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DS1922L-F5
Temperature Logger iButton With 8kB Datalog Memory
GENERAL DESCRIPTIONThe DS1922L/T temperature logger iButtons® are
rugged, self-sufficient systems that measuretemperature and record the result in a protected
memory section. The recording is done at a user-
defined rate. A total of 8192 8-bit readings or 409616-bit readings taken at equidistant intervals rangingfrom 1s to 273hrs can be stored. In addition to this,
there are 512 bytes of SRAM for storing application-specific information and 64 bytes for calibration data.mission to collect data can be programmed to
begin immediately, or after a user-defined delay orafter a temperature alarm. Access to the memory andcontrol functions can be password protected. The
DS1922L/T is configured and communicates with ahost computing device through the serial 1-Wire®
protocol, which requires only a single data lead and a
ground return. Every DS1922L/T is factory-laseredwith a guaranteed unique 64-bit registration numberthat allows for absolute traceability. The durable
stainless-steel package is highly resistant toenvironmental hazards such as dirt, moisture, and
shock. Accessories permit the DS1922L/T to be
mounted on almost any object, including containers,pallets, and bags.
APPLICATIONSTemperature Logging in Cold Chain, Food Safety,
Bio Science, and Pharmaceutical and MedicalProducts
High-Temperature Logging (Process Monitoring,Industrial Temperature Monitoring)
ORDERING INFORMATIONSee page 11 for Common iButton Features and
Examples of Accessories.
Commands, Registers, and Modes are capitalized for
clarity.
SPECIAL FEATURESAutomatically Wakes up, Measures Tempera-
ture, and Stores Values in 8kB of DatalogMemory in 8- or 16-Bit FormatDigital Thermometer Measures Temperature with
8-Bit (0.5°C) or 11-Bit (0.0625°C) ResolutionAccuracy Better than ±0.5°C from -10°C to+65°C (DS1922L), ±0.5°C from +20°C to +75°C
(DS1922T) with Software CorrectionWater resistant - 1 meter under water for 30 daysSampling Rate from 1s up to 273hrsProgrammable Recording Start Delay AfterElapsed Time or Upon a Temperature Alarm TripPointProgrammable High and Low Trip Points forTemperature AlarmsQuick Access to Alarmed Devices Through
1-Wire Conditional Search Function512 Bytes of General-Purpose Plus 64 Bytes ofCalibration MemoryTwo-Level Password Protection of All Memoryand Configuration RegistersCommunicates to Host with a Single Digital
Signal at up to 15.4kbps at Standard Speed orup to 125kbps in Overdrive Mode Using 1-WireProtocolOperating Range: DS1922L: -40 to +85°C;DS1922T: 0 to +125°C
PIN CONFIGURATION
DS1922L, DS1922T
Temperature Logger iButton
With 8kB Datalog MemoryiButton and 1-Wire are registered trademarks of Dallas Semiconductor.
DS1922L/DS1922T
ABSOLUTE MAXIMUM RATINGSI/O Voltage to GND-0.3V, +6V
I/O Sink Current20mAOperating Temperature Range (DS1922L)-40°C to +85°C
Operating Temperature Range (DS1922T)0°C to +125°CJunction Temperature+150°C
Storage Temperature Range (DS1922L)-40°C to +85°C*Storage Temperature Range (DS1922T)0°C to +125°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 the absolute maximum rating conditions for extended periods may affect device.
*Storage or operation above +50°C significantly reduces battery life.
ELECTRICAL CHARACTERISTICS (VPUP = 3.0V to 5.25V, TA = -40°C to +85°C)
DS1922L/DS1922T
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 the data pin could be 800pF when VPUP is first applied. If a 2.2k� resistor is used to pull up the data line, 2.5µs
after VPUP has been applied the parasite capacitance will not affect normal communications.
Note 4:VTL, VTH are a function of the internal supply voltage.
Note 5:Voltage below which, during a falling edge on I/O, a logic '0' is detected.
Note 6:The voltage on I/O needs to be less or equal to VILMAX whenever the master drives the line low.
Note 7:Voltage above which, during a rising edge on I/O, a logic '1' is detected.
Note 8:After VTH is crossed during a rising edge on I/O, the voltage on I/O has to drop by VHY to be detected as logic '0'.
Note 9:The I-V characteristic is linear for voltages less than 1V.
