Maxim Integrated DS1921H User manual

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19-4887; Rev 11/13
SPECIAL FEATURES
 Digital thermometer measures temperature in
1/8°C increments with ±1°C accuracy
 Built-in real-time clock (RTC) and timer has
accuracy of ±2 minutes per month from 0°C
to +45°C
 Water resistant or waterproof if placed inside
DS9107 iButton® capsule (Exceeds Water
Resistant 3 ATM requirements)
 Automatically wakes up and measures
temperature at user-programmable intervals
from 1 to 255 minutes
 Logs consecutive temperature measurements
in 2KB of datalog memory
 Records a long-term temperature histogram
with 1/2°C resolution
 Programmable temperature-high and
temperature-low alarm trip points
 Records up to 24 time stamps and durations
when temperature leaves the range specified
by the trip points
 512 bytes of general-purpose battery-backed
SRAM
 Communicates to host with a single digital
signal at 15.4kbits or 125kbits per second
using 1-Wire® protocol
 Fixed range: H: +15°C to +46°C;
Z: -5°C to +26°C
COMMON iButton DEVICE
FEATURES
 Digital identification and information by
momentary contact
 Unique, factory-lasered and tested 64-bit
registration number (8-bit family code + 48-
bit serial number + 8-bit CRC tester) assures
absolute traceability because no two parts are
alike
 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
PIN CONFIGURATION
IO
GND
0.51
5.89
16.25
17.35
D6 21
3B2000FBC52B
1-Wire



Thermochron
®
All dimensions are shown in millimeters.
ORDERING INFORMATION
PART
TEMP RANGE
PIN-PACKAGE
DS1921H-F5#
+15°C to +46°C
F5 Can
DS1921Z-F5#
-5°C to +26°C
F5 Can
#Denotes a RoHS-compliant device that may include lead(Pb) that is
exempt under the RoHS requirements.
EXAMPLES OF ACCESSORIES
DS9096P Self-Stick Adhesive Pad
DS9101 Multi-Purpose Clip
DS9093RA Mounting Lock Ring
DS9093A Snap-In Fob
DS9092 iButton Device Probe
Thermochron, iButton, and 1-Wire are registered trademarks of Maxim Integrated Products, Inc.
DS1921H/DS1921Z
High-Resolution Thermochron
iButton Devices

DS1921H/Z
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iButton DEVICE DESCRIPTION
The DS1921H/Z Thermochron® iButton devices are rugged, self-sufficient systems that measure
temperature and record 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 255 minutes can be stored. The
histogram provides 64 data bins with a resolution of 0.5°C. If the temperature leaves a user-
programmable range, the DS1921H/Z will also record when this happened, for how long the temperature
stayed outside the permitted range, and if the temperature was too high or too low. Additional 512 bytes
of general-purpose battery-backed SRAM allow storing information pertaining to the object to which the
DS1921H/Z is associated. Data is transferred serially via the 1-Wire protocol, which requires only a
single data lead and a ground return. Every DS1921H/Z 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 hazards such as dirt, moisture, and shock. Accessories
permit the DS1921H/Z to be mounted on almost any object, including containers, pallets, and bags.
APPLICATION
The DS1921Z is an ideal device to monitor the temperature of any object it is attached to or shipped with,
such as fresh produce, medical drugs and supplies. It is also ideal for use in refrigerators. The DS1921H
is intended for monitoring the body temperature of humans and animals and for monitoring temperature
critical processes such as curing, powder coating, and painting. Alternatively, the DS1921H can be used
for monitoring the temperature of clean rooms, and computer and equipment rooms. It can also aid in
calculating the proportional share of heating cost of each party in buildings with central heating. The
DS1921H has a fixed range of +15°C to +46°C. The DS1921Z has a fixed range of -5°C to +26°C. The
high resolution makes the DS1921H and DS1921Z suitable for scientific research and development. The
general-purpose battery-backed SRAM can store information such as shipping manifests, dates of
manufacture, or other relevant data written as ASCII or encrypted files. Note that the initial sealing level
of DS1921H/Z achieves the equivalent of IP56. Aging and use conditions can degrade the integrity of the
seal over time, so for applications with significant exposure to liquids, sprays, or other similar
environments, it is recommended to place the Thermochron in the DS9107 iButton capsule. The DS9107
provides a watertight enclosure that has been rated to IP68 (refer to Application Note 4126).
OVERVIEW
The block diagram in Figure 1 shows the relationships between the major control and memory sections of
the DS1921H/Z. The device has seven main data components: 1) 64-bit lasered ROM; 2) 256-bit
scratchpad; 3) 4096-bit general-purpose SRAM; 4) 256-bit register page of timekeeping, control, and
counter registers; 5) 96 bytes of alarm time stamp and duration logging memory; 6) 128 bytes of
histogram memory; and 7) 2048 bytes of datalog memory. Except for the ROM and the scratchpad, all
other memory is arranged in a single linear address space. All memory reserved for logging purposes,
counter registers and several other registers are read-only for the user. The timekeeping and control
registers 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: 1) Read ROM; 2) Match ROM; 3) Search ROM; 4)
Conditional Search ROM; 5) Skip ROM; 6) Overdrive-Skip ROM; or 7) Overdrive-Match ROM. Upon
completion of an Overdrive ROM command 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 in Figure 13. After a ROM function command is
successfully executed, the memory functions become accessible and the master may 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.

