must be disabled (IAD bit set to “0”).
7 of 29
DS2438
THRESHOLD REGISTER FORMAT Table 6
TH2
TH1
0
0
0
0
0
MSb
0
LSb
TH2
TH1
THRESHOLD
0
0
None (default)
0
1
±2 LSB
1
0
±4 LSB
1
1
±8 LSB
The Charging Current Accumulator (CCA) is a two-byte nonvolatile read/write counter which represents
the total charging current the battery has encountered in its lifetime. It is only updated when current
through RSENS, is positive; i.e., when the battery is being charged. Similarly, the Discharge Current
Accumulator (DCA) is a two-byte nonvolatile counter which represents the total discharging current the
battery has encountered over its lifetime.
The CCA and DCA can be configured to function in any of three modes: disabled, enabled with shadowto-EEPROM, and enabled without shadow-to-EEPROM. When the CCA and DCA are disabled (by
setting either the IAD bit or the CA bit in the Status/Configuration Register to “0”), the memory in page
07h is free for general purpose data storage. When the CCA and DCA are enabled (by setting both IAD
and CA to “1”), page 07h is reserved for these registers, and none of the bytes in page 07h should be
written to via the 1-Wire bus. When the CCA and DCA are enabled, their values are automatically
shadowed to EEPROM memory by setting the EE bit in the Status/Configuration Register to “1”. When
these registers are configured to shadow to EEPROM, the information will accumulate over the lifetime
of the battery pack and will not be lost when the battery becomes discharged. Shadow-to-EEPROM is
disabled when the EE bit is “0”. Table 7 illustrates the format of the CCA and DCA registers. Table 8
summarizes the modes of operation for ICA, CCA and DCA.
CCA/DCA REGISTER FORMAT Table 7
27
26
25
MSb
215
24
23
22
21
(unit = 15.625 mVHr)
214
213
212
211
210
20
LSB
LSb
29
28
MSB
ICA/CCA/DCA MODES OF OPERATION Table 8
IAD Bit
CA Bit
EE Bit
ICA
CCA/DCA
CCA/DCA Copyto-EEPROM
0
X
X
Inactive
Inactive
Inactive
1
0
X
Active
Inactive
Inactive
1
1
0
Active
Active
Inactive
1
1
1
Active
Active
Active
8 of 29
DS2438
Figure 2 illustrates the activity of the ICA, CCA, and DCA over a sample charge/discharge cycle of a
battery pack, assuming the DS2438 is configured for the ICA to function and the CCA/DCA to function
and shadow data to EEPROM. To simplify the illustration of the accumulators, they are treated as analog
values, although they are digital counters in the DS2438. Note that when the battery becomes fully
discharged, i.e., the ICA value reaches 0, the CCA and DCA register values are maintained.
CURRENT ACCUMULATOR ACTIVITY Figure 2
SENSE RESISTOR SELECTION
The selection of RSENS involves a tradeoff. On the one hand, the impedance of RSENS must be minimized
to avoid excessive voltage drop during peak current demands. On the other hand, the impedance of RSENS
should be maximized to achieve the finest resolution for current measurement and accumulation. Table 9
below lists several example RSENS values, the LSb of the current calculation ( 1/(4096 * RSENS) ) and the
LSb of the remaining capacity calculation ( 1/(2048 * RSENS) ). The user should carefully consider
voltage drop at maximum current and required current measurement/accumulation resolution when
selecting RSENS.
SENSE RESISTOR TRADEOFFS Table 9
SENSE RESISTOR
VALUE (RSENS)
25 mW
CURRENT lsb
9.76 mA
REMAINING
CAPACITY lsb
19.53 mAHr
MAX REMAINING
CAPACITY VALUE
5000 mAhr
50 mW
4.88 mA
9.76 mAHr
2500 mAhr
100 mW
2.44 mA
4.88 mAHr
1250 mAhr
200 mW
1.22 mA
2.44 mAHr
625 mAhr
OPERATION - ELAPSED TIME METER
An internal oscillator is used as the timebase for the timekeeping functions. The elapsed time functions
are double buffered, allowing the master to read elapsed time without the data changing while it is being
read. To accomplish this, a snapshot of the counter data is transferred to holding registers which the user
accesses. This occurs after the 8th bit of the Recall Memory command.
The elapsed time meter (ETM) is a 4-byte binary counter with 1-second resolution. The ETM can
accumulate 136 years of seconds before rolling over. Time/date is represented by the number of seconds
since a reference point, which is determined by the user. For example, 12:00 A.M., January 1, 1970 could
be used as a reference point.
9 of 29
DS2438
Two other time-related functions are available. The first is the Disconnect Timestamp, which is written to
by the DS2438 whenever it senses that the DQ line has been low for approximately 2 seconds. This
condition would signal that the battery pack has been removed from the system; the time when that
occurs is written into the Disconnect Timestamp register, so that upon replacement into the system, the
system can determine how long the device has been in storage, to facilitate self-discharge corrections to
the remaining battery capacity. After the disconnect has been detected, the DS2438 reverts to a sleep
mode, during which nothing is active except the real time clock. Some applications may prefer that the
data converters and current accumulators continue operation following a pack disconnect. Thus, a
version of the DS2438 (part number DS2438A) is offered for those applications. Other than not reverting
to a low-power sleep mode following disconnect, there are no specification differences between the
DS2438 and the DS2438A.
