DS1921G ,Thermochron iButtonElectrical Characteristics for Accuracy Specification)form of a histogram. Up to 2048 temperature v ..
DS1921G ,Thermochron iButtonElectrical Characteristics(V = +2.8V to +5.25V, T = -40°C to +85°C.)PUP APARAMETER SYMBOL CONDITION ..
DS1921G-F5 ,2.8 to 5.25 V, thermochron buttonFEATURES Digital identification and information by All dimensions are shown in millimeters. mom ..
DS1921Z ,High Resolution Thermochron iButton Range H: +15°C to +46°C; Z: -5°C to +26°CFEATURES #Denotes a RoHS-compliant device that may include lead(Pb) that is Digital identificatio ..
DS1922L-F5 ,Temperature Logger iButton With 8kB Datalog MemoryFEATURES®The DS1922L/T temperature logger iButtons are Automatically Wakes up, Measures Tempera-r ..
DS1923-F5 ,Hygrochron Temperature/Humidity Logger iButton with 8kB Data Log MemoryFEATURES iButton DESCRIPTION Digital Hygrometer Measures Humidity with 8-Bit The DS1923 tempera ..
DS1921G
Thermochron iButton
DS1921GThermochron iButton DeviceEVALUATION KIT AVAILABLE
General DescriptionThe Thermochron®iButton®device (DS1921G) is a
rugged, self-sufficient system that measures temperature
and records the result in a protected memory section.
The recording is done at a user-defined rate, both as a
direct storage of temperature values as well as in the
form of a histogram. Up to 2048 temperature values
taken at equidistant intervals ranging from 1 to 255min
can be stored. The histogram provides 63 data bins with
a resolution of 2.0°C. If the temperature leaves a user-
programmable range, the DS1921G also records when
this happened, for how long the temperature stayed out-
side the permitted range, and if the temperature was too
high or too low. An additional 512 bytes of battery-
backed SRAM allow storing information pertaining to the
object to which the DS1921G is associated. Data is
transferred serially through the 1-Wire®protocol, which
requires only a single data lead and a ground return.
Every DS1921G is factory lasered with a guaranteed
unique, electrically readable, 64-bit registration number
that allows for absolute traceability. The durable stainless
steel package is highly resistant to environmental haz-
ards such as dirt, moisture, and shock. Accessories per-
mit the DS1921G to be mounted on almost any object
including containers, pallets, and bags.
Applications
Benefits and FeaturesHigh Accuracy, Full-Featured Digital Temperature
Logger Simplifies Temperature Data Collection and
Dissemination of Electronic Temperature RecordAccuracy ±1°C from -30°C to +70°C (See the
Electrical Characteristicsfor Accuracy Specification)Digital Thermometer Measures Temperature in
0.5°C IncrementsBuilt-In Real-Time Clock (RTC) and Timer Has
Accuracy of ±2 Minutes per Month from 0°C to +45°CAutomatically Wakes Up and Measures
Temperature at User-Programmable Intervals from
1 Minute to 255 MinutesLogs Consecutive Temperature Measurements in
2kB of Datalog MemoryRecords a Long-Term Temperature Histogram with
2.0°C ResolutionProgrammable Temperature High and Temperature
Low Alarm Trip PointsRecords Up to 24 Timestamps and Durations
When Temperature Leaves the Range Specified
by the Trip PointsIndividually Calibrated in a NIST-Traceable ChamberComplies to Standard EN12830512 Bytes of General-Purpose Battery-Backed SRAM
Rugged Construction Survives Harsh EnvironmentsWater Resistant Enclosure (IP56) or Waterproof if
Placed Inside DS9107 iButton Capsule (Exceeds
Water Resistant 3 ATM Requirements)CE, FCC, and UL913 Certifications
Simple Serial Port Interfaces to Most Microcontrollers
for Rapid Data TransferCommunicates to Host with a Single Digital Signal
Up to 15.4kbps at Standard Speed or Up to
125kbps in Overdrive Mode Using 1-Wire Protocol
Ordering Information
PART TEMP RANGE PIN-PACKAGE DS1921G-F5# -40°C to +85°C F5 Can
DS1921G-F5N# -40°C to +85°C F5 Can
Examples of Accessories
PART ACCESSORY DS9096P Self-Stick Adhesive Pad
DS9101 Multipurpose Clip
DS9093RA Mounting Lock Ring
DS9093A Snap-In Fob
DS9092 iButton Probe
Pin Configuration appears at end of data sheet.
Common iButton Device Featuresappear at end of data
sheet.Thermochron, iButton, and 1-Wire are registered trademarks of
#Denotes a RoHS-compliant device that include lead(Pb) that
is exempt under the RoHS requirements.
A NIST traceability certificate is available for the DS1921G-F5N.
See Application Note 4629 for details.
Temperature Logging in Cold Chain, Food Safety,
Pharmaceutical, and Medical Products
DS1921GThermochron iButton Device
Absolute Maximum Ratings
Electrical Characteristics(VPUP= +2.8V to +5.25V, TA= -40°C to +85°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
IO Voltage Range Relative to GND..........................-0.5V to +6V
IO Sink Current....................................................................20mA
Operating Temperature Range..........................-40°C to +85°C*
Storage Temperature Range..............................-40°C to +50°C*
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
IO PIN: GENERAL DATA 1-Wire Pullup Resistance RPUP (Notes 1, 2) 2.2 k
Input Capacitance CIO (Notes 3, 4) 100 800 pF
Input Load Current ILIO pin at VPUP (Note 5) 10 μA
VPUP > 4.5V 1.14 2.70 High-to-Low Switching Threshold
(Notes 4, 6, 7, 8) VTL 0.71 2.70 V
Input Low Voltage VIL (Notes 1, 6, 9) 0.30 V
VPUP > 4.5V 1.00 2.70 Low-to-High Switching Threshold
(Notes 4, 6, 7, 10) VTH 0.66 2.70 V
Output Low Voltage at 4mA VOL (Notes 6, 11) 0.4 V
Standard speed, RPUP = 2.2k 5
Overdrive speed, RPUP = 2.2k 2 Recovery Time (Notes 1, 4) tREC
Overdrive speed, directly prior to reset
pulse; RPUP = 2.2k5
μs
Standard speed 65 Time-Slot Duration (Notes 1, 12) tSLOTOverdrive speed 8 μs
IO PIN: 1-Wire RESET, PRESENCE-DETECT CYCLE Standard speed, VPUP > 4.5V 480 640
Standard speed
540 640
Overdrive speed, VPUP > 4.5V 48 80 Reset Low Time (Notes 1,12) tRSTL
Overdrive speed
58 80 μs
Standard speed 15 60 Presence-Detect High
Time (Note 12) tPDH Overdrive speed
1.1 6 μs
Standard speed 60
270 Overdrive speed, VPUP > 4.5V
7.5 24 Presence-Detect Low
Time (Note 12) tPDL
Overdrive speed
7.5 32μs
Standard speed 60 75Presence-Detect
Sample Time (Notes 1, 4) tMSP Overdrive speed 6 8.6 μs
*Storage or operation above +50°C significantly reduces battery life.