DS1921H/Z
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DS1921H/Z BLOCK DIAGRAM Figure 1
Internal
Timekeeping &
Control Reg. &
Counters
3V Lithium
General-Purpose
SRAM
Register Page
Alarm Time Stamp
and Duration
Logging Memory
Datalog
Memory
Histogram Memory
32.768kHz
Oscillator
Control
Logic
Temperature
Sensor
256-Bit
Scratchpad
Memory
Function
Control
ROM
Function
Control
64-Bit
Lasered
ROM
Parasite
Powered
Circuitry
1-Wire
Port
IO
PARASITE POWER
The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power
whenever the IO input is high. IO will provide sufficient power as long as the specified timing 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 DS1921 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. Application Note 5057:
OneWireViewer Tips and Tricks explains how to check the battery status.
64-BIT LASERED ROM
Each DS1921 contains a unique ROM code that is 64 bits long. The first eight bits are a 1-Wire family
code. The next 36 bits are a unique serial number. The next 12 bits, called temperature range code, allow
distinguishing the DS1921H and DS1921Z from each other and from other DS1921 versions. The last
eight 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 in Figure 4. The polynomial
is X8 + X5 + X4 + 1. Additional information about the Maxim 1-Wire Cyclic Redundancy Check is
available in Application Note 27.
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 eighth 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 CRC value. Shifting in the eight bits of CRC returns the shift register to all 0s.

DS1921H/Z
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HIERARCHICAL STRUCTURE FOR 1-Wire PROTOCOL Figure 2
1-Wire net
Other
Devices
Bus
Master
Command
Level:
1-Wire ROM Function
Commands
DS1921-Specific
Memory/Control
Function Commands
DS1921
Available
Commands:
Read ROM
Match ROM
Search ROM
Skip ROM
Overdrive Skip
Overdrive Match
Conditional Search
ROM
Write Scratchpad
Read Scratchpad
Copy Scratchpad
Read Memory
Read Memory w/CRC
Clear Memory
Convert Temperature
Data Field
Affected:
64-bit Reg. #
64-bit Reg. #
64-bit Reg. #
N/A
OD-Flag
64-bit Reg. #, OD-Flag
64-bit Reg. #, Cond. Search settings,
device status
256-bit scratchpad, flags
256-bit scratchpad
4096-bit SRAM, registers, flags
All memory
All memory
Mission Time Stamp, Mission Samples
Counter, Start Delay, Sample
Rate, Alarm Time Stamps and
Durations, Histogram Memory
Memory address 211h
Cmd.
Codes:
33h
55h
F0h
CCh
3Ch
69h
ECh
0Fh
AAh
55h
F0h
A5h
3Ch
44h
64-BIT LASERED ROM Figure 3
MSB
LSB
8-Bit
CRC Code
12-Bit Temperature
Range Code 36-Bit Serial Number 8-Bit Family Code (21h)
MSB LSB
MSB LSB
MSB LSB
MSB LSB
DEVICE TEMP.
RANGE (°C)
RESOLUTION
(°C)
TEMP. RANGE CODE HEX.
EQUIVALENT
DS1921H-F5 +15 to +46 0.125 0100 1111 0010 4F2
DS1921Z-F5 -5 to +26 0.125 0011 1011 0010 3B2

DS1921H/Z
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1-Wire CRC GENERATOR Figure 4
X
0
X
1
X
2
X
3
X
4
X
5
X
6
X
7
X
8
Polynomial = X
8
+ X
5
+ X
4
+ 1
1st
STAGE
2nd
STAGE
3rd
STAGE
4th
STAGE
6th
STAGE
5th
STAGE
7th
STAGE
8th
STAGE
INPUT DATA
MEMORY
The memory map of the DS1921H/Z is shown in Figure 5. The 4096-bit general-purpose SRAM make up
pages 0 through 15. The timekeeping, control, and counter registers fill page 16, called Register Page (see
Figure 6). Pages 17 to 19 are assigned to storing the alarm time stamps and durations. The temperature
histogram bins begin at page 64 and use up to four pages. The datalog memory covers 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 supervision of
the on-chip control logic.
DS1921H/Z MEMORY MAP Figure 5
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 Time Stamps 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
Datalog Memory (64 Pages)
Pages 128 to 191
1800h to
1FFFh
(Reserved for Future Extensions) Pages 192 to 255