The other timestamp is the End-of-Charge timestamp, which is written to by the DS2438 whenever it
senses that charging is finished (when current changes direction). This timestamp allows the user to
calculate the amount of time the battery has been in a discharge or storage state, again to facilitate selfdischarge calculations.
The format of the ETM, Disconnect, and End-of-Charge registers are as shown in Table 10. Refer to the
“Memory Map” section for the address location of the time-related registers.
TIME REGISTER FORMAT Table 10
27
26
25
24
MSb
215
214
213
212
21
211
210
222
221
220
219
218
29
229
228
227
226
LSB
28
LSb
217
(unit = 1s)
230
20
LSb
(unit = 1s)
MSb
231
22
(unit = 1s)
MSb
223
23
216
LSb
225
224
MSB
64-BIT LASERED ROM
Each DS2438 contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code
(DS2438 code is 26h). The next 48 bits are a unique serial number. The last 8 bits are a CRC of the first
56 bits. (See Figure 3.) The 64-bit ROM and ROM Function Control section allow the DS2438 to operate
as a 1-Wire device and follow the 1-Wire protocol detailed in the section “1-Wire Bus System.” The
functions required to control sections of the DS2438 are not accessible until the ROM function protocol
has been satisfied. This protocol is described in the ROM function protocol flow chart (Figure 5). The
1-Wire bus master must first provide one of four ROM function commands: 1) Read ROM, 2) Match
ROM, 3) Search ROM, or 4) Skip ROM. After a ROM function sequence has been successfully executed,
the functions specific to the DS2438 are accessible and the bus master may then provide any one of the
six memory and control function commands.
10 of 29
DS2438
64-BIT LASERED ROM FORMAT Figure 3
8-BIT CRC CODE
MSb
48-BIT SERIAL NUMBER
LSb
MSb
LSb
8-BIT FAMILY CODE (26h)
MSb
LSb
CRC Generation
The DS2438 has an 8-bit CRC stored in the most significant byte of the 64-bit ROM. The bus master can
compute a CRC value from the first 56 bits of the 64-bit ROM and compare it to the value stored within
the DS2438 to determine if the ROM data has been received error-free by the bus master. The equivalent
polynomial function of this CRC is:
CRC = X8 + X5 + X4 +1
The DS2438 also generates an 8-bit CRC value using the same polynomial function shown above and
provides this value to the bus master to validate the transfer of data bytes. In each case where a CRC is
used for data transfer validation, the bus master must calculate a CRC value using the polynomial
function given above and compare the calculated value to either the 8-bit CRC value stored in the 64-bit
ROM portion of the DS2438 (for ROM reads) or the 8-bit CRC value computed within the DS2438
(which is read as a 9th byte when a scratchpad is read). The comparison of CRC values and decision to
continue with an operation are determined entirely by the bus master. There is no circuitry inside the
DS2438 that prevents a command sequence from proceeding if the CRC stored in or calculated by the
DS2438 does not match the value generated by the bus master. Proper use of the CRC as outlined in the
flowchart of Figure 6 can result in a communication channel with a very high level of integrity.
The 1-Wire CRC can be generated using a polynomial generator consisting of a shift register and XOR
gates as shown in Figure 4. Additional information about the Dallas 1-Wire Cyclic Redundancy Check is
available in Application Note 27 entitled “Understanding and Using Cyclic Redundancy Checks with
Dallas Semiconductor Touch Memory Products.”
The shift register bits are initialized to 0. Then starting with the least significant bit of the family code,
1 bit at a time is shifted in. After the 8th bit of the family code has been entered, the serial number is
entered. After the 48th bit of the serial number has been entered, the shift register contains the CRC
value.
1-WIRE CRC CODE Figure 4
11 of 29
DS2438
ROM FUNCTIONS FLOWCHART Figure 5
12 of 29
DS2438
MEMORY/CONTROL FUNCTIONS FLOWCHART Figure 6
NO
13 of 29
YES
DS2438
MEMORY/CONTROL FUNCTIONS FLOWCHART Figure 6 (continued)
MEMORY MAP
The DS2438’s memory is organized as shown in Figure 7. The memory consists of a scratchpad RAM
and storage SRAM/EEPROM. The scratchpad helps insure data integrity when communicating over the
1-Wire bus. Data is first written to the scratchpad where it can be read back. After the data has been
verified, a copy scratchpad command will transfer the data to the appropriate page in memory (pages 0-2
are primarily volatile SRAM, pages 3-7 are EEPROM). This process insures data integrity when
modifying the memory.
The DS2438’s memory is organized as 64 bytes of memory, in eight 8-byte pages. Each page has its own
scratchpad space, organized as 8 bytes of memory. When reading a scratchpad, there is a 9th byte which
may be read with a Read Scratchpad command. This byte contains a cyclic redundancy check (CRC)
byte, which is the CRC over all of the 8 bytes in the currently selected scratchpad. This CRC is
implemented in the fashion described in the section titled “CRC Generation.”