DS1921GThermochron iButton Device
Note 1:System requirement.
Note 2:Maximum allowable pullup resistance is a function of the number of 1-Wire devices in the system and 1-Wire recovery
times. The specified value here applies to systems with only one device and with the minimum 1-Wire recovery times. For
more heavily loaded systems, an active pullup such as that found in the DS2480B may be required.
Note 3:Capacitance on IO could be 800pF when power is first applied. If a 2.2kΩresistor is used to pull up the data line, 2.5µs
after VPUPhas been applied, the parasite capacitor does not affect normal communication.
Note 4:These values are derived from simulation across process, voltage, and temperature and are not production tested.
Note 5:Input load is to ground.
Note 6:All voltages are referenced to ground.
Note 7:VTLand VTHare functions of the internal supply voltage, which is a function of VPUPand the 1-Wire recovery times. The
VTHand VTLmaximum specifications are valid at VPUP= 5.25V. In any case, VTL< VTH< VPUP.
Note 8:Voltage below which, during a falling edge of IO, a logic 0 is detected.
Note 9:The voltage on IO must be less than or equal to VILMAXwhenever the master drives the line low.
Note 10:Voltage above which, during a rising edge on IO, a logic 1 is detected.
Note 11:The I-V characteristic is linear for voltages less than 1V.
Note 12:Numbers in boldare notin compliance with the published iButton device standards. See the Comparison Table.
Note 13:εin Figure 15 represents the time required for the pullup circuitry to pull the voltage on the IO pin up from VILto VTH. The
actual maximum duration for the master to pull the line low is tW1LMAX+ tF- εand tW0LMAX+ tF- ε, respectively.
Note 14:δin Figure 15 represents the time required for the pullup circuitry to pull the voltage on the IO pin up from VILto the input
high threshold of the bus master. The actual maximum duration for the master to pull the line low is tRLMAX+ tF.
Note 15:This number was derived from a test conducted by Cemagref in Antony, France, in July 2000.
http://www.cemagref.fr/English/index.htm Test Report No. E42
Note 16:Total accuracy is Δϑplus 0.25°C quantization due to the 0.5°C digital resolution of the device.
Electrical Characteristics (continued)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
IO PIN: 1-Wire WRITE Standard speed 60 120
Overdrive speed, VPUP > 4.5V 6
15Write-Zero Low Time
(Notes 1, 12, 13) tW0L
Overdrive speed
8.515μs
Standard speed 5 15 Write-One Low Time
(Notes 1, 13) tW1L
Overdrive speed 1 2
μs
IO PIN: 1-Wire READ Standard speed 5 15 - Read Low Time (Notes 1, 14) tRL Overdrive speed 1 2 - μs
Standard speed tRL + 15 Read Sample Time
(Notes 1, 14) tMSR Overdrive speed tRL + 2 μs
REAL-TIME CLOCK Frequency Deviation F -5°C to +46°C -48 +46 ppm
TEMPERATURE CONVERTER Tempcore Operating Range TTC -40 +85 °C
Conversion Time tCONV 19 90 ms
Thermal Response Time
Constant RESP (Note 15) 130 s
-40°C to < -30°C -1.3 +1.3
-30°C to +70°C -1.0 +1.0 Conversion Error
(Notes 16, 17)
> +70°C to +85°C -1.3 +1.3
°C
Number of Conversions NCONV (Notes 4, 18) (See the lifetime graphs.) —
DS1921GThermochron iButton Device
Note 17:WARNING:Maxim data-logger products are 100% tested and calibrated at time of manufacture to ensure that they meet
all data sheet parameters, including temperature accuracy. As with any sensor-based product, user shall be responsible
for occasionally rechecking the temperature accuracy of the product to ensure it is still operating properly. Furthermore, as
with all products of this type, when deployed in the field and subjected to handling, harsh environments, or other
hazards/use conditions, there may be some extremely small but nonzero logger failure rate. In applications where the fail-
ure of any logger is a concern, user shall assure that redundant (or other primary) methods of testing and determining the
handling methods, quality, and fitness of the articles and products are implemented to further mitigate any risk.
Note 18:The number of temperature conversions (= samples) possible with the built-in energy source depends on the operating and
storage temperature of the device. When not in use for a mission, the RTC oscillator should be turned off and the device
should be stored at a temperature not exceeding +25°C. Under this condition the shelf life time is 10 years minimum.
Comparison Table
LEGACY VALUES DS1921G VALUES
STANDARD SPEED (µs) OVERDRIVE SPEED (µs) STANDARD SPEED (µs) OVERDRIVE SPEED (µs) PARAMETER
MIN MAX MIN MAX MIN MAX MIN MAX tSLOT (including
tREC)61 (undefined) 7 (undefined) 65* (undefined) 8* (undefined)
tRSTL 480 (undefined) 48 80
540 640
58 80 tPDH 15 60 2 6 15 60
1.1 6
tPDL 60 240 8 24 60
2707.5 32tW0L 60 120 6 16 60 120
8.515
iButton Can Physical Specification
SIZE See the Package Information section.
WEIGHT Ca. 3.3g
*Intentional change; longer recovery time between time slots.
Note:Numbers in boldare notin compliance with the published iButton device standards.
Electrical Characteristics (continued)
DS1921GThermochron iButton Device
RTC Deviation vs. TemperatureTEMPERATURE (°C)
RTC DEVIATION (MINUTES/MONTH)
UPPER LIMIT
LOWER LIMIT
Minimum Product Lifetime vs. Temperature at Different Sample RatesMINIMUM PRODUCT LIFETIME (YEARS)
TEMPERATURE (°C)
EVERY MINUTE
EVERY 3 MINUTES
EVERY 10 MINUTES
NO SAMPLES
OSCILLATOR OFF
DS1921GThermochron iButton Device
Minimum Product Lifetime vs. Sample Rate at Different TemperaturesMINUTES BETWEEN SAMPLES
MINIMUM PRODUCT LIFETIME (YEARS)
+15°C
+40°C
+60°C
+70°C
+85°C
+45°C
+50°C
+55°C
-20°C
-40°C
Accuracy LimitsACCURACY (
TEMPERATURE (°C)
UPPER LIMIT
LOWER LIMIT
DS1921GThermochron iButton Device
Detailed DescriptionThe DS1921G is an ideal device to monitor the temper-
ature of any object it is attached to or shipped with,
such as perishable goods or containers of temperature-
sensitive chemicals. The general-purpose battery-
backed SRAM can store an electronic copy of shipping
information, date of manufacture and other important
data written as clear as well as encrypted files. Note
that the initial sealing level of the DS1921G achieves
IP56. Aging and use conditions can degrade the
integrity of the seal over time, therefore, for applications
with significant exposure to liquids, sprays, or other
similar environments, it is recommended to place the
Thermochron in the DS9107 capsule. The DS9107 pro-
vides a watertight enclosure that has been rated to IP68
(refer to Application Note 4126:Understanding the IP
(Ingress Protection) Ratings of iButton Data Loggers
and Capsule).