DS1921H/Z
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DS1921H/Z REGISTER PAGE MAP Figure 6
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
Function
Access*
0200h
0
10 Seconds
Single Seconds
0201h
0
10 Minutes
Single Minutes
Real-
0202h
0
12/24
20h.
AM/PM
10h.
Single Hours
Time
Clock
R/W; R/W**
0203h
0
0
0
0
0
Day of Week
Registers
0204h
0
0
10 Date
Single Date
0205h
CENT
0
0
10m.
Single Months
0206h
10 Years
Single Years
0207h
MS
10 Seconds Alarm
Single Seconds Alarm
Real-
0208h
MM
10 Minutes Alarm
Single Minutes Alarm
Time
0209h MH 12/24 10ha.
A/P
10h.
alm.
Single Hours Alarm
Clock
Alarm R/W; R/W**
020Ah
MD
0
0
0
0
Day of Week Alarm
Registers
020Bh
Temperature Low Alarm Threshold
Temp.
R/W; R/W**
020Ch
Temperature High Alarm Threshold
Alarms
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)
(N/A)
R; R**
0210h
(no function, reads 00h)
(N/A)
R; R**
0211h
Temperature Read Out (Forced Conversion)
Temp.
R; R**
0212h
Low Byte
Start
R/W; R/W**
0213h
High Byte
Delay
0214h
TCB
MEMCLR
MIP
SIP
0
TLF
THF
TAF
Status
R/W; R/W
0215h
Minutes
0216h
Hours
Mission
0217h
Date
Time
R; R
0218h
Month
Stamp
0219h
Year
021Ah
Low Byte
Mission
021Bh
Center Byte
Samples
R; R
021Ch
High Byte
Counter
021Dh
Low Byte
Device
021Eh
Center Byte
Samples
R; R
021Fh
High Byte
Counter
*The first entry in column ACCESS is valid between missions. The second 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, will end the mission and overwrite selected writeable registers.
TIMEKEEPING
The RTC/alarm and calendar information is accessed by reading/writing the appropriate bytes in the
register page, address 200h to 206h. Note that some bits are set to 0. These bits will 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.

DS1921H/Z
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RTC and RTC Alarm Register Bitmap
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
0200h
0
10 Seconds
Single Seconds
0201h
0
10 Minutes
Single Minutes
0202h
0
12/24
20h.
AM/PM
10h.
Single Hours
0203h
0
0
0
0
0
Day of Week
0204h
0
0
10 Date
Single Date
0205h
CENT
0
0
10m.
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 10ha.
A/P
10h.
alm.
Single Hours Alarm
020Ah
MD
0
0
0
0
Day of Week Alarm
RTC/Calendar
The RTC of the DS1921H/Z 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, bit 5 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).
To distinguish between the days of the week the DS1921H/Z includes a counter with a range from 1 to 7.
The assignment of 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 standard).
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.
The DS1921H/Z is Y2K-compliant. Bit 7 (CENT) of the Months Register at address 205h serves as a
century flag. When the Year Register rolls over from 99 to 00 the century flag will toggle. 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.
RTC Alarms
The DS1921H/Z also contains a RTC alarm function. The alarm registers are located in registers 207h to
20Ah. The most significant bit of each of the alarm registers is a mask bit. When all of the mask bits are
logic 0, an alarm will occur once per week when the values stored in timekeeping registers 200h to 203h
match the values stored in the time of day alarm registers. Any alarm will set the Timer Alarm Flag
(TAF) in the device's Status Register (address 214h). The bus master may set the Search Conditions in the
Control Register (address 20Eh) to identify devices with timer alarms by means of the Conditional Search
function (see ROM Function Commands).