14 of 29
DS2438
Page 0 (00h)
The first page contains the most frequently accessed information of the DS2438, and most locations are
volatile read-only bytes with the exception of the Status/Configuration Register (Byte 0) and the
Threshold Register (Byte 7). The Status/Configuration Register is a nonvolatile read/write byte which
defines which features of the DS2438 are enabled and how they will function. The register is formatted as
follows:
X
ADB
NVB
TB
AD
MSb
EE
CA
IAD
LSb
IAD = Current A/D Control Bit. “1” = the current A/D and the ICA are enabled, and current
measurements will be taken at the rate of 36.41 Hz; “0” = the current A/D and the ICA have been
disabled. The default value of this bit is a “1” (current A/D and ICA are enabled).
CA = Current Accumulator Configuration. “1” = CCA/DCA is enabled, and data will be stored and can
be retrieved from page 7, bytes 4-7; “0” = CCA/DCA is disabled, and page 7 can be used for general
EEPROM storage. The default value of this bit is a “1” (current CCA/DCA are enabled).
EE = Current Accumulator Shadow Selector bit. “1” = CCA/DCA counter data will be shadowed to
EEPROM each time the respective register is incremented; “0”= CCA/DCA counter data will not be
shadowed to EEPROM. The CCA/DCA could be lost as the battery pack becomes discharged. If the CA
bit in the status/configuration register is set to “0”, the EE bit will have no effect on the DS2438
functionality. The default value of this bit is a “1” (current CCA/DCA data shadowed to EEPROM).
AD = Voltage A/D Input Select Bit. “1” = the battery input (VDD) is selected as the input for the
DS2438 voltage A/D converter; “0” = the general purpose A/D input (VAD) is selected as the voltage
A/D input. For either setting, a Convert V command will initialize a voltage A/D conversion. The default
value of this bit is a “1” (VDD is the input to the A/D converter).
TB = Temperature Busy Flag. “1” = temperature conversion in progress; “0” = temperature conversion
complete.
NVB = Nonvolatile Memory Busy Flag. “1” = Copy from Scratchpad to EEPROM in progress; “0” =
Nonvolatile memory not busy. A copy to EEPROM may take from 2 ms to 10 ms (taking longer at lower
supply voltages).
ADB = A/D Converter Busy Flag. “1” = A/D conversion in progress on battery voltage; “0” = conversion
complete, or no measurement being made. An A/D conversion takes approximately 10 ms.
X = Don’t care
Bytes 1 and 2 of page 0 contain the last completed temperature conversion in the format described in the
“Operation - Measuring Temperature” section. Bytes 3-4 contain the last completed voltage A/D
conversion result and bytes 5-6 contain the instantaneous current data. Byte 7 contains the Threshold
Register. Refer to the appropriate section for the data format of these locations.
NOTE:
The data in the scratchpad of the status and threshold register will determine the operation of the device.
15 of 29
DS2438
Page 1 (01h)
The second page, Page 1, contains the ICA, elasped time meter, and current offset data. Both the ETM
and ICA are volatile read/write locations so that they may be set, changed, or cleared by the host
software. Bytes 0-3 contain the ETM data, formatted as described in the “Operation - Elapsed Time
Meter” section. Byte 4 contains the 8-bit ICA. Bytes 5 and 6 contain the Offset Register data. Byte 7 is
reserved and will read out as all “1”s.
Page 2 (02h)
The third page of memory (Page 2) contains the Disconnect (first 4 bytes) and End of Charge (remaining
4 bytes) timestamps. This page is volatile and read/write. Refer to the “Operation – Elapsed Time Meter”
section for the formatting of these locations.
Pages 3-7 (03h - 07h)
The remainder of the memory in the DS2438 (Pages 3 through 7) is backed with EEPROM. This memory
provides 40 bytes of user memory which may be used to carry any information the user wishes to store.
Additionally, the CCA/DCA information is stored in bytes 4-7 of page 7 if the DS2438 is configured
appropriately. If the CCA/DCA is used, page 7 should not be written to or current accumulator data will
be overwritten. See “Operation-Current Accumulators” for details.