OverviewFigure 1 shows the relationships between the major
control and memory sections of the DS1921G. The
device has seven main data components: 64-bit
lasered ROM; 256-bit scratchpad; 4096-bit general-
purpose SRAM; 256-bit register page of timekeeping,
control, and counter registers; 96 bytes of alarm time-
stamp and duration logging memory; 126bytes of
histogram memory; and 2048 bytes of data-log mem-
GENERAL-PURPOSE
SRAM
ALARM TIMESTAMP AND
DURATION LOGGING
MEMORY
REGISTER PAGE
MEMORY
FUNCTION
CONTROL
64-BIT
LASERED
ROM
256-BIT
SCRATCHPAD
CONTROL
LOGIC
32.768kHz
OSCILLATOR
3V LITHIUM
TEMPERATURE
CORE
DATA-LOG MEMORY
INTERNAL
TIMEKEEPING,
CONTROL REGISTERS,
AND COUNTERS
HISTOGRAM
MEMORY
ROM
FUNCTION
CONTROL
1-Wire PORTPARASITE-POWERED
CIRCUITRY
DS1921G
Figure 1. Block Diagram
DS1921GThermochron iButton Device
AVAILABLE COMMANDS:COMMAND CODES:DATA FIELD AFFECTED:READ ROM
MATCH ROM
SEARCH ROM
SKIP ROM
OVERDRIVE-SKIP ROM
OVERDRIVE-MATCH ROM
CONDITIONAL SEARCH ROM
33h
55h
F0h
CCh
3Ch
69h
ECh
64-BIT ROM
64-BIT ROM
64-BIT ROM
N/A
OD-FLAG
64-BIT ROM, OD-FLAG
64-BIT ROM, CONDITIONAL SEARCH
SETTINGS, DEVICE STATUS
1-Wire ROM
FUNCTION COMMANDS
WRITE SCRATCHPAD
READ SCRATCHPAD
COPY SCRATCHPAD
READ MEMORY
READ MEMORY WTH CRC
CLEAR MEMORY
CONVERT TEMPERATURE
0Fh
AAh
55h
F0h
A5h
3Ch
44h
256-BIT SCRATCHPAD, FLAGS
256-BIT SCRATCHPAD
4096-BIT SRAM, REGISTERS, FLAGS
ALL MEMORY
ALL MEMORY
MISSION TIMESTAMP, MISSION SAMPLES COUNTER,
START DELAY, SAMPLE RATE, ALARM TIMESTAMPS
AND DURATIONS, HISTOGRAM MEMORY
MEMORY ADDRESS 211h
DS1921G-SPECIFIC
MEMORY/CONTROL
FUNCTION COMMANDS
COMMAND LEVEL:BUS
MASTER
1-Wire NETOTHER DEVICES
DS1921G
Figure 2. Hierarchical Structure for 1-Wire Protocol
ory. Except for the ROM and the scratchpad, all other
memory is arranged in a single linear address space.
All memory reserved for logging purposes, including
counter registers and several other registers, is read-
only for the user. The timekeeping and control regis-
ters are write protected while the device is
programmed for a mission.
The hierarchical structure of the 1-Wire protocol is
shown in Figure 2. The bus master must first provide
one of the seven ROM function commands: Read ROM,
Match ROM, Search ROM, Conditional Search ROM,
Skip ROM, Overdrive-Skip ROM, or Overdrive-Match
ROM. Upon completion of an Overdrive ROM com-
mand byte executed at standard speed, the device
enters overdrive mode, where all subsequent communi-
cation occurs at a higher speed. The protocol required
for these ROM function commands is described in
Figure 13. After a ROM function command is success-
fully executed, the memory functions become accessi-
ble and the master can provide any one of the seven
available commands. The protocol for these memory
function commands is described in Figure 10. All data
is read and written least significant bit first.
Parasite PowerFigure 1 shows the parasite-powered circuitry. This cir-
cuitry “steals” power whenever the IO input is high. IO
provides sufficient power as long as the specified tim-
ing and voltage requirements are met. The advantages
of parasite power are two-fold: 1) By parasiting off this
input, battery power is not consumed for 1-Wire ROM
function commands, and 2) if the battery is exhausted
for any reason, the ROM may still be read normally. The
remaining circuitry of the DS1921G is solely operated
by battery energy. As a consequence, if the battery is
exhausted, all memory data is lost including the data of
the last mission, and no new mission can be started.
Refer to Application Note 5057: OneWireViewer Tips
and Tricksfor how to check the battery status.
DS1921GThermochron iButton Device
MSB
8-BIT
CRC CODE48-BIT SERIAL NUMBER
MSBMSBLSB
LSB
LSB
8-BIT FAMILY CODE
(21h)
MSBLSB
Figure 3. 64-Bit Lasered ROM
1ST
STAGE
2ND
STAGE
3RD
STAGE
4TH
STAGE
7TH
STAGE
8TH
STAGE
6TH
STAGE
5TH
STAGE0X1X2X3X4
POLYNOMIAL = X8 + X5 + X4 + 1
INPUT DATA5X6X7X8
Figure 4. 1-Wire CRC Generator
64-Bit Lasered ROMEach DS1921G contains a unique ROM code that is 64
bits long. The first 8 bits are a 1-Wire family code. The
next 48 bits are a unique serial number. The last 8 bits
are a cyclic redundancy check (CRC) of the first 56 bits
(see Figure 3 for details). The 1-Wire CRC is generated
using a polynomial generator consisting of a shift regis-
ter and XOR gates as shown in Figure 4. The polynomi-
al is X8+ X5 + X4+ 1. Additional information about the
1-Wire CRC is available in Application Note 27:
Understanding and Using Cyclic Redundancy Checks
with Maxim iButton Products.
The Shift register bits are initialized to 0. Then, starting
with the least significant bit of the family code, one bit at
a time is shifted in. After the 8th bit of the family code
has been entered, the serial number is then entered.
After the 48th bit of the serial number has been entered,
the Shift register contains the CRC value. Shifting in the
8 bits of CRC returns the Shift register to all zeros.
MemoryFigure 5 shows the DS1921G memory map. The 4096-bit
general-purpose SRAM makes up pages 0 to 15. The
timekeeping, control, and counter registers fill page 16,
called register page (see Figure 6). Pages 17, 18, and
19 are assigned to storing the alarm timestamps and
durations. The temperature histogram bins begin at page
64 and use up to four pages. The data-log memory cov-
ers pages 128 to 191. Memory pages 20 to 63, 68 to
127, and 192 to 255 are reserved for future extensions.