Note 10:The earliest recognition of a negative edge is possible at tREH after VTH has been previously reached.
Note 11:Highlighted numbers are NOT in compliance with the published iButton standards. See comparison table below.
Note 12:Interval during the negative edge on I/O at the beginning of a Presence Detect pulse between the time at which the voltage is
90% of VPUP and the time at which the voltage is 10% of VPUP.
Note 13:� represents the time required for the pullup circuitry to pull the voltage on I/O up from VIL to VTH.
Note 14:� represents the time required for the pullup circuitry to pull the voltage on I/O up from VIL to the input high threshold of the bus
master.
Note 15:To conserve battery power, use 8-bit temperature logging whenever possible.
Note 16:This number was derived from a test conducted by Cemagref in Antony, France, in July of 2000.
Note 17:Includes +0.1/-0.2oC calibration chamber measurement uncertainty.
Note 18:Assumes using calibration memory with calibration equations for error compensation. Includes +0.1/-0.2oC calibration chamber
measurement uncertainty. Guaranteed by design.1) Intentional change, longer recovery time requirement due to modified 1-Wire front end.
PHYSICAL SPECIFICATIONSizeSee mechanical drawing
WeightCa. 3.3 gramsSafetyMeets UL#913 (4th Edit.); Intrinsically Safe Apparatus,
approval under Entity Concept for use in Class I,
DS1922L/DS1922T
DS1922L MINIMUM PRODUCT LIFETIME VS. TEMPERATURE, SLOW SAMPLING
DS1922L/DS1922T
DS1922L MINIMUM PRODUCT LIFETIME VS. TEMPERATURE, FAST SAMPLING
DS1922L/DS1922T
DS1922T MINIMUM PRODUCT LIFETIME VS. TEMPERATURE, SLOW SAMPLING
DS1922L/DS1922T
DS1922T MINIMUM PRODUCT LIFETIME VS. TEMPERATURE, FAST SAMPLING
DS1922L/DS1922T
DS1922L MINIMUM PRODUCT LIFETIME VS. SAMPLE RATE
DS1922L/DS1922T
DS1922T MINIMUM PRODUCT LIFETIME VS. SAMPLE RATE
DS1922L/DS1922T
DS1922L TEMPERATURE ACCURACYNOTE: The graphs are based on 11-bit data.
DS1922T TEMPERATURE ACCURACYNOTE: The graphs are based on 11-bit data.
DS1922L/DS1922T
COMMON iButton FEATURESDigital identification and information by momentary contact.Unique factory-lasered 64-bit registration number assures error free device selection and absolute traceability
because no two parts are alike.Built-in multidrop controller for 1-Wire net.Chip-based data carrier compactly stores information.Data can be accessed while affixed to object.Button shape is self-aligning with cup-shaped probes.Durable stainless-steel case engraved with registration number withstands harsh environments.Easily affixed with self-stick adhesive backing, latched by its flange, or locked with a ring pressed onto its rim.Presence detector acknowledges when reader first applies voltage.Meets UL#913 (4th Edit.); Intrinsically Safe Apparatus: approved under Entity Concept for use in Class I,Division 1, Group A, B, C, and D Locations (application pending).
EXAMPLES OF ACCESSORIESDS9096PSelf-Stick Adhesive Pad
DS9101Multipurpose ClipDS9093RAMounting Lock Ring
DS9093ASnap-In FobDS9092iButton Probe
APPLICATIONThe DS1922L is an ideal device to monitor for extended periods of time the temperature of any object it is attached
to or shipped with, such as fresh produce, medical drugs and supplies and for use in refrigerators and freezers.With its shifted temperature range, the DS1922T is suited to monitor processes that require temperatures close to
the boiling point of water, such as pasteurization of food items. Software for setup and data retrieval through the1-Wire interface is available for free download from the iButton website (www.ibutton.com). This software also
includes drivers for the serial and USB port of a PC, and routines to access the general-purpose memory forstoring application- or equipment-specific data files.