MEMORY MAP Figure 7
PAGE
BYTE
0
R/W
R/W
NV
YES
R
3
4
5
6
7
CONTENTS
STATUS/
CONFIGURATION
TEMPERATURE
LSB
TEMPERATURE
MSB
VOLTAGE LSB
VOLTAGE MSB
CURRENT LSB
CURRENT MSB
THRESHOLD
0
1
2
3
4
5
6
7
ETM BYTE 0
ETM BYTE 1
ETM BYTE 2
ETM BYTE 3
ICA
OFFSET LSB
OFFSET MSB
RESERVED
0
DISCONNECT
BYTE 0
DISCONNECT
BYTE 1
DISCONNECT
BYTE 2
DISCONNECT
BYTE 3
END OF CHARGE
BYTE 0
END OF CHARGE
BYTE 1
END OF CHARGE
BYTE 2
END OF CHARGE
BYTE 3
1
2
0
1
1
2
2
3
4
5
6
7
PAGE
BYTE
0
CONTENTS
USER BYTE
R/W
R/W
NV
YES
NO
1
USER BYTE
R/W
YES
R
NO
2
USER BYTE
R/W
YES
R
R
R
R
R/W
NO
NO
NO
NO
YES
3
4
5
6
7
USER BYTE
USER BYTE
USER BYTE
USER BYTE
USER BYTE
R/W
R/W
R/W
R/W
R/W
YES
YES
YES
YES
YES
R/W
R/W
R/W
R/W
R/W
R/W
R/W
NO
NO
NO
NO
NO
YES
YES
R/W
R/W
NO
NO
0
1
2
3
4
5
6
7
USER BYTE
USER BYTE
USER BYTE
USER BYTE
USER BYTE
USER BYTE
USER BYTE
USER BYTE
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
YES
YES
YES
YES
YES
YES
YES
YES
·
·
·
·
·
R/W
NO
·
·
·
·
·
R/W
NO
·
·
·
·
·
R/W
NO
0
USER BYTE
R/W
YES
R/W
NO
1
USER BYTE
R/W
YES
R/W
NO
2
USER BYTE
R/W
YES
R/W
NO
3
USER BYTE
R/W
YES
R/W
NO
4
USER BYTE/
CCA LSB
USER BYTE/
CCA MSB
USER BYTE/
DCA LSB
USER BYTE/
DCA MSB
R/W
YES
R/W
YES
R/W
YES
R/W
YES
3
4
7
5
6
7
16 of 29
DS2438
1-Wire BUS SYSTEM
The 1-Wire bus is a system which has a single bus master and one or more slaves. The DS2438 behaves
as a slave. The discussion of this bus system is broken down into three topics: hardware configuration,
transaction sequence, and 1-Wire signaling (signal types and timing).
HARDWARE CONFIGURATION
The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to
drive it at the appropriate time. To facilitate this, each device attached to the 1-Wire bus must have open
drain or 3-state outputs. The 1-Wire port of the DS2438 (DQ pin) is open drain with an internal circuit
equivalent to that shown in Figure 8. A multidrop bus consists of a 1-Wire bus with multiple slaves
attached. The 1-Wire bus requires a pull-up resistor of approximately 5 kW.
HARDWARE CONFIGURATION Figure 8
The idle state for the 1 wire bus is high. If for any reason a transaction needs to be suspended, the bus
MUST be left in the idle state if the transaction is to resume. Infinite recovery time can occur between
bits so long as the 1-Wire bus is in the inactive (high) state during the recovery period. If this does not
occur and the bus is left low, all components on the bus will be reset. See Wire-1 Reset Pulse Timing
(Figure 9).
TRANSACTION SEQUENCE
The protocol for accessing the DS2438 via the 1-Wire port is as follows:
§
§
§
§
Initialization
ROM Function Command
Memory Function Command
Transaction/Data
INITIALIZATION
All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence
consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the
slave(s). The presence pulse lets the bus master know that the DS2438 is on the bus and is ready to
operate. For more details, see the “1-Wire Signaling” section.
17 of 29
DS2438
ROM FUNCTION COMMANDS
Once the bus master has detected a presence, it can issue one of the four ROM function commands. All
ROM function commands are 8-bits long. A list of these commands follows (refer to flowchart in
Figure 5):
Read ROM [33h]
This command allows the bus master to read the DS2438’s 8-bit family code (26h), unique 48-bit serial
number, and 8-bit CRC. This command can only be used if there is a single DS2438 on the bus. If more
than one slave is present on the bus, a data collision will occur when all slaves try to transmit at the same
time (open-drain will produce a wired-AND result).
Match ROM [55h]
The Match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a
specific DS2438 on a multidrop bus. Only the DS2438 that exactly matches the 64-bit ROM sequence
will respond to the following memory function command. All slaves that do not match the 64-bit ROM
sequence will wait for a reset pulse. This command can be used with a single or multiple devices on the
bus.
Skip ROM [CCh]
This command can save time in a single-drop bus system by allowing the bus master to access the
memory functions without providing the 64-bit ROM code. If more than one slave is present on the bus
and a read command is issued following the Skip ROM command, data collision will occur on the bus as
multiple slaves transmit simultaneously (open drain pull-downs will produce a wired-AND result).
Search ROM [F0h]
When a system is initially brought up, the bus master might not know the number of devices on the 1Wire bus or their 64-bit ROM codes. The search ROM command allows the bus master to use a process
of elimination to identify the 64-bit ROM codes of all slave devices on the bus.
Example of a ROM Search
The ROM search process is the repetition of a simple three-step routine: read a bit, read the complement
of the bit, then write the desired value of that bit. The bus master performs this simple, three-step routine
on each bit of the ROM. After one complete pass, the bus master knows the contents of the ROM in one
device. The remaining number of devices and their ROM codes may be identified by additional passes.
The following example of the ROM search process assumes four different devices are connected to the
same 1-Wire bus. The ROM data of the four devices is as shown (LSb first):
ROM1 = 00110101...