DS1921H/Z
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RTC Alarm Control
ALARM REGISTER MASK BITS
(Bit 7 of 207h to 20Ah)
MS MM MH MD
1 1 1 1 Alarm once per second.
0 1 1 1 Alarm when seconds match (once per minute).
0 0 1 1 Alarm when minutes and seconds match (once every hour).
0 0 0 1 Alarm when hours, minutes and seconds match (once every day).
0 0 0 0
Alarm when day, hours, minutes, and seconds match (once every
week).
TEMPERATURE CONVERSION
The DS1921H and DS1921Z measure temperatures with a resolution of 1/8th of a degree Celsius.
Temperature values are represented in a single byte as an unsigned binary number, which translates into a
range of 32°C. The possible values are 0000 0000 (00h) through 1111 1111 (FFh). The codes 01h to FEh
are considered valid temperature readings. Since the DS1921H and DS1921Z have different starting
temperatures, the meaning of a binary temperature code depends on the device.
If a temperature conversion yields a temperature that is out-of-range, it will be recorded as 00h (if too
low) or FFh (if too high). Since out-of-range results are accumulated in histogram bins 0 and 63 the data
in these bins is of limited value (see the Temperature Logging and Histogram section). For this reason the
specified temperature range of the DS1921H and DS1921Z is considered to begin at code 04h and end at
code FBh, which corresponds to histogram bins 1 to 62.
With T[7..0] representing the decimal equivalent of a temperature reading, the temperature value is
calculated as
ϑ (°C) = T[7…0] / 8 + 14.500 (DS1921H)
ϑ (°C) = T[7…0] / 8 - 5.500 (DS1921Z)
This equation is valid for converting temperature readings stored in the datalog memory as well as for
data read from the forced temperature conversion readout Register (address 211h).
To specify the high or low temperature alarm thresholds, this equation needs to be resolved to
T[7…0] = 8 * ϑ (°C) -116 (DS1921H)
T[7…0] = 8 * ϑ (°C) + 44 (DS1921Z)
A value of 23°C, for example, thus translates into 68 decimal or 44h for the DS1921H, and 228 decimal
or E4h for the DS1921Z. This corresponds to the binary patterns 0100 0100 and 1110 0100 respectively,
which could be written to a Temperature Alarm Register (address 020Bh and 020Ch, respectively).
Temperature Alarm Register Map
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
020Bh
Temperature Low Alarm Threshold
020Ch
Temperature High Alarm Threshold

DS1921H/Z
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SAMPLE RATE
The 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 may 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 (Status Register bit EM = 0), writing a non-zero value to the Sample Rate
Register will start a mission. For a full description of the correct sequence of steps to start a temperature-
logging mission see sections Missioning or Missioning Example.
Sample Rate Register Map
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
020Dh
Sample Rate
CONTROL REGISTER
The DS1921H/Z 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 Control Register (address 20Eh). This register can be read and written. If the
device is programmed for a mission, writing to the Control Register will end the mission and change the
register contents.
Control Register Bitmap
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
020Eh
EOSC
EMCLR
0
EM
RO
TLS
THS
TAS
The functional assignments of the individual bits are explained in the table below. Bit 5 has no function.
It always reads 0 and cannot be written to 1.
Control Register Details
BIT DESCRIPTION
BIT(S)
DEFINITION
EOSC: Enable Oscillator
b7
This bit controls the crystal oscillator of the RTC. When set to logic 0,
the oscillator will start operation. When written to logic 1, the oscillator
will stop and the device is in a low-power data retention mode. This bit
must be 0 for normal operation. The RTC must have advanced at
least 1 second before a mission start will be accepted.
EMCLR: Memory Clear
Enable
b6
This bit needs to be set to logic 1 to enable the Clear Memory function,
which is invoked as a memory function command. The Time-Stamp,
Histogram Memory as well as the Mission Time Stamp, Mission
Samples Counter, Mission Start Delay and Sample Rate will be cleared
only if the Clear Memory command is issued with the next access to
the device. The EMCLR bit will return to 0 as the next memory function
command is executed.
EM: Enable Mission
b4
This bit controls whether the DS1921H/Z will begin a mission as soon as
the sample rate is written. To enable the device for a mission, this bit
must be 0.
RO: Rollover
Enable/Disable
b3
This bit controls whether the datalog memory is overwritten with new
data or whether data logging is stopped once the memory 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 will be stored in the datalog memory once it is filled
with data. This will not stop the mission. The device will continue
measuring temperatures and updating the histogram and alarm time
stamps and durations.

DS1921H/Z
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BIT DESCRIPTION
BIT(S)
DEFINITION
TLS: Temperature Low
Alarm Search
b2
If this bit is 1, the device will respond to a Conditional Search command
if during a mission the temperature has reached or is lower than the Low
Temperature Threshold stored at address 020Bh.
THS: Temperature High
Alarm Search
b1
If this bit is 1, the device will respond to a Conditional Search command
if during a mission the temperature has reached or is higher than the
High Temperature Threshold stored at address 020Ch.
TAS: Timer Alarm Search
b0
If this bit is 1, the device will respond to a Conditional Search command
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 may be advisable to set the TAS bit to a 0, in most cases.
Mission Start Delay Counter
The content of the Mission Start Delay Counter determines how many minutes the device will wait before
starting the logging process. The mission start delay value is stored as unsigned 16-bit integer number at
addresses 212h (low byte) and 213h (high byte). The maximum delay is 65535 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 DS1921H/Z 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 RO bit in the control register (address
020Eh) must be set to 0 to prevent overwriting of the recorded temperature log after the datalog memory
is full. See Mission Start and Logging Process description and flow chart for details.
Status Register
The Status Register holds device status information and alarm flags. The register is located at address
214h. Writing to this register will not necessarily end a mission.
Status Register Bitmap
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
0214h
TCB
MEMCLR MIP SIP 0 TLF THF TAF
The functional assignments of the individual bits are explained in the table 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.
Status Register Details
BIT DESCRIPTION
BIT(S)
DEFINITION
TCB: Temperature Core
Busy
b7 If this bit reads 0 the DS1921H/Z is currently performing a temperature
conversion, either self-initiated because of a mission being in progress
or initiated by a command when a mission is not in progress. The TCB
bit goes low just before a conversion starts and returns to high just after
the result is latched into the readout register at address 0211h.
MEMCLR: Memory
Cleared
b6 If this bit reads 1, the memory pages 17 and higher (alarm time
stamps/durations, temperature histogram, excluding datalog memory),
as well as the Mission Time Stamp, Mission Samples Counter, Mission
Start Delay and Sample Rate have been cleared to 0 from executing a
Clear Memory function command. The MEMCLR bit will return to 0 as
soon as writing a non-0 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.