OVERVIEWThe block diagram in Figure 1 shows the relationships between the major control and memory sections of the
DS1922L/T. The device has six main data components: 1) 64-bit lasered ROM, 2) 256-bit scratchpad, 3) 512-bytegeneral-purpose SRAM, 4) two 256-bit register pages of timekeeping, control, status, and counter registers and
passwords, 5) 64 bytes of calibration memory, and 6) 8192 bytes of data-logging memory. Except for the ROM andthe scratchpad, all other memory is arranged in a single linear address space. The data-logging memory, counter
registers, and several other registers are read-only for the user. Both register pages are write-protected while thedevice is programmed for a mission. The password registers, one for a read password and another one for a
read/write password can only be written, never read.
Figure 1 shows the hierarchical structure of the 1-Wire protocol. The bus master must first provide one of the eightROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, 4) Conditional Search ROM, 5) Skip
ROM, 6) Overdrive-Skip ROM, 7) Overdrive-Match ROM or 8) Resume. Upon completion of an Overdrive ROMcommand byte executed at standard speed, the device will enter Overdrive mode, where all subsequent
communication occurs at a higher speed. The protocol required for these ROM function commands is described inFigure 11. After a ROM function command is successfully executed, the memory and control functions become
accessible and the master may provide any one of the nine available commands. The protocol for these memoryand control function commands is described in Figure 9. All data is read and written least significant bit first.
DS1922L/DS1922T
Figure 1. DS1922L/T Block Diagram3V Lithium
PARASITE POWERThe block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power whenever the I/O
input is high. I/O will provide sufficient power as long as the specified timing and voltage requirements are met. Theadvantages of parasite power are two-fold: 1) by parasiting off this input, battery power is conserved; and 2) if the
battery is exhausted for any reason, the ROM may still be read.
64-BIT LASERED ROMEach DS1922L/T contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code. Thenext 48 bits are a unique serial number. The last 8 bits are a 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 register and XOR gates as shown inFigure 4. The polynomial is X8 + X5 + X4 + 1. Additional information about the Dallas 1-Wire Cyclic Redundancy
Check (CRC) is available in Application Note 27 and in the Book of DS19xx iButton Standards.
The shift register bits are initialized to 0. Then starting with the least significant bit of the family code, one bit at atime is shifted in. After the 8th bit of the family code has been entered, then the serial number followed by the
temperature range code is entered. After the range code has been entered, the shift register contains the CRCvalue. Shifting in the 8 bits of CRC returns the shift register to all 0s.
DS1922L/DS1922T
Figure 2. Hierachical Structure for 1-Wire Protocol
Figure 3. 64-Bit Lasered ROMMSBLSB
MSBLSBMSBLSBMSB LSB
Figure 4. 1-Wire CRC Generator
DS1922L/DS1922T
MEMORYThe memory map of the DS1922L/T is shown in Figure 5. The 512 bytes general-purpose SRAM are located inpages 0 through 15. The various registers to set-up and control the device fill page 16 and 17, called register pages
1 and 2 (details in Figure 6). Pages 18 and 19 provide storage space for calibration data. They can alternatively beused as extension of the general-purpose memory. The "datalog" logging memory starts at address 1000h (page
128) and extends over 256 pages. The memory pages 20 to 127 are reserved for future extensions. Thescratchpad is an additional page that acts as a buffer when writing to the SRAM memory or the register page. The
data memory can be written at any time. The calibration memory holds data from the device calibration that can beused to further improve the accuracy of 11-bit temperature readings. See the Software Correction Algorithm forTemperature section for details. The last byte of the calibration memory page stores an 8-bit CRC of the
preceeding 31 bytes. Page 19 is an exact copy of the data in page 18. While the calibration memory can be
overwritten by the user, this is not recommended. See section Security by Password for ways to protect thememory. The access type for the register pages is register-specific and depends on whether the device is pro-
grammed for a mission. Figure 6 shows the details. The datalog memory is read-only for the user. It is writtensolely under supervision of the on-chip control logic. Due to the special behavior of the write access logic (write
scratchpad, copy scratchpad) it is recommended to only write full pages at a time. This also applies to the registerpages and the calibration memory. See the Address Register and Transfer Status section for details.