ROM2 = 10101010...
ROM3 = 11110101...
ROM4 = 00010001...
The search process is as follows:
1. The bus master begins the initialization sequence by issuing a reset pulse. The slave devices respond
by issuing simultaneous presence pulses.
2. The bus master will then issue the search ROM command on the 1-Wire bus (F0h).
18 of 29
DS2438
3. The bus master reads a bit from the 1-Wire bus. Each device will respond by placing the value of the
first bit of their respective ROM data onto the 1-Wire bus. ROM1 and ROM4 will place a 0 onto the
1-Wire bus, i.e., pull it low. ROM2 and ROM3 will place a 1 onto the 1-Wire bus by allowing the line
to stay high. The result is the logical AND of all devices on the line; therefore the bus master sees a 0.
The bus master reads another bit. Since the Search ROM data command is being executed, all of the
devices on the 1-Wire bus respond to this second read by placing the complement of the first bit of
their respective ROM data onto the 1-Wire bus. ROM1 and ROM4 will place a 1 onto the 1-wire,
allowing the line to stay high. ROM2 and ROM3 will place a 0 onto the 1-wire, thus it will be pulled
low. The bus master again observes a 0 for the complement of the first ROM data bit. The bus master
has determined that there are some devices on the 1-Wire bus that have a 0 in the first position and
others that have a 1.
The data obtained from the two reads of the three-step routine have the following interpretations:
00 - There are still devices attached which have conflicting bits in this position.
01 - All devices still coupled have a 0 bit in this bit position.
10 - All devices still coupled have a 1 bit in this bit position.
11 - There are no devices attached to the 1-Wire bus.
4. The bus master writes a 0. This deselects ROM2 and ROM3 for the remainder of this search pass,
leaving only ROM1 and ROM4 connected to the 1-Wire bus.
5. The bus master performs two more reads and receives a 0 bit followed by a 1 bit. This indicates that
all devices still coupled to the bus have 0s as their second ROM data bit.
6. The bus master then writes a 0 to keep both ROM1 and ROM4 coupled.
7. The bus master executes two reads and receives two 0 bits. This indicates that both 1 bits and 0 bits
exist as the 3rd bit of the ROM data of the attached devices.
8. The bus master writes a 0 bit. This deselects ROM1, leaving ROM4 as the only device still connected.
9. The bus master reads the remainder of the ROM bits for ROM4 and continues to access the part if
desired. This completes the first pass and uniquely identifies one part on the 1-Wire bus.
10. The bus master starts a new ROM search sequence by repeating steps 1 through 7.
11. The bus master writes a 1 bit. This decouples ROM4, leaving only ROM1 still coupled.
12. The bus master reads the remainder of the ROM bits for ROM1 and communicates to the underlying
logic if desired. This completes the second ROM search pass, in which another of the ROMs was
found.
13. The bus master starts a new ROM search by repeating steps 1 through 3.
14. The bus master writes a 1 bit. This deselects ROM1 and ROM4 for the remainder of this search pass,
leaving only ROM2 and ROM3 coupled to the system.
15. The bus master executes two read time slots and receives two 0s.
19 of 29
DS2438
16. The bus master writes a 0 bit. This decouples ROM3, and leaving only ROM2.
17. The bus master reads the remainder of the ROM bits for ROM2 and communicates to the underlying
logic if desired. This completes the third ROM search pass, in which another of the ROMs was found.
18. The bus master starts a new ROM search by repeating steps 13 through 15.
19. The bus master writes a 1 bit. This decouples ROM2, leaving only ROM3.
20. The bus master reads the remainder of the ROM bits for ROM3 and communicates to the underlying
logic if desired. This completes the fourth ROM search pass, in which another of the ROMs was
found.
Note that the bus master learns the unique ID number (ROM data pattern) of one 1-Wire device on each
ROM Search operation. The time required to derive the part’s unique ROM code is:
960 ms + (8 + 3 x 64) 61 ms = 13.16 ms
The bus master is therefore capable of identifying 75 different 1-Wire devices per second.
MEMORY COMMAND FUNCTIONS
The following command protocols are summarized in Table 11, and by the flowchart of Figure 6.
Write Scratchpad [4Ehxxh]
This command writes to the scratchpad page xxh of the DS2438. The entire 8-byte scratchpad space may
be written, but all writing begins with the byte present at address 0 of the selected scratchpad. After
issuing this command, the user must send the page number of the scratchpad to be written; then the user
may begin writing data to the DS2438 scratchpad. Writing may be terminated at any point by issuing a
reset. Valid page numbers for writing are 00h-07h.
Read Scratchpad [BEhxxh]
This command reads the contents of the scratchpad page xxh on the DS2438. After issuing this command,
the user must send the page number of the scratchpad to be read, and then may begin reading the data,
always beginning at address 0 of the selected scratchpad. The user may read through the end of the
scratchpad space (byte 07h), with any reserved data bits reading all logic 1s, then read the CRC of the
data, and after which the data read will be all logic 1s. If not all locations are to be read, the master may
issue a reset to terminate reading at any time. Valid page numbers are 00h - 07h.