DS1921H/Z
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BIT DESCRIPTION
BIT(S)
DEFINITION
MIP: Mission in Progress
b5
If this bit reads 1 the DS1921H/Z has been set up for a mission and this
mission is still in progress. A mission is started if the EM bit of the
Control Register (address 20Eh) is 0 and a non-zero 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 will end with the first
write attempt (Copy Scratchpad command) to any register in the
address range of 200h to 213h. Alternatively, a mission can be ended by
directly writing to the Status Register and setting the MIP bit to 0. The
MIP bit cannot be set to 1 by writing to the status register.
SIP: Sample in Progress
b4
If this bit reads 1 the DS1921H/Z is currently performing a temperature
conversion as part of a mission in progress. The mission samples occur
on the seconds rollover from 59 to 00. The SIP bit will change from 0 to
1 approximately 250ms before the actual temperature conversion begins
allowing 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
may reveal invalid data.
TLF: Temperature Low
Flag
b2 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.
THF: Temperature High
Flag
b1
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.
TAF: Timer Alarm Flag
b0
If this bit reads 1, a RTC alarm has occurred (see section
TIMEKEEPING 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.
MISSION TIME STAMP
The Mission Time Stamp indicates the time and date of the first temperature conversion of a mission.
Subsequent temperature conversions will 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 Time Stamp Register Bitmap
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
0215h
0
10 Minutes
Single Minutes
0216h
0
12/24
20h.
AM/PM
10h.
Single Hours
0217h
0
0
10 Date
Single Date
0218h
0
0
0
10m.
Single Months
0219h
10 Years
Single Years
MISSION SAMPLES COUNTER
The Mission Samples Counter 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.

DS1921H/Z
12 of 45
Mission Samples Counter Register Map
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
021Ah
Low Byte
021Bh
Center Byte
021Ch
High Byte
DEVICE SAMPLES COUNTER
The Device Samples Counter 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 16777215, which is higher than the expected
lifetime of the DS1921H/Z devices. This counter cannot be reset under software control.
Device Samples Counter Register Map
ADDR
b7
b6
b5
b4
b3
b2
b1
b0
021Dh
Low Byte
021Eh
Center Byte
021Fh
High Byte
TEMPERATURE LOGGING AND HISTOGRAM
Once setup for a mission, the DS1921H/Z logs the temperature measurements simultaneously byte after
byte in the datalog memory as well as in histogram form in the histogram memory. The datalog memory
is able to store 2048 temperature values measured at equidistant time points. The first temperature value
of a mission is written to address location 1000h of the datalog memory, the second value to address
location 1001h and so on. Knowing the starting time point (Mission Time Stamp), the interval between
temperature measurements, the Mission Samples Counter, and the rollover setting, one can reconstruct
the time and date of each measurement stored in the datalog.
There are two alternatives to the way the DS1921H/Z will behave after the 2048 bytes of datalog memory
is filled with data. With rollover disabled (RO = 0), the device will fill the datalog memory with the first
2048 mission samples. Additional mission samples are not logged in the datalog, but the histogram, and
temperature alarm memory continue to update. With rollover enabled (RO = 1), the datalog will wrap
around, and overwrite previous data starting at 1000h for the every 2049th mission sample. In this mode
the device stores the last 2048 mission samples.
For the temperature histogram, the DS1921H/Z provides 64 bins that begin at memory address 0800h.
Each bin consists of a 16-bit, non-rolling-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
087Eh for bin 63, 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 counted as 00h or FFh.