The scratchpad is an additional page that acts as a
buffer when writing to the SRAM or the register page.
The memory pages 17 and higher are read only for the
user. They are written to or erased solely under the
supervision of the on-chip control logic.
DS1921GThermochron iButton Device
32-BYTE INTERMEDIATE STORAGE SCRATCHPAD ADDRESS
0000h to 01FFh
GENERAL-PURPOSE SRAM (16 PAGES) PAGES 0 to 15
0200h to 021Fh
32-BYTE REGISTER PAGE PAGE 16
0220h to 027Fh
ALARM TIMESTAMPS AND DURATIONS PAGES 17 to 19
0280h to 07FFh (RESERVED FOR FUTURE EXTENSIONS) PAGES 20 to 63
0800h to 087Fh
TEMPERATURE HISTOGRAM MEMORY PAGES 64 to 67
0880h to 0FFFh (RESERVED FOR FUTURE EXTENSIONS) PAGES 68 to 127
1000h to 17FFh
DATA-LOG MEMORY (64 PAGES) PAGES128 to 191
1800h to 1FFFh (RESERVED FOR FUTURE EXTENSIONS) PAGES 192 to 255
Figure 5. Memory Map
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0FUNCTIONACCESS*0200h 0 10 Seconds Single Seconds
0201h 0 10 Minutes Single Minutes
0202h 0 12/24 20 Hour
AM/PM 10 Hour Single Hours
0203h 0 0 0 0 0 Day of Week
0204h 0 0 10 Date Single Date
0205h CENT 0 0 10
Months Single Months
0206h 10 Years Single Years
RTC
Registers R/W R/W**
0207h MS 10 Seconds Alarm Single Seconds Alarm
0208h MM 10 Minutes Alarm Single Minutes Alarm
0209h MH 12/24
20 Hour
AM/PM
Alarm
10 Hour
Alarm Single Hours Alarm
020Ah MD 0 0 0 0 Day of Week Alarm
RTC Alarm
Registers R/W R/W**
020Bh Temperature Low Alarm Threshold
020Ch Temperature High Alarm Threshold
Temperature
Alarms R/W R/W**
020Dh Number of Minutes Between Temperature Conversions Sample Rate R/W R**
020Eh EOSC EMCLR 0 EM RO TLS THS TAS Control R/W R/W**
020Fh (No function, reads 00h) — R R**
Figure 6. Register Pages Map
*The left entry in the ACCESS column is valid between missions. The right entry shows the applicable access mode while a
mission is in progress.
**While a mission is in progress, these addresses can be read. The first attempt to write to these registers (even read-only
ones), however, ends the mission and overwrites selected writable registers.
DS1921GThermochron iButton Device
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0FUNCTIONACCESS*0210h (No function, reads 00h) — R R**
0211h Temperature Read-Out (Forced Conversion) Temperature R R**
0212h Low Byte
0213h High Byte
Mission Start
Delay R/W R/W**
0214h TCB MEMCLR MIP SIP 0 TLF THF TAF Status R/W R/W
0215h Minutes
0216h Hours
0217h Date
0218h Month
0219h Year
Mission
Timestamp R R
021Ah Low Byte
021Bh Center Byte
021Ch High Byte
Mission
Samples
Counter
R R
021Dh Low Byte
021Eh Center Byte
021Fh High Byte
Device
Samples
Counter
R R
Figure 6. Register Pages Map (continued)
*The left entry in the ACCESS column is valid between missions. The right entry shows the applicable access mode while a
mission is in progress.
**While a mission is in progress, these addresses can be read. The first attempt to write to these registers (even read-only
ones), however, ends the mission and overwrites selected writable registers.
Detailed Register Descriptions
TimekeepingThe RTC/alarm and calendar information is accessed
by reading/writing the appropriate bytes in the register
page, address 0200h to 0206h. Note that some bits are
set to 0. These bits always read 0 regardless of how
they are written. The contents of the time, calendar, and
alarm registers are in the binary-coded decimal (BCD)
format.
RTC/CalendarThe RTC of the DS1921G can run in either 12hr or 24hr
mode. Bit 6 of the Hours register (address 0202h) is
defined as the 12hr or 24hr mode select bit. When high,
the 12hr mode is selected. In the 12hr mode, bit 5 is the
AM/PM bit with logic 1 being PM. In the 24hr mode, bit
5 is the 20hr bit (20hr to 23hr).
To distinguish between the days of the week, the
DS1921G includes a counter with a range from 1 to 7.
The assignment of a counter value to the day of week is
arbitrary. Typically, the number 1 is assigned to a
Sunday (U.S. standard) or to a Monday (European stan-
dard).
The calendar logic is designed to automatically com-
pensate for leap years. For every year value that is
either 00 or a multiple of four, the device adds a 29th of
February. This works correctly up to (but not including)
the year 2100.
The DS1921G is Y2K compliant. Bit 7 (CENT) of the
Months register at address 0205h serves as a century
flag. When the Year register rolls over from 99 to 00, the
century flag toggles. It is recommended to write the
century bit to a 1 when setting the RTC to a time/date
between the years 2000 and 2099.
DS1921GThermochron iButton Device
RTC AlarmsThe DS1921G also contains an RTC alarm function. The
RTC Alarm registers are located in registers 0207h to
020Ah. The most significant bit of each of the alarm
registers is a mask bit. When all the mask bits are logic
0, an alarm occurs once per week when the values
stored in timekeeping registers 0200h to 0203h match
the values stored in the RTC Alarm registers. Any alarm
sets the timer alarm flag (TAF) in the device’s Status
register (address 214h). The bus master can set the
search conditions in the Control register (address
20Eh) to identify devices with timer alarms by means of
the conditional search function (see the ROM Function
Commandssection).
RTC Alarm Control
ALARM REGISTER MASK BITS
(BIT 7 OF 0207h TO 20Ah)
MS MMMH MD
FUNCTION 1 1 1 1 Alarm once per second. 1 1 1 Alarm when seconds match (once per minute). 0 1 1 Alarm when minutes and seconds match (once every hour). 0 0 1 Alarm when hours, minutes, and seconds match (once every day). 0 0 0 Alarm when day, hours, minutes, and seconds match (once every week).
RTC and RTC Alarm Registers Map
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 00200h 0 10 Seconds Single Seconds
0201h 0 10 Minutes Single Minutes
0202h 0 12/24 20 Hour
AM/PM 10 Hour Single Hours
0203h 0 0 0 0 0 Day of Week
0204h 0 0 10 Date Single Date
0205h CENT 0 0 10 Months Single Months
0206h 10 Years Single Years
0207h MS 10 Seconds Alarm Single Seconds Alarm
0208h MM 10 Minutes Alarm Single Minutes Alarm
0209h MH 12/24
20 Hour
AM/PM
Alarm
10 Hour
Alarm Single Hours Alarm
020Ah MD 0 0 0 0 Day of Week Alarm
DS1921GThermochron iButton Device
Temperature ConversionThe DS1921G measures temperatures with a resolution
of 0.5°C. Temperature values are represented in a sin-
gle byte as an unsigned binary number, which trans-
lates into a theoretical range of 128°C. The range,
however, has been limited to values from 0000 0000
(00h) through 1111 1010 (FAh). The codes 01h to F9h
are considered valid temperature readings.