Copy Scratchpad [48hxxh]
This command copies the scratchpad page xxh into the EEPROM / SRAM memory page xxh of the
DS2438. After issuing this command, the user must write a page number to direct which page of memory
the scratchpad is to be copied. Valid page numbers are 00h - 07h. During the copy function, the NVB bit
in the Status/Configuration register will be set to a “l”. When the copy is complete, this bit will reset to
“0”. If the bus master issues read time slots following this command, the DS2438 will output “0” on the
bus as long as it is busy copying the scratchpad to SRAM/EEPROM; it will return a “1” when the copy
process is complete.
20 of 29
DS2438
Recall Memory [B8hxxh]
This command recalls the stored values in EEPROM / SRAM page xxh to the scratchpad page xxh. This
command must proceed a Read SPxx command in order to read any page of memory on the DS2438.
Valid page numbers are 00h - 07h.
Convert T [44h]
This command begins a temperature conversion. No further data is required. The temperature conversion
will be performed, setting the TB flag in the Status/Configuration register to a “1” during conversion.
When the temperature conversion is done, the TB flag will clear to a “0”. If the bus master issues read
time slots following this command, the DS2438 will output “0” on the bus as long as it is busy making a
temperature conversion; it will return a “1” when the temperature conversion is complete.
Convert V [B4h]
This command instructs the DS2438 to initiate a voltage analog-to-digital conversion cycle. This sets the
ADB flag (see Status/Configuration register discussion in the Memory Map section). The voltage supply
that is measured is defined by the AD bit of the Status/Configuration register. When the A/D conversion
is done, the ADB flag is cleared and the current voltage value is placed in the VOLTAGE REGISTER of
page 00h. While an A/D conversion is taking place, all other memory functions are still available for use.
If the bus master issues read time slots following this command, the DS2438 will output “0” on the bus as
long as it is busy making a voltage measurement; it will return a “1” when the conversion is complete.
DS2438 COMMAND SET Table 11
INSTRUCTION
DESCRIPTION
Read Scratchpad
Reads bytes from
DS2438 Scratchpad
page xxh
Writes bytes to
DS2438
Scratchpad page xxh
BEh <page
00h-07h>
Rx
<read up to nine
bytes of data>
4Eh <page
00h-07h>
Tx
<write up to eight
bytes of data>
Copies entire contents
of Scratchpad page
xxh
to 8-byte EEPROM/
SRAM page xxh
Copies entire contents
of EEPROM/SRAM
page xxh to
Scratchpad
page xxh
48h<page
00h-07h>
Idle or Rx of NVB bit
{NVB bit in Status
Register = 1 until
copy complete (210 ms, typ)}
B8h<page
00h-07h>
Idle
Idle
{TB bit in Status
Register = 1 until
conversion
complete}
{ADB bit in Status
Register = 1 until
conversion
complete}
Write Scratchpad
Copy Scratchpad
Recall Memory
PROTOCOL
1-WIRE BUS
MASTER STATUS
AFTER ISSUING
PROTOCOL
MEMORY COMMANDS
1-WIRE BUS
DATA AFTER
ISSUING
PROTOCOL
REGISTER COMMANDS
Convert T
Initiates temperature
conversion
44h
Idle or Rx of TB bit
Convert V
Initiates voltage A/D
conversion
B4h
Idle or Rx of ADB bit
21 of 29
DS2438
NOTES:
1. Temperature conversion takes up to 10 ms.
2. A/D conversion takes up to 4 ms.
3. EEPROM writes take up to 10 ms.
SAMPLE COMMAND SEQUENCE Table 12
Example: Bus Master enables the ICA, CCA, and DCA on a single DS2438 and configures it such that
the CCA/DCA information is shadowed to EEPROM. The voltage A/D is configured such that the
DS2438 will perform voltage measurements on the battery (VDD) voltage.
MASTER MODE
TX
RX
TX
TX
TX
TX
RX
TX
TX
RX
TX
RX
TX
TX
RX
TX
RX
DATA (LSB FIRST)
Reset
Presence
CCh
4Eh00h
0Fh
Reset
Presence
CCh
BEh00h
<9 data bytes>
Reset
Presence
CCh
48h00h
Read Slots
Reset
Presence
COMMENTS
Read pulse
Presence pulse
Skip ROM
Issue Write SP 00h command
Sets ICA, CA, EE, AD Bits active
Reset pulse
Presence pulse
Skip ROM
Issue Read SP 00h command
Read scratchpad data and CRC
Reset pulse
Presence pulse
Skip ROM
Issue Copy SP 00h command
DS2438 returns a “1” when Copy SP is complete
Reset pulse
Presence Pulse, done
22 of 29
DS2438
SAMPLE COMMAND SEQUENCE Table 13
Example: Bus Master issues a temperature and voltage conversion, then reads the temperature, battery
voltage, battery current, all on a single DS2438.