Figure 5. DS1922L/T Memory Map0000H to
001FHPage 0
0020H to
01FFHPages 1
to 15
0200H to
021FHPage 16
0220H to
023FHPage 17
0240H to
025FHPage 18
0260H to
027FHPage 19
0280H to
0FFFHPages 20 to 127
1000H to
2FFFHPages 128
to 383
DS1922L/DS1922T
Figure 6. DS1922L/T Register Pages Map
Note: The first entry in column ACCESS TYPE is valid between missions. The second entry shows the applicable
DS1922L/DS1922T
TIMEKEEPING AND CALENDARThe real-time clock (RTC)/alarm and calendar information is accessed by reading/writing the appropriate bytes inthe register page, address 200h to 205h. For readings to be valid, all RTC registers must be read sequentially
starting at address 0200h. Some of the RTC bits are set to 0. These bits will always read 0 regardless of how theyare written. The number representation of the real-time clock registers is BCD format (binary-coded decimal).
Real-Time Clock and RTC Alarm Register BitmapThe RTC of the DS1922L/T can run in either 12-hour or 24-hour mode. Bit 6 of the Hours Register (address 202h)
is defined as the 12- or 24-hour mode select bit. When high, the 12-hour mode is selected. In the 12-hour mode, bit5 is the AM/PM bit with logic 1 being PM. In the 24-hour mode, bit 5 is the 20-hour bit (20 to 23 hours). The CENT
bit, bit 7 of the Months Register, can be written by the user. This bit changes its state when the years countertransitions from 99 to 00.
The calendar logic is designed to automatically compensate for leap years. For every year value that is either 00 or
a multiple of 4 the device will add a 29th of February. This will work correctly up to (but not including) the year 2100.
SAMPLE RATEThe content of the Sample Rate Register (addresses 0206h, 0207h) specifies the time elapse (in seconds if EHSS
= 1, or minutes if EHSS = 0) between two temperature logging events. The sample rate may be any value from 1 to16383, coded as an unsigned 14-bit binary number. If EHSS = 1, the shortest time between logging events is 1
second and the longest (sample rate = 3FFFh) is 4.55 hours. If EHSS = 0, the shortest is 1 minute and the longesttime is 273.05 hours ( sample rate = 3FFFh). The EHSS bit is located in the RTC Control Register at address
0212h. It is important that the user sets the EHSS bit accordingly while setting the Sample Rate Register.
Asample rate of 0000h is not valid and must be avoided under all circumstances. This will cause the device
to enter into an unrecoverable state.
Sample Rate Register BitmapDuring a mission, there is only read access to these registers. Bits cells marked "0" always read 0 and cannot be
written to 1.
TEMPERATURE CONVERSIONThe DS1922L measures temperatures in the range of -40°C to +85°C. With the DS1922T the temperature rangebegins at 0°C and ends at +125°C. Temperature values are represented as a 8- or 16-bit unsigned binary number
with a resolution of 0.5°C in the 8-bit mode and 0.0625°C in the 16-bit mode.
The higher temperature byte TRH is always valid. In the 16-bit mode only the three highest bits of the lower byteTRL are valid. The five lower bits all read zero. TRL is undefined if the device is in 8-bit temperature mode. An out-
of-range temperature reading will be indicated as 00h or 0000h when too cold and FFh or FFE0h when too hot.
DS1922L/DS1922T
Latest Temperature Conversion Result Register BitmapTRLTRHWith TRH and TRL representing the decimal equivalent of a temperature reading the temperature value iscalculated as
�(°C) = TRH/2 - 41 + TRL/512(16 bit mode, TLFS = 1, see address 0213h)
�(°C) = TRH/2 - 41(8 bit mode, TLFS = 0, see address 0213h)
This equation is valid for converting temperature readings stored in the datalog memory as well as for data read
from the Latest Temperature Conversion Result Register. The "-41" applies to the DS1922L. For the DS1922T use"-1" instead of "-41".
To specify the temperature alarm thresholds, the equation above needs to be resolved to
TALM = 2 * � (°C) + 82
The "+82" applies to the DS1922L. For the DS1922T use "+2" instead of "+82".
Since the temperature alarm threshold is only one byte, the resolution or temperature increment is limited to 0.5°C.The TALM value needs to be converted into hexadecimal format before it can be written to one of the temperaturealarm threshold registers (Low
Alarm address 0208h; High Alarm address 0209h). Independent of theconversion mode (8 or 16 bit) only the most significant byte of a temperature conversion is used to determine
whether an alarm will be generated.