If a temperature conversion yields a temperature that is
out of range, it is recorded as 00h (if too low) or FAh (if
too high). Since out-of-range results are accumulated in
histogram bins 0 and 62 (see the Temperature Logging
and Histogramsection), the data in these bins is of lim-
ited value. For this reason the specified temperature
range of the DS1921G is considered to begin at code
04h and end at code F7h, which corresponds to his-
togram bins 1 to 61.
With T[7…0] representing the decimal equivalent of a tem-
perature reading, the temperature value is calculated as
ϑ(°C) = T[7…0]/2 - 40.0
This equation is valid for converting temperature read-
ings stored in the data-log memory as well as for data
read from the Forced Temperature Conversion Readout
register (address 0211h).
To specify the temperature alarm thresholds, this equa-
tion needs to be resolved to
T[7…0] = 2 x ϑ(°C) + 80.0
A value of 23°C, for example, thus translates into 126
decimal or 7Eh. This corresponds to the binary patterns
0111 1110, which could be written to a Temperature
Alarm register (address 020Bh and 020Ch, respectively).
Sample RateThe content of the Sample Rate register (address
020Dh) determines how many minutes the temperature
conversions are apart from each other during a mission.
The sample rate can be any value from 1 to 255, coded
as an unsigned 8-bit binary number. If the memory has
been cleared (Status register bit MEMCLR = 1) and a
mission is enabled (Control register bit EM= 0), writing
a nonzero value to the Sample Rate register starts a mis-
sion. For a full description of the correct sequence of
steps to start a temperature-logging mission, see the
Missioningor Mission Example: Prepare and Start a
New Missionsections.
Temperature Alarm Register Map
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0 020Bh Temperature Low Alarm Threshold
020Ch Temperature High Alarm Threshold
Sample Rate Register Map
ADDRRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0 020Dh Sample Rate
DS1921GThermochron iButton Device
Control RegisterThe DS1921G is set up for its operation by writing
appropriate data to its special function registers that
are located in the register page. Several functions that
are controlled by a single bit only are combined into a
single byte called the Control register (address 020Eh).
This register can be read and written. If the device is
programmed for a mission, writing to the Control regis-
ter ends the mission and changes the register contents.
The functional assignments of the individual bits are
explained below. Bit 5 has no function. It always reads
0 and cannot be written to 1.
Bit 7: Enable Oscillator (EOSC). This bit controls thecrystal oscillator of the RTC. When set to logic 0, the
oscillator starts operation. When written to logic 1, the
oscillator stops and the device is in a low-power data-
retention mode. This bit must be 0 for normal opera-
tion.The RTC must have advanced at least 1 second
before a Mission Start is accepted.
Bit 6: Memory Clear Enable (EMCLR). This bit needsto be set to logic 1 to enable the Clear Memory func-
tion, which is invoked as a memory function command.
The timestamp, histogram memory as well as the
Mission Timestamp, Mission Samples Counter, Mission
Start Delay, and Sample Rate are cleared only if the
Clear Memory command is issued with the next
access to the device. The EMCLR bit returns to 0 asthe next memory function command is executed.
Bit 4: Enable Mission (EM). This bit controls whetherthe DS1921G begins a mission as soon as the sample
rate is written. To enable the device for a mission, this
bit must be 0.
Bit 3: Rollover Enable/Disable (RO). This bit controlswhether the data-log memory is overwritten with new
data or whether data logging is stopped once the mem-
ory is filled with data during a mission. Setting this bit to
a 1 enables the rollover and data logging continues at
the beginning, overwriting previously collected data.
Clearing this bit to 0 disables the rollover and no further
temperature values are stored in the data-log memory
once it is filled with data. This does not stop the mis-
sion. The device continues measuring temperatures
and updating the histogram and alarm timestamps and
durations.
Bit 2: Temperature Low Alarm Search (TLS). If thisbit is 1, the device responds to a Conditional Search
ROM command if, during a mission, the temperature
has reached or is lower than the Low Temperature
Threshold stored at address 020Bh.
Bit 1: Temperature High Alarm Search (THS). If thisbit is 1, the device responds to a Conditional Search
ROM command if, during a mission, the temperature
has reached or is higher than the High Temperature
Threshold stored at address 020Ch.
Bit 0: Timer Alarm Search (TAS). If this bit is 1, thedevice responds to a Conditional Search ROM com-
mand if, during a mission, a timer alarm has occurred.
Since a timer alarm cannot be disabled, the TAF flag
usually reads 1 during a mission. Therefore, it is advis-
able to set the TAS bit to a 0, in most cases.
Mission Start Delay CounterThe content of the Mission Start Delay Counter register
determines how many minutes the device waits before
starting the logging process. The Mission Start Delay
value is stored as an unsigned 16-bit integer number at
addresses 0212h (low byte) and 0213h (high byte). The
maximum delay is 65,535 minutes, equivalent to 45
days, 12 hours, and 15 minutes.
For a typical mission, the Mission Start Delay is 0. If a
mission is too long for a single DS1921G to store all
temperature readings at the selected sample rate, one
can use several devices, staggering the Mission Start
Delay to record the full period. In this case, the rollover
enable (RO) bit in the Control register (address 020Eh)
must be set to 0 to prevent overwriting of the recorded
temperature log after the data-log memory is full. See
the Mission Start and Logging Processsection and
Figure 11 for details.
Control Register Map
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0 020Eh EOSC EMCLR 0 EM RO TLS THS TAS
DS1921GThermochron iButton Device
Status RegisterThe Status register holds device status information
and alarm flags. The register is located at address
0214h. Writing to this register does not necessarily
end a mission.
The functional assignments of the individual bits are
explained below. The bits MIP, TLF, THF, and TAF can
only be written to 0. All other bits are read-only. Bit 3
has no function.
Bit 7: Temperature Core Busy (TCB). If this bit reads0, the DS1921G is currently performing a temperature
conversion. This temperature conversion is either self-
initiated because of a mission being in progress or initi-
ated by a command when a mission is not in progress.
The TCBbit goes low just before a conversion starts
and returns to high just after the result is latched into
the Read-Out register at address 0211h.
Bit 6: Memory Cleared (MEMCLR). If this bit reads 1,the memory pages 17 and higher (alarm timestamps/
durations, temperature histogram, excluding data-log
memory), as well as the Mission Timestamp, Mission
Samples Counter, Mission Start Delay, and Sample
Rate have been cleared to 0 from executing a Clear
Memory function command. The MEMCLR bit returns to
0 as soon as writing a nonzero value to the Sample
Rate register starts a new mission, provided that the EM
bit is also 0. The memory has to be cleared in order
for a mission to start.