DS1921H/Z
13 of 45
HISTOGRAM BIN AND TEMPERATURE CROSS-REFERENCE Figure 7
TEMPERATURE
READING
DS1921H TEMP.
EQUIV. IN °C
DS1921Z TEMP.
EQUIV. IN °C
HISTOGRAM BIN
NUMBER
HISTOGRAM BIN
ADDRESS
00h
14.500
-5.500
0 800h to 801h
01h
14.625
-5.375
0 800h to 801h
02h
14.750
-5.250
0 800h to 801h
03h
14.875
-5.125
0 800h to 801h
04h 15.000 -5.000 1 802h to 803h
05h 15.125 -4.875 1 802h to 803h
06h 15.250 -4.750 1 802h to 803h
07h 15.375 -4.625 1 802h to 803h
08h 15.500 -4.500 2 804h to 805h
F7h 45.375 25.375 61 87Ah to 87Bh
F8h 45.500 25.500 62 87Ch to 87Dh
F9h 45.625 25.625 62 87Ch to 87Dh
FAh 45.750 25.750 62 87Ch to 87Dh
FBh 45.875 25.875 62 87Ch to 87Dh
FCh
46.000
26.000
63 87Eh to 87Fh
FDh
46.125
26.125
63 87Eh to 87Fh
FEh
46.250
26.250
63 87Eh to 87Fh
FFh
46.375
26.375
63 87Eh to 87Fh
Since each data bin is 2 bytes it can increment up to 65535 times. Additional measurements for a bin that
has already reached its maximum value will not be counted; the bin counter will remain 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 temperature readings fall into the same bin.
TEMPERATURE ALARM LOGGING
For some applications it may be essential to not only record temperature over time and the temperature
histogram, but also record when exactly the temperature has exceeded a predefined tolerance band and
for how long the temperature stayed outside the desirable range. The DS1921H/Z can log high and low
durations. The tolerance band is specified by means of the Temperature Alarm Threshold Registers,
addresses 20Bh and 20Ch in the register page. One can set a high and a low temperature threshold. See
section Temperature Conversion 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 DS1921H/Z will not record any temperature alarm. If the temperature during a
mission reaches or exceeds either threshold, the DS1921H/Z will generate an alarm and set either the
Temperature High Flag (THF) or the Temperature Low Flag (TLF) in the Status Register (address 214h).
This way, if the search conditions (address 20Eh) are set accordingly, the master can quickly identify
devices with temperature alarms by means of the Conditional Search function (see ROM Function
Commands). The device also generates a time stamp of when the alarm occurred and begins recording the
duration of the alarming temperature.
Time stamps and durations where the temperature leaves the tolerance band are stored in the address
range 0220h to 027Fh, as shown in Figure 8. This allocation allows recording 24 individual alarm events

DS1921H/Z
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and periods (12 periods for too hot and 12 for too cold). The date and time of each of these periods can be
determined from the Mission Time Stamp and the time distance between each temperature reading.
ALARM TIME STAMPS AND DURATIONS ADDRESS MAP Figure 8
ADDRESS
DESCRIPTION
ALARM EVENT
0220h Mission Samples Counter Low Byte
0221h Mission Samples Counter Center Byte Low Alarm 1
0222h Mission Samples Counter High Byte
0223h Alarm Duration Counter
0224h to 0227h Alarm Time Stamp and Duration Low Alarm 2
0228h to 024Fh Alarm Time Stamp and Durations Low Alarms 3 to 12
0250h Mission Samples Counter Low Byte
0251h Mission Samples Counter Center Byte High Alarm 1
0252h Mission Samples Counter High Byte
0253h Alarm Duration Counter
0254h to 0257h Alarm Time Stamp and Duration High Alarm 2
0258h to 027Fh Alarm Time Stamp and Durations High Alarms 3 to 12
The alarm time stamp is a copy of the Mission Samples Counter when the alarm first occurred. The least
significant byte is stored at the lower address. One address higher than the time stamp the DS1921H/Z
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 tolerance band, the device will issue another time stamp
at the next higher alarm location and open 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 time
stamp will not increment any further. Should the temperature again cross this threshold, it will be
recorded at the next available alarm location. This algorithm is implemented for the low as well as for the
high temperature threshold.
MISSIONING
The typical task of the DS1921H/Z is recording the temperature of a temperature-sensitive object. Before
the device can perform this function, it needs to be configured. This procedure is called missioning.
First of all, DS1921H/Z needs to have its RTC set to valid time and date. This reference time may be
UTC (also called GMT, Greenwich Mean Time) or any other time standard that was chosen for the
application. The clock must be running (EOSC = 0) for at least one second. Setting a RTC alarm is
optional. The memory assigned to storing alarm time stamps and durations, temperature histogram, as
well as the Mission Time Stamp, Mission Samples Counter, Mission Start Delay, and Sample Rate must
be cleared using the Memory Clear 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 EM
flag 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.
Next, the low temperature and high temperature thresholds to specify the temperature tolerance band
must be defined. How to convert a temperature value into the binary code to be written to the threshold
registers is described under Temperature Conversion earlier in this document.