MASTER MODE
TX
RX
TX
TX
TX
RX
TX
TX
TX
RX
TX
TX
TX
RX
TX
TX
RX
DATA (LSB FIRST)
Reset
Presence
CCh
44h
Reset
Presence
CCh
B4h
Reset
Presence
CCh
B8h00h
Reset
Presence
CCh
BEh00h
<9 data bytes>
TX
RX
Reset
Presence
COMMENTS
Reset pulse
Presence pulse
Skip ROM
Issue Convert Temperature command, Read Slots
Reset pulse
Presence pulse
Skip ROM
Issue Convert Voltage command, Read Slots
Reset pulse
Presence pulse
Skip ROM
Issue Recall Memory page 00h command
Reset pulse
Presence pulse
Skip ROM
Issue Read SP 00h command
Read scratchpad data and CRC. This page contains
temperature, voltage, and current measurements.
Reset pulse
Presence pulse, done
23 of 29
DS2438
SAMPLE COMMAND SEQUENCE Table 14
Example: Assuming a single DS2438 is configured for its current accumulators to function, this sequence
allows the Bus Master to read the three current accumulators.
MASTER MODE
TX
RX
TX
TX
TX
RX
TX
TX
RX
DATA (LSB FIRST)
Reset
Presence
CCh
B8h01h
Reset
Presence
CCh
BEh01h
<9 data bytes>
TX
RX
TX
TX
TX
RX
TX
TX
RX
Reset
Presence
CCh
B8h07h
Reset
Presence
CCh
B8h07h
<9 data bytes>
TX
RX
Reset
Presence
COMMENTS
Reset pulse
Presence pulse
Skip ROM
Issue Recall Memory page 01h command
Reset pulse
Presence pulse
Skip ROM
Issue Read SP 01h command
Read scratchpad data and CRC. The ICA is
located in byte 04h
Reset pulse
Presence pulse
Skip ROM
Issue Recall Memory page 07h command
Reset pulse
Presence pulse
Skip ROM
Issue Read SP 07h command
Read scratchpad data and CRC. The CCA is
located in bytes 04h-05h and the DCA is located in
bytes 06h-07h.
Reset pulse
Presence pulse, done
I/O SIGNALING
The DS2438 requires strict protocols to insure data integrity. The protocol consists of several types of
signaling on one line: reset pulse, presence pulse, write 0, write 1, read 0, and read 1. All of these signals,
with the exception of the presence pulse, are initiated by the bus master.
The initialization sequence required to begin any communication with the DS2438 is shown in Figure 9.
A reset pulse followed by a presence pulse indicates the DS2438 is ready to send or receive data given the
correct ROM command and memory function command. The bus master transmits (Tx) a reset pulse (a
low signal for a minimum of 480 ms). The bus master then releases the line and goes into a receive
mode(Rx). The 1-Wire bus is pulled to a high state via the 5 kW pull-up resistor. After detecting the
rising edge on the I/O pin, the DS2438 waits 15-60 ms and then transmits the presence pulse (a low signal
for 60-240 ms). DS2438 data is read and written through the use of time slots to manipulate bits and a
command word to specify the transaction.
24 of 29
DS2438
INITIALIZATION PROCEDURE “RESET AND PRESENCE PULSES” Figure 9
RESISTOR
MASTER
DS2438
480 ms £ tRSTL < ¥ *
480 ms £ tRSTH < ¥ (includes recovery time)
15 ms £ tPDH < 60 ms
60 ms £ tPDL < 240 ms
Write Time Slots
A write time slot is initiated when the host pulls the data line from a high (inactive) logic level to a low
logic level. There are two types of write time slots: Write 1 time slots and Write 0 time slots. All write
time slots must be a minimum of 60 ms in duration with a minimum of a 1 ms recovery time between
individual write cycles.
The DS2438 samples the I/O line in a window of 15 ms to 60 ms after the I/O line falls. If the line is high,
a Write 1 occurs. If the line is low, a Write 0 occurs (See Figure 10).
For the host to generate a Write 1 time slot, the data line must be pulled to a logic low level and then
released, allowing the data line to pull up to a high level within 15 microseconds after the start of the
write time slot. For the host to generate a Write 0 time slot, the data line must be pulled to a logic low
level and remain low for the duration of the write time slot.
Read Time Slots
The host generates read time slots when data is to be read from the DS2438. A read time slot is initiated
when the host pulls the data line from a logic high level to logic low level. The data line must remain at a
low logic level for a minimum of 1 ms; output data from the DS2438 is then valid within the next 14 ms
maximum.
The host therefore must stop driving the I/O pin low in order to read its state 15 ms from the start of the
read slot. (see Figure 10). By the end of the read time slot, the I/O pin will pull back high via the external
pull-up resistor. All read time slots must be a minimum of 60 ms in duration with a minimum of a 1 ms
recovery time between individual read slots.