Bit 5: Mission in Progress (MIP). If this bit reads 1, theDS1921G has been set up for a mission and this mis-
sion is still in progress. A mission is started if the EMbit
of the Control register (address 20Eh) is 0 and a nonze-
ro value is written to the Sample Rate register, address
20Dh. The MIP bit returns from logic 1 to logic 0 when a
mission is ended. A mission ends with the first write
attempt (Copy Scratchpad command) to any register in
the address range of 200h to 213h. Alternatively, a mis-
sion can be ended by directly writing to the Status reg-
ister and setting the MIP bit to 0. The MIP bit cannot be
set to 1 by writing to the Status register.
BIT 4: Sample in Progress (SIP). If this bit reads 1, theDS1921G is currently performing a temperature conver-
sion as part of a mission in progress. The mission sam-
ples occur on the seconds rollover from 59 to 00. The
SIP bit changes from 0 to 1 approximately 250ms
before the actual temperature conversion begins allow-
ing the circuitry of the chip to wake up. A temperature
conversion including a wake-up phase takes maximum
875ms. During this time, read accesses to the memory
pages 17 and higher are permissible but can reveal
invalid data.
Bit 2: Temperature Low Flag (TLF). Logic 1 in the
temperature low flag bit indicates that a temperature
measurement during a mission revealed a temperature
equal to or lower than the value in the Temperature Low
Threshold register. The temperature low flag can be
cleared at any time by writing this bit to 0. This flag
must be cleared before starting a new mission.
Bit 1: Temperature High Flag (THF). Logic 1 in the
temperature high flag bit indicates that a temperature
measurement during a mission revealed a temperature
equal to or higher than the value in the Temperature
High Threshold register. The temperature high flag can
be cleared at any time by writing this bit to 0. This flag
must be cleared before starting a new mission.
Bit 0: Timer Alarm Flag (TAF). If this bit reads 1, aRTC alarm has occurred (see the Timekeeping section
for details). The timer alarm flag can be cleared at any
time by writing this bit to logic 0. Since the timer alarm
cannot be disabled, the TAF flag usually reads 1 during
a mission. This flag should be cleared before starting a
new mission.
Status Register Map
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0 0214hTCB MEMCLR MIP SIP 0 TLF THF TAF
DS1921GThermochron iButton Device
Mission Timestamp Register Map
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0 0215h 0 10 Minutes Single Minutes
0216h 0 12/24 20 Hour
AM/PM 10 Hour Single Hours
0217h 0 0 10 Date Single Date
0218h 0 0 0 10 Months Single Months
0219h 10 Years Single Years
Mission Samples Counter Register Map
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0 021Ah Low Byte
021Bh Center Byte
021Ch High Byte
Device Samples Counter Register Map
ADDRESSBIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0 021Dh Low Byte
021Eh Center Byte
021Fh High Byte
Mission TimestampThe Mission Timestamp register indicates the time and
date of the first temperature conversion of a mission.
Subsequent temperature conversions take place as
many minutes apart from each other as specified by the
value in the Sample Rate register. Mission samples
occur on minute boundaries.
Mission Samples CounterThe Mission Samples Counter register indicates how
many temperature measurements have taken place
during the current mission in progress (if MIP = 1) or
during the latest mission (if MIP = 0). The value is
stored as an unsigned 24-bit integer number. This
counter is reset through the Clear Memory command.
Device Samples CounterThe Device Samples Counter register indicates how
many temperature measurements have taken place
since the device was assembled at the factory. The
value is stored as an unsigned, 24-bit integer number.
The maximum number that can be represented in this
format is 16,777,215, which is higher than the expected
lifetime of the DS1921G. This counter cannot be reset
under software control.
Temperature Logging and HistogramOnce set up for a mission, the DS1921G logs the tem-
perature measurements simultaneously byte after byte
in the data-log memory as well as in histogram form in
the histogram memory. The data-log memory is able to
store 2,048 temperature values measured at equidis-
tant time points. The first temperature value of a mission
is written to address location 1000h of the data-log
memory, the second value to address location 1001h
and so on. Knowing the starting time point (Mission
Timestamp register), the interval between temperature
measurements, the Mission Samples Counter register,
and the rollover setting, one can reconstruct the time
and date of each measurement stored in the data log.
There are two alternatives to the way the DS1921G
behaves after the 2048 bytes of data-log memory is
filled with data. With rollover disabled (RO = 0), the
device fills the data-log memory with the first 2048 mis-
sion samples. Additional mission samples are not
logged in the data-log, but the histogram and tempera-
ture alarm RAM continue to update. With rollover
enabled (RO = 1), the data log wraps around and over-
writes previous data starting at 1000h for the every
2049thmission sample. In this mode, the device stores
DS1921GThermochron iButton Device
For the temperature histogram, the DS1921G provides
63 bins that begin at memory address 0800h. Each bin
consists of a 16-bit, nonrolling-over binary counter that
is incremented each time a temperature value acquired
during a mission falls into the range of the bin. The least
significant byte of each bin is stored at the lower
address. Bin 0 begins at memory address 0800h, bin 1
at 0802h, and so on up to 087Ch for bin 62, as shown
in Figure 7. The number of the bin to be updated after a
temperature conversion is determined by cutting off the
two least significant bits of the binary temperature
value. Out-of-range values are range locked and count-
ed as 00h or FAh.
Since each data bin is 2 bytes, it can increment up to
65,535 times. Additional measurements for a bin that
has already reached its maximum value are not count-
ed; the bin counter remains at its maximum value. With
the fastest sample rate of one sample every minute, a
2-byte bin is sufficient for up to 45 days if all tempera-
ture readings fall into the same bin.