DS1921H/Z
15 of 45
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 will enable devices to
participate in the conditional search if certain events such as timer or temperature alarm have occurred.
Details on the search conditions are found in the section ROM Function Commands later in this document
and in the Control Register description.
The setting of the RO bit (rollover enable) 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). If the estimated duration of a mission is 10 days (= 14400 minutes) for example, then the
2048-byte capacity of the datalog memory would be sufficient to store a new value every 7 minutes. If the
datalog memory of the DS1921H/Z 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 temperature 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 may be any value from 1 to 255, coded as an unsigned 8-bit binary
number. As soon as the sample rate is written, the DS1921H/Z will set the MIP flag and clear the
MEMCLR flag. After as many minutes as specified by the Mission Start Delay are over, the device will
wait for the next minute boundary, then wake up, copy the current time and date to the Mission Time
Stamp Register, and make the first temperature 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 may read the
memory of the DS1921H/Z to watch the mission as it progresses. Care should be taken to avoid memory
access conflicts. See section Memory Access Conflicts for details.
MEMORY/CONTROL FUNCTION COMMANDS
The Memory/Control Function Flow Chart (Figure 10) describes the protocols necessary for accessing
the memory and the special function registers of the DS1921H/Z. An example on how to use these and
other functions to set up the DS1921H/Z for a mission is included at the end of this document, preceding
the Electrical Characteristics section. The communication between master and DS1921H/Z takes place
either at regular speed (default, OD = 0) or at Overdrive Speed (OD = 1). If not explicitly set into the
Overdrive mode, the DS1921H/Z assumes regular speed. Internal memory access during a mission has
priority over external access through the 1-Wire interface. This can affect the Read Memory commands
described below. See section Memory Access Conflicts for details.
ADDRESS REGISTERS AND TRANSFER STATUS
Because of the serial data transfer, the DS1921H/Z 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 will be
written or from which data will be sent to the master upon a Read command. 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 only has read access to this register. The lower 5 bits of the E/S Register 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 determine
the address within the scratchpad, where intermediate storage of data will begin. This address is called

DS1921H/Z
16 of 45
byte offset. If the target address for a Write command is 13Ch, for example, then the scratchpad will store
incoming data beginning at the byte offset 1Ch and will be full after only 4 bytes. The corresponding
ending offset in this example is 1Fh. For best economy of speed and efficiency, the target address for
writing should point to the beginning of a new page, (i.e., the byte offset will be 0). Thus, the full 32-byte
capacity of the scratchpad is available, resulting also in the ending offset of 1Fh. However, it is possible
to write 1 or several contiguous bytes somewhere within a page. The ending offset together with the
Partial and Overflow Flag is mainly a means to support the master checking the data integrity after a
Write command. The highest valued bit of the E/S Register, called AA or Authorization Accepted,
indicates that a valid copy command for the scratchpad has been received and executed. Writing data to
the scratchpad clears this flag.
ADDRESS REGISTERS Figure 9
Bit # 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
WRITING WITH VERIFICATION
To write data to the DS1921H/Z, the scratchpad has to 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 scratchpad data, the DS1921H/Z sends the
requested target address TA1 and TA2 and the contents of the E/S Register. 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 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 DS1921H/Z has received these bytes, it
will copy the data to the requested location beginning at the target address.
Write Scratchpad Command [0Fh]
After issuing the Write Scratchpad command, the master must first provide the 2-byte target address,
followed by the data to be written to the scratchpad. The data will be written to the scratchpad starting at
the byte offset (T4:T0). The ending offset (E4:E0) will be the byte offset at which the master stops writ-
ing data. Only full data bytes are accepted. If the last data byte is incomplete, its content will be ignored
and the partial byte flag (PF) will be set.
When executing the Write Scratchpad command, the CRC generator inside the DS1921H/Z (see Figure
16) calculates a CRC of the entire data stream, starting at the command code and ending at the last data