Temperature Conversion Examples
Temperature Alarm Threshold Examples
TEMPERATURE SENSOR ALARMThe DS1922L/T has two Temperature Alarm Threshold Registers (address 0208h, 0209h) to store values, whichdetermine whether a critical temperature has been reached. A temperature alarm is generated if the device
measures an alarming temperature AND the alarm signaling is enabled. The bits ETLA and ETHA that enable thetemperature alarm are located in the Temperature Sensor Control Register. The temperature alarm flags TLF and
THF are found in the Alarm Status Register at address 0214h.
Temperature Sensor Control Register BitmapDuring a mission, there is only read access to this register. Bits 2 to 7 have no function. They always read 0 and
cannot be written to 1.
DS1922L/DS1922T
Register Details
RTC CONTROLTo minimize the power consumption of a DS1922L/T, the RTC oscillator should be turned off when device is not in
use. The oscillator on/off bit is located in the RTC control register. This register also includes the EHSS bit, which
determines whether the sample rate is specified in seconds or minutes.
RTC Control Register BitmapDuring a mission, there is only read access to this register. Bits 2-7 have no function. They always read 0 and
cannot be written to 1.
Register Details
MISSION CONTROLThe DS1922L/T is set up for its operation by writing appropriate data to its special function registers, which are
located in the two register pages. The settings in the Mission Control Register determine which format (8 or 16 bits)is to be used and whether old data may be overwritten by new data, once the datalog memory is full. An additional
control bit can be set to tell the DS1922L/T to wait with logging data until a temperature alarm is encountered.
Mission Control Register BitmapDuring a mission, there is only read access to this register. Bits 6 and 7 have no function. They always read 1 andcannot be written to 0. Bits 1 and 3 control functions that are not available with the DS1922L and DS1922T. Bit 1
must be set to 0. Under this condition the setting of bit 3 becomes a “don’t care”.
DS1922L/DS1922T
Register Details
ALARM STATUSThe fastest way to determine whether a programmed temperature threshold was exceeded during a mission isthrough reading the Alarm Status Register. In a networked environment that contains multiple DS1922L/T iButtons
the devices that encountered an alarm can quickly be identified by means of the Conditional Search command (seeROM Function Commands). The temperature alarm will only occur if enabled (see Temperature Sensor Alarm).
The BOR alarm is always enabled.
Alarm Status Register BitmapThere is only read access to this register. Bits 4 to 6 have no function. They always read 1. Bits 2 and 3 have no
function with the DS1922L and DS1922T. They always read 0. The alarm status bits are cleared simultaneouslywhen the Clear Memory function is invoked. See Memory and Control Functions for details.
Register Details
DS1922L/DS1922T
GENERAL STATUSThe information in the general status register tells the host computer whether a mission-related command wasexecuted successfully. Individual status bits indicate whether the DS1922L/T is performing a mission, waiting for a
temperature alarm to trigger the logging of data or whether the data from the latest mission has been cleared.
General Status Register BitmapThere is only read access to this register. Bits 0, 2, 5, 6, and 7 have no function.
Register Details
MISSION START DELAYThe content of the Mission Start Delay Counter tells how many minutes will have to expire from the time a mission
was started until the first measurement of the mission will take place (SUTA = 0) or until the device will start testingthe temperature for a temperature alarm (SUTA = 1). The Mission Start Delay is stored as an unsigned 24-bit
integer number. The maximum delay is 16777215 minutes, equivalent to 11650 days or roughly 31 years. If thestart delay is non-zero and the SUTA bit is set to 1, first the delay has to expire before the device starts testing for
temperature alarms to begin logging data.
Mission Start Delay CounterDuring a mission, there is only read access to these registers.
For a typical mission, the Mission Start Delay is 0. If a mission is too long for a single DS1922L/T to store all
readings at the selected sample rate, one can use several devices and set the Mission Start Delay for the seconddevice to start recording as soon as the memory of the first device is full, and so on. The RO-bit in the Mission
Control Register (address 0213h) must be set to 0 to prevent overwriting of collected data once the datalogmemory is full.
DS1922L/DS1922T
MISSION TIME STAMPThe Mission Time Stamp indicates the date and time of the first temperature sample of the mission. There is onlyread access to the Mission Time Stamp Register.
Mission Time Stamp Registers Bitmap
MISSION PROGRESS INDICATORDepending on settings in the Mission Control Register (address 0213h) the DS1922L/T will log temperature in 8-bit
or 16-bit format. The Mission Samples Counter together with the starting address and the logging format (8 or 16
bits) provides the information to identify valid blocks of data that have been gathered during the current (MIP = 1)or latest mission (MIP = 0). See Datalog Memory Usage for an illustration.