Temperature Alarm LoggingFor some applications it is essential to not only record
temperature over time and the temperature histogram,
but also record when exactly the temperature exceed-
ed a predefined tolerance band and for how long the
temperature stayed outside the desirable range. The
DS1921G can log high and low durations. The toler-
ance band is specified by means of the Temperature
Alarm Threshold registers, addresses 20Bh and 20Ch
in the register page. One can set a high temperature
and low temperature threshold. See the Temperature
Conversionsection for the data format the temperature
has to be written in. As long as the temperature values
stay within the tolerance band (i.e., are higher than the
low threshold and lower than the high threshold), the
DS1921G does not record any temperature alarm. If the
temperature during a mission reaches or exceeds
either threshold, the DS1921G generates an alarm and
sets either the temperature high flag (THF) or the tem-
perature low flag (TLF) in the Status register (address
TEMPERATURE READING TEMPERATURE
EQUIVALENT IN °C HISTOGRAM BIN NUMBER HISTOGRAM BIN ADDRESS 00h -40.0 or lower0 800h to 801h
01h -39.50 800h to 801h
02h -39.00 800h to 801h
03h -38.50 800h to 801h
04h -38.0 1 802h to 803h
05h -37.5 1 802h to 803h
06h -37.0 1 802h to 803h
07h -36.5 1 802h to 803h
08h -36.0 2 804h to 805h
… … … …
F3h +81.5 60 878h to 879h
F4h +82.0 61 87Ah to 87Bh
F5h +82.5 61 87Ah to 87Bh
F6h +83.0 61 87Ah to 87Bh
F7h +83.5 61 87Ah to 87Bh
F8h +84.062 87Ch to 87Dh
F9h +84.562 87Ch to 87Dh
FAh +85.0 or higher62 87Ch to 87Dh
Figure 7. Histogram Bin and Temperature Cross-Reference
DS1921GThermochron iButton Device
214h). This way, if the search conditions (address
20Eh) are set accordingly, the master can quickly iden-
tify devices with temperature alarms by means of the
conditional search function (see the ROM Function
Commandssection). The device also generates a time-
stamp of when the alarm occurred and begins record-
ing the duration of the alarming temperature.
Timestamps and durations where the temperature
leaves the tolerance band are stored in the address
range 0220h to 027Fh, as shown in Figure 8. This allo-
cation allows recording 24 individual alarm events and
periods (12 periods for too hot and 12 for too cold). The
date and time of each of these periods can be deter-
mined from the Mission Timestamp register and the
time distance between each temperature reading.
The alarm timestamp is a copy of the Mission Samples
Counter register when the alarm first occurred. The
least significant byte is stored at the lower address.
One address higher than the timestamp, the DS1921G
maintains a 1-byte duration counter that stores the
number of samples the temperature was found to be
beyond the threshold. If this counter has reached its
limit after 255 consecutive temperature readings and
the temperature has not yet returned to within the toler-
ance band, the device issues another timestamp at the
next higher alarm location and opens another counter
to record the duration. If the temperature returns to
normal before the counter has reached its limit, the
duration counter of the particular timestamp does not
increment any further. Should the temperature again
cross this threshold, it is recorded at the next available
alarm location. This algorithm is implemented for the
low temperature thresholds as well as for the high tem-
perature threshold.
MissioningThe typical task of the DS1921G is recording the tem-
perature of a temperature-sensitive object. Before the
device can perform this function, it needs to be config-
ured. This procedure is called missioning.
First, the DS1921G must have its RTC set to a valid time
and date. This reference time can be UTC (also called
GMT, Greenwich Mean Time) or any other time stan-
dard that was chosen for the application. The clock
must be running (EOSC= 0) for at least one second.
Setting an RTC alarm is optional. The memory assigned
to store the alarm timestamps and durations, tempera-
ture histogram, Mission Timestamp, Mission Samples
Counter, Mission Start Delay, and Sample Rate must be
cleared using the Clear Memory command. In case
there were temperature alarms in the previous mission,
the TLF and THF flags need to be cleared manually. To
enable the device for a mission, the EMflag must be
set to 0. These are general settings that have to be
made regardless of the type of object to be monitored
and the duration of the mission.
ADDRESS DESCRIPTION ALARM EVENT 0220h Mission Samples Counter, Low Byte
0221h Mission Samples Counter, Center Byte
0222h Mission Samples Counter, High Byte
0223h Alarm Duration Counter
Low Alarm 1
0224h to 0227h Alarm Timestamp and Duration Low Alarm 2
0228h to 024Fh Alarm Timestamp and Durations Low Alarms 3 to 12
0250h Mission Samples Counter, Low Byte
0251h Mission Samples Counter, Center Byte
0252h Mission Samples Counter, High Byte
0253h Alarm Duration Counter
High Alarm 1
0254h to 0257h Alarm Timestamp and Duration High Alarm 2
0258h to 027Fh Alarm Timestamp and Durations High Alarms 3 to 12
Figure 8. Alarm Timestamps and Durations Address Map
DS1921GThermochron iButton Device
Next, the low temperature and high temperature thresh-
olds that specify the temperature tolerance band must
be defined. The Temperature Conversionsection
describes how to convert a temperature value into the
binary code to be written to the threshold registers.
The state of the search condition bits in the Control
register does not affect the mission. If multiple devices
are connected to form a 1-Wire net, the setting of the
search condition enables these devices to participate
in the conditional search if certain events, such as
timer or temperature alarms, have occurred. Details
on the search conditions are found in the ROM
Function Commandssection and in the Control regis-
ter description.
The setting of the rollover-enable bit (RO) and sample
rate depends on the duration of the mission and the
monitoring requirements. If the most recent temperature
history is important, the rollover should be enabled
(RO= 1). Otherwise, one should estimate the duration
of the mission in minutes and divide the number by
2048 to calculate the value of the sample rate (number
of minutes between temperature conversions). For
example, if the estimated duration of a mission is 10
days (14,400min), then the 2048-byte capacity of the
data-log memory would be sufficient to store a new
value every 7min. If the DS1921G’s data-log memory is
not large enough to store all temperature readings, one
can use several devices and set the Mission Start Delay
to values that make the second device start recording
as soon as the memory of the first device is full and so
on. The RO bit needs to be set to 0 to disable rollover
that would otherwise overwrite the recorded tempera-
ture log.
After the RO bit and the Mission Start Delay are set, the
Sample Rate register is the last element of data that is
written. The sample rate can be any value from 1 to
255, coded as an unsigned 8-bit binary number. As
soon as the sample rate is written, the DS1921G sets
the MIP flag and clears the MEMCLR flag. After as
many minutes as specified by the Mission Start Delay
are over, the device waits for the next minute boundary,
then wakes up, copies the current time and date to the
Mission Timestamp register, and makes the first tem-
perature conversion of the mission. This increments
both the Mission Samples Counter and Device Samples
Counter. All subsequent temperature measurements
are taken on minute boundaries specified by the value
in the Sample Rate register. One can read the memory
of the DS1921G to watch the mission as it progresses.
Care should be taken to avoid memory access con-
flicts. See the Memory Access Conflicts section for
details.
Address Registers and
Transfer StatusBecause of the serial data transfer, the DS1921G
employs three address registers, called TA1, TA2, and
E/S (Figure 9). Registers TA1 and TA2 must be loaded
with the target address to which the data is written or
from which data is sent to the master upon a read com-
mand. Register E/S acts like a byte counter and transfer
status register. It is used to verify data integrity with
write commands. Therefore, the master has only read
access to this register. The lower 5 bits of the E/S regis-
ter indicate the address of the last byte that has been
written to the scratchpad. This address is called Ending
Offset. Bit 5 of the E/S register, called PF or partial byte
flag, is set if the number of data bits sent by the master
is not an integer multiple of 8. Bit 6 is always a 0. Note
that the lowest 5 bits of the target address also deter-
mine the address within the scratchpad where interme-
diate storage of data begins. This address is called
byte offset. If the target address for a write command is
13Ch, for example, then the scratchpad stores incom-
ing data beginning at the byte offset 1Ch and is full
after only 4 bytes. The corresponding ending offset in
this example is 1Fh. For the best economy of speed
and efficiency, the target address for writing should
point to the beginning of a new page, i.e., the byte off-
set is 0. Thus, the full 32-byte capacity of the scratch-
pad is available, resulting also in the ending offset of
1Fh. However, it is possible to write one or several con-
tiguous bytes somewhere within a page. The ending
offset together with the partial and overflow flag are a
means to support the master checking the data integri-
ty after a write command. The highest valued bit of the
E/S register, called authorization accepted (AA), indi-
cates that a valid copy command for the scratchpad
has been received and executed. Writing data to the
scratchpad clears this flag.