DS1921H/Z
17 of 45
byte sent by the master. This CRC is generated using the CRC16 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 may end the Write
Scratchpad command at any time. However, if the ending offset is 11111b, the master may send 16 read
time slots and will receive an inverted CRC16 generated by the DS1921H/Z.
The range 200h to 213h of the register page is protected during a mission. See Figure 6, Register
Page Map, for the access type of the individual registers between and during missions.
Read Scratchpad Command [AAh]
This command is used to verify scratchpad data and target address. After issuing the Read Scratchpad
command, the master begins reading. The first 2 bytes will be the target address. The next byte will be the
ending offset/data status byte (E/S) followed by the scratchpad data beginning at the byte offset (T4:T0),
as shown in Figure 9. Regardless of the actual ending offset, the master may read data until the end of the
scratchpad after which it will receive an inverted CRC16 of the command 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 will read logical 1s from the DS1921H/Z until a reset pulse is issued.
Copy Scratchpad [55h]
This command is used to copy data from the scratchpad to the writable memory sections. Applying Copy
Scratchpad to the Sample Rate Register can start a mission provided that several preconditions are met.
See Mission Start and Logging Process description and the flow chart in Figure 11 for details. After
issuing 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
(Authorization Accepted) flag will be set and the copy will begin. A pattern of alternating 1s and 0s will
be 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 will be 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 will be copied, starting at the target address. Anywhere from 1 to 32
bytes may be copied to memory with this command. The AA flag will remain at logic 1 until it is cleared
by the next Write Scratchpad command. Note that Copy Scratchpad when applied to the address range
200h to 213h during a mission will end the mission.
Read Memory [F0h]
The Read Memory command may 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 may continue until the end of memory, at which point logic 0s will be read. It is im-
portant to realize that the target address registers will contain the address provided. The ending offset/data
status byte is unaffected.
The hardware of the DS1921H/Z provides a means to accomplish error-free writing to the memory sec-
tion. To safeguard data in the 1-Wire environment when reading 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 eliminate having to read a page multiple times to verify whether if the received data is cor-
rect. (See Application Note 114 for the recommended file structure.)

DS1921H/Z
18 of 45
MEMORY/CONTROL FUNCTION FLOW CHART Figure 10-1
Master TX Memory or
Control Fkt. Command
0FH
Write
Scratchpad
DS1921 sets Scratch-
pad Offset = (T4:T0)
and Clears (PF, AA)
Master TX Data Byte
to Scratchpad Offset
DS1921 sets (E4:E0)
= Scratchpad Offset
Master
TX Reset?
Scratch-
pad Offset =
11111b?
Master RX CRC16 of
Command, Address, Data
DS1921 Incre-
ments Scratch-
pad Offset
Master RX "1"s
Master
TX Reset?
Master
TX Reset?
Partial
Byte Written?
PF = 1
AAH
Read
Scratchpad
Master RX Ending
Offset with Data
Status (E/S)
Master
TX Reset?
Scratch-
pad Offset =
11111b?
Master RX CRC16 of
Command, Address Data,
E/S Byte, and Data Starting
at the Target Address
DS1921 Incre-
ments Scratch-
pad Offset
Master RX "1"s
Master
TX Reset?
DS1921 sets Scratch-
pad Offset = (T4:T0)
Master RX Data Byte
from Scratchpad Offset
From ROM Functions
Flow Chart (Figure 12)
To ROM Functions
Flow Chart (Figure 12)
N
Y
N
Y
N
Y
N
Y
N
Y
N
Y
Y
N
Y
N
Y
N
Y
DS1921 sets
EMCLR = 0
Master TX
TA1 (T7:T0), TA2 (T15:T8)
DS1921 sets
EMCLR = 0
Master RX
TA1 (T7:T0), TA2 (T15:T8)
To Figure 10
2
nd
Part
N
From Figure 10
2
nd
Part

DS1921H/Z
19 of 45
MEMORY/CONTROL FUNCTION FLOW CHART Figure 10-2
To Figure 10
3rd Part
55H
Copy
Scratchpad
Master TX
E/S Byte
Authorization
Code Match?
DS1921 Copies Scratchpad
Data to Memory
Copying
Finished
Master
TX Reset?
Master
TX Reset?
AA = 1
F0H
Read
Memory
Master
TX Reset?
End of
Memory?
Master RX "0"s
DS1921 sets Memory
Address = (T15:T0)
Master RX Data Byte
from Memory Address
Master TX
TA1 (T7:T0), TA2 (T15:T8)
Master
RX "1"s
DS1921 TX "0"
DS1921 TX "1"
Master
TX Reset?
Master
RX "1"s
Master TX
TA1 (T7:T0), TA2 (T15:T8)
DS1921 Incre-
ments Address
Counter
N
Y
N
Y
N
Y
N
Y
N
Y
N
Y
N
Y
N
Y
N
Y
DS1921 sets
EMCLR = 0
DS1921 sets
EMCLR = 0
From Figure 10
1st Part
To Figure 10
1st Part
From Figure 10
3rd Part

DS1921H/Z
20 of 45
MEMORY/CONTROL FUNCTION FLOW CHART Figure 10-3
A5H
Read Mem.
w/CRC
DS1921 sets Memory
Address = (T15:T0)
Master RX Data Byte
from Memory Address
End of
Memory?
Y
Master
TX Reset?
CRC OK?
DS1921 Incre-
ments Address
Counter
End of Page?
Master RX CRC16 of
Command, Address, Data