Mission Samples Counter Register MapThere is only read access to this register.
The number read from the Mission Samples Counter indicates how often the DS1922L/T woke up during a mission
to measure temperature. The number format is 24-bit unsigned integer. The Mission Samples Counter is resetthrough the Clear Memory command.
OTHER INDICATORSThe Device Samples Counter is similar to the Mission Samples Counter. During a mission this counter increments
whenever the DS1922L/T wakes up to measure and log data and when the device is testing for a temperaturealarm in SUTA mode. Between missions the counter increments whenever the Forced Conversion command is
executed. This way the Device Samples Counter functions like a gas gauge for the battery that powers the iButton.
Device Samples Counter Register MapThere is only read access to this register.
The Device Samples Counter is reset to zero when the iButton is assembled. The counter will increment a couple
of times during final test. The number format is 24-bit unsigned integer. The maximum number that can berepresented in this format is 16777215.
The Device Configuration Byte is used to allow the master to distinguish between the DS2422 chip, and differentversions of the DS1922 iButtons. The table below shows the codes assigned to the various devices.
DS1922L/DS1922T
Device Configuration ByteDS2422DS1922LDS1922TThere is only read access to this register.
SECURITY BY PASSWORDThe DS1922L/T is designed to use two passwords that control read access and full access. Reading from or writing
to the scratchpad as well as the forced conversion command does not require a password. The password needs tobe transmitted right after the command code of the memory or control function. If password checking is enabled the
password transmitted is compared to the passwords stored in the device. The data pattern stored in the Password
Control register determines whether password checking is enabled.
Password Control RegisterDuring a mission, there is only read access to this register.
To enable password checking, the EPW bits need to form a binary pattern of 10101010 (AAh). The default pattern
of EPW is different from AAh. If the EPW pattern is different from AAh, any pattern will be accepted, as long as ithas a length of exactly 64 bits. Once enabled, changing the passwords and disabling password checking requires
the knowledge of the current full-access password.
Before enabling password checking, passwords for read-only access as well as for full access (read/write/control)need to be written to the password registers. Setting up a password or enabling/disabling the password checking is
done in the same way as writing data to a memory location, only the address is different. Since they are located inthe same memory page, both passwords can be redefined at the same time.
Read Access Password RegisterThere is only write access to this register. Attempting to read the password will report all zeros. The passwordcannot be changed while a mission is in progress.
The Read Access Password needs to be transmitted exactly in the sequence RP0, RP1… RP62, RP63. This
password only applies to the function “Read Memory with CRC”. The DS1922L/T will deliver the requested dataonly if the password transmitted by the master was correct or if password checking is not enabled.
Full Access Password RegisterThere is only write access to this register. Attempting to read the password will report all zeros. The password
DS1922L/DS1922T
The Full Access Password needs to be transmitted exactly in the sequence FP0, FP1… FP62, FP63. It will affectthe functions “Read Memory with CRC”, “Copy Scratchpad”, “Clear Memory”, “Start Mission”, and “Stop Mission”.
The DS1922L/T executes the command only if the password transmitted by the master was correct or if passwordchecking is not enabled
Due to the special behavior of the write access logic, the Password Control Register and both passwords must be
written at the same time. When setting up new passwords, always verify (read back) the scratchpad before sendingthe copy scratchpad command. After a new password is successfully copied from the scratchpad to its memory
location, erase the scratchpad by filling it with new data (write scratchpad command). Otherwise a copy of thepasswords will remain in the scratchpad for public read access.
DATALOG MEMORY USAGEOnce setup for a mission, the DS1922L/T logs the temperature measurements at equidistant time points entry after
entry in its datalog memory. The datalog memory is able to store 8192 entries in 8-bit format or 4096 entries in 16-bit format (Figure 7). In 16-bit format, the higher 8 bits of an entry are stored at the lower address. Knowing the
starting time point (Mission Time Stamp) and the interval between temperature measurements one can reconstructthe time and date of each measurement.