Writing with VerificationTo write data to the DS1921G, the scratchpad must be
used as intermediate storage. First, the master issues
the Write Scratchpad command to specify the desired
target address, followed by the data to be written to the
scratchpad. In the next step, the master sends the
Read Scratchpad command to read the scratchpad
and to verify data integrity. As preamble to the scratch-
pad data, the DS1921G sends the requested target
address TA1 and TA2 and the contents of the E/S regis-
ter. If the PF flag is set, data did not arrive correctly in
the scratchpad. The master does not need to continue
reading; it can start a new trial to write data to the
scratchpad. Similarly, a set AA flag indicates that the
DS1921GThermochron iButton Device
write command was not recognized by the device. If
everything went correctly, both flags are cleared and
the ending offset indicates the address of the last byte
written to the scratchpad. Now the master can continue
verifying every data bit. After the master has verified the
data, it has to send the Copy Scratchpad command.
This command must be followed exactly by the data of
the three address registers TA1, TA2, and E/S as the
master has read them verifying the scratchpad. As
soon as the DS1921G has received these bytes, it
copies the data to the requested location beginning at
the target address.
Memory/Control Function
CommandsThe Memory/Control Function Flowchart(Figure 10)
describes the protocols necessary for accessing the
memory and the special function registers of the
DS1921G. An example on how to use these and other
functions to set up the DS1921G for a mission is includ-
ed in the Mission Example: Prepare and Start a New
Missionsection. The communication between master
and DS1921G takes place either at standard speed
(default, OD = 0) or at overdrive speed (OD = 1). If not
explicitly set into the overdrive mode, the DS1921G
assumes standard speed. Internal memory access dur-
ing a mission has priority over external access through
the 1-Wire interface. This affects the read memory com-
mands described below. See the Memory Access
Conflictssection for details.
Write Scratchpad [0Fh]After issuing the Write Scratchpad command, the mas-
ter must first provide the 2-byte target address, fol-
lowed by the data to be written to the scratchpad. The
data is written to the scratchpad starting at the byte off-
set T[4:0]. The ending offset E[4:0] is the byte offset at
which the master stops writing data. Only full data
bytes are accepted. If the last data byte is incomplete,
its content is ignored and the partial byte flag (PF) is
set.
When executing the Write Scratchpad command, the
CRC generator inside the DS1921G (see Figure 16) cal-
culates a CRC of the entire data stream, starting at the
command code and ending at the last data byte sent
by the master. This CRC is generated using the CRC-
16 polynomial by first clearing the CRC generator and
then shifting in the command code (0Fh) of the Write
Scratchpad command, the target addresses TA1 and
TA2 as supplied by the master, and all the data bytes.
The master can end the Write Scratchpad command at
any time. However, if the ending offset is 11111b, the
master can send 16 read time slots and receive an
inverted CRC-16 generated by the DS1921G.
Note:The range 200h to 213h of the register page is
protected during a mission. See Figure 6 for the
access type of the individual registers between and
during missions.
BIT NUMBER 7 6 5 4 3 2 1 0 TARGET ADDRESS (TA1) T7 T6 T5 T4 T3 T2 T1 T0
TARGET ADDRESS (TA2) T15 T14 T13 T12 T11 T10 T9 T8
ENDING ADDRESS WITH
DATA STATUS (E/S)
(READ-ONLY)
AA 0 PF E4 E3 E2 E1 E0
Figure 9. Address Registers
DS1921GThermochron iButton Device
Read Scratchpad [AAh]This command is used to verify scratchpad data and
target addresses. After issuing the Read Scratchpad
command, the master begins reading. The first 2 bytes
are the target address. The next byte is the ending off-
set/data status byte (E/S) followed by the scratchpad
data beginning at the byte offset T[4:0], as shown in
Figure 9. Regardless of the actual ending offset, the
master can read data until the end of the scratchpad
after which it receives an inverted CRC-16 of the com-
mand code, target addresses TA1 and TA2, the E/S
byte, and the scratchpad data starting at the target
address. After the CRC is read, the bus master reads
logical “1”s from the DS1921G until a reset pulse is
issued.
Copy Scratchpad [55h]This command is used to copy data from the scratch-
pad to the writable memory sections. Applying a Copy
Scratchpad command to the Sample Rate register can
start a mission provided that several preconditions are
met. See the Mission Start and Logging Processsec-
tion and the flowchart in Figure 11 for details. After issu-
ing the Copy Scratchpad command, the master must
provide a 3-byte authorization pattern, which can be
obtained by reading the scratchpad for verification.
This pattern must exactly match the data contained in
the three address registers (TA1, TA2, E/S, in that
order). If the pattern matches, the AA flag is set and the
copy begins. A pattern of alternating “1”s and “0”s is
transmitted after the data has been copied until the
master issues a reset pulse. While the copy is in
progress, any attempt to reset the part is ignored. Copy
typically takes 2µs per byte.
The data to be copied is determined by the three
address registers. The scratchpad data from the begin-
ning offset through the ending offset is copied, starting
at the target address. Anywhere from 1 to 32 bytes can
be copied to memory with this command. The AA flag
remains at logic 1 until it is cleared by the next Write
Scratchpad command. Note that the Copy Scratchpad
command, when applied to the address range 200h to
213h during a mission, ends the mission.
Read Memory [F0h]The Read Memory command can be used to read the
entire memory. After issuing the command, the master
must provide the 2-byte target address. After the 2
bytes, the master reads data beginning from the target
address and can continue until the end of memory, at
which point logic “0”s are read. It is important to realize
that the target address registers contain the address
provided. The ending offset/data status byte is unaf-
fected.
The hardware of the DS1921G provides a means to
accomplish error-free writing to the memory section. To
safeguard data in the 1-Wire environment when read-
ing and to simultaneously speed up data transfers, it is
recommended to packetize data into data packets of
the size of one memory page each. Such a packet
would typically store a 16-bit CRC with each page of
data to ensure rapid, error-free data transfers that elim-
inate having to read a page multiple times to verify if
the received data is correct (refer to Application Note
114:1-Wire File Structurefor the recommended file
structure).