25 of 29
DS2438
READ / WRITE TIMING DIAGRAM Figure 10
Write-One Time Slot
DS2438
SAMPLING WINDOW
RESISTOR
MASTER
60 ms £ tSLOT < 120 ms
1 ms £ tLOW1 < 15 ms
1 ms £ tREC < ¥
Write-Zero Time Slot
DS2438
60 ms £ tLOW0 < tSLOT < 120 ms
1 ms £ tREC < ¥
Read-Data Time Slot
RESISTOR
MASTER
DS2438
60 ms £ tSLOT < 120 ms
1 ms £ tLOWR < 15 ms
0 £ tRELEASE < 45 ms
1 ms £ tREC < ¥
tRDV = 15 ms
tSU < 1ms
26 of 29
DS2438
ABSOLUTE MAXIMUM RATINGS*
Voltage on VDD and VAD, Relative to Ground
Voltage on VSENS+, VSENS-, Relative to Ground
Voltage on Any Other Pin Relative to Ground
Operating Temperature
Storage Temperature
Soldering Temperature
-0.3V to + 12V
<±300 mV
-0.3V to + 7.0V
-40°C to +85°C
-55°C to +125°C
See J-STD-020A specification
* 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
(-40°C to +85°C; 2.4V £ VDD £ 10.0V)
PARAMETER
SYMBOL
CONDITION
MIN
TYP MAX
UNITS
NOTES
Supply Voltage
VDD
2.4
10.0
V
1
Data Pin
DQ
-0.3
+5.5
V
1
2.4
5.5
DQ Pull-up Voltage
DC ELECTRICAL CHARACTERISTICS
(-40°C to +85°C; 2.4V £ VDD £ 10.0V)
PARAMETER
SYMBOL
CONDITION
Input Logic High
VIH
2.0
Input Logic Low
VIL
-0.3
Shutdown Current
IDD1
DQ=0, RTC
Active
25
Active Current
IDD
DQ=1, ICA
Active or
Temperature or
Voltage
Conversions or
EEPROM write
in progress
50
Input Resistance
RI
DQ
500
27 of 29
MIN
TYP MAX
0.5
UNITS
NOTES
V
1
V
1
mA
100
mA
kW
2
DS2438
ELECTRICAL CHARACTERISTICS: DIGITAL THERMOMETER
(-40°C to +85°C; 2.4V £ VDD £ 10.0V)
PARAMETER
Thermometer Error (TACTUALTMEASURED)
Conversion Time
SYMBOL
CONDITION
MIN
TYP MAX
TERR
tCONVT
3
UNITS
±2
°C
10
ms
NOTES
ELECTRICAL CHARACTERISTICS: VOLTAGE A/D CONVERTER
(-40°C to +85°C; 2.4V £ VDD £ 10.0V)
PARAMETER
A/D Error
SYMBOL
CONDITION
MIN
TYP MAX
VDDERR
VDD input
±10
±50
VADERR
VAD input
±25
±75
UNITS
NOTES
mV
VAD Input Range
VADR
1.5V
10.0
V
1
VDD Input Range
VDDR
2.4
10.0
V
1
Conversion Time
tCONVV
10
ms
+85
°C
3
No Missing Code
Temperature Range
-40
ELECTRICAL CHARACTERISTICS: CURRENT A/D CONVERTER
(-40°C to +85°C; 2.4V £ VDD £ 10.0V)
PARAMETER
SYMBOL
Current Measurement Input
Range (VSENS+ - VSENS-)
IADRANGE
Current Measurement Error
IADERR
CONDITION
MIN
TYP
|VSENS+ - VSENS-|
£ 125 mV
MAX
UNI
TS
250
mV
±2
LSB
1
%
NOTES
3
ELECTRICAL CHARACTERISTICS: RTC COUNTER
(-40°C to +85°C; 2.4V £ VDD £ 10.0V)
PARAMETER
Clock Error
SYMBOL
CONDITION
RTCERR
0°C to +70°C
Resolution
MIN
1
1
28 of 29
TYP MAX
3
UNITS
%
sec
NOTES
DS2438
AC ELECTRICAL CHARACTERISTICS: NV MEMORY
(-40°C to +85°C; 2.4V £ VDD £ 10.0V)
PARAMETER
NV Write Cycle Time
SYMBOL
CONDITION
MIN
tWR
TYP MAX
2
10
UNITS
NOTES
ms
EEPROM Writes (copy
scratchpad command)
NEEWR
-20ºC to +55ºC
50k
writes
EEPROM data retention
tEEDR
-20ºC to +55ºC
10
years
AC ELECTRICAL CHARACTERISTICS: 1-WIRE INTERFACE
(-40°C to +85°C; 2.4V £ VDD £ 10.0V)
PARAMETER
SYMBOL
CONDITION
MIN
TYP MAX
120
UNITS
Time Slot
tSLOT
60
Recovery Time
tREC
1
Write 0 Low Time
tLOW0
60
120
ms
Write 1 Low Time
tLOW1
1
15
ms
Read Data Valid
tRDV
15
ms
Reset Time High
tRSTH
480
Reset Time Low
tRSTL
480
980
ms
Presence Detect High
tPDH
15
60
ms
Presence Detect Low
tPDL
60
240
ms
DQ Capacitance
CDQ
25
pF
NOTES
ms
ms
ms
NOTES:
1. All voltages are referenced to GND.
2. Input load is to GND.
3. Current measurement accuracy is ±2 LSb or 1%, whichever is greater.
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied.
Maxim 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
© Maxim Integrated
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