There are two alternatives to the way the DS1922L/T will behave after the datalog memory is filled with data. The
user can program the device to either stop any further recording (disable “rollover”) or overwrite the previouslyrecorded data (enable “rollover”), one entry at a time, starting again at the beginning of the respective memory
section. The contents of the Mission Samples Counter in conjunction with the sample rate and the Mission TimeStamp will then allow reconstructing the time points of all values stored in the datalog memory. This gives the exact
history over time for the most recent measurements taken. Earlier measurements cannot be reconstructed.
Figure 7. Temperature Logging
DS1922L/DS1922T
MISSIONINGThe typical task of the DS1922L/T iButton is recording temperature. Before the device can perform this function, itneeds to be set up properly. This procedure is called missioning.
First of all, DS1922L/T needs to have its real-time clock set to valid time and date. This reference time may be the
local time, or, when used inside of a mobile unit, UTC (also called GMT, Greenwich Mean Time) or any other timestandard that was agreed upon. The real-time clock oscillator must be running (EOSC = 1). The memory assigned
to store the Mission Time Stamp, Mission Samples Counter, and Alarm Flags must be cleared using the MemoryClear command. To enable the device for a mission, the ETL-bit must be set to 1. These are general settings that
have to be made in any case, regardless of the type of object to be monitored and the duration of the mission.
If alarm signaling is desired, the temperature alarm low and high thresholds must be defined. How to convert atemperature value into the binary code to be written to the threshold registers is described under Temperature
Conversion earlier in this document. In addition, the temperature alarm must be enabled for the low- and/or high-threshold. This makes the device respond to a Conditional Search command (see ROM Function Commands),
provided that an alarming condition has been encountered.
The setting of the RO bit (rollover enable) and sample rate depends on the duration of the mission and themonitoring requirements. If the most recently logged data 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 8192 (8-bit format)or 4096 (16-bit format) to calculate the value of the sample rate (number of minutes between conversions). If the
estimated duration of a mission is 10 days (= 14400 minutes), for example, then the 8192-byte capacity of thedatalog memory would be sufficient to store a new 8-bit value every 1.8 minutes (110 seconds). If the datalog
memory of the DS1922L/T is not large enough to store all readings, one can use several devices and set theMission Start Delay to values that make the second device start logging 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 logged data.
After the RO bit and the Mission Start Delay are set, the sample rate needs to be written to the Sample RateRegister. The sample rate may be any value from 1 to 16383, coded as an unsigned 14-bit binary number.
A
sample rate of all zeros is not valid and must be avoided under all circumstances. This will cause thedevice to enter into an unrecoverable state. The fastest sample rate is one sample per second (EHSS = 1,
Sample Rate = 0001h) and the slowest is one sample every 273.05 hours (EHSS = 0, Sample Rate = 3FFFh). Toget one sample every 6 minutes, for example, the sample rate value needs to be set to 6 (EHSS = 0) or 360
decimal (equivalent to 0168h at EHSS = 1).
If there is a risk of unauthorized access to the DS1922L/T or manipulation of data, one should define passwords forread access and full access. Before the passwords become effective, their use needs to be enabled. See Security
by Password for more details.
The last step to begin a mission is to issue the Start Mission command. As soon as it has received this command,the DS1922L/T sets the MIP flag and clear the MEMCLR flag. With the immediate/delayed start mode (SUTA = 0),
after as many minutes as specified by the Mission Start Delay are over, the device will wake up, copy the currentdate and time to the mission time stamp register, and log the first entry of the mission. This increments both the
Mission Samples Counter and Device Samples Counter. All subsequent log entries will be made as specified bythe value in the Sample Rate Register and the EHSS bit.
If the Start Upon Temperature Alarm mode is chosen (SUTA = 1) and temperature logging is enabled (ETL = 1) the
DS1922L/T first waits until the start delay is over. Then the device wakes up in intervals as specified by the samplerate and EHSS bit and measure the temperature. This will increment the Device Samples Counter only. Only after
an alarming temperature is encountered will the DS1922L/T set the mission time stamp. The first sample of themission is logged one sample period after the temperature alarm occurred. From then on, both the Mission
Samples Counter and Device Samples Counter increment at the same time. All subsequent log entries are madeas specified by the value in the Sample Rate Register and the EHSS bit.
The general-purpose memory operates independently of the other memory sections and is not write-protected
during a mission. All memory of the DS1922L/T can be read at any time, e. g., to watch the progress of a mission.Attempts to read the passwords will read 00h bytes instead of the data that is stored in the password registers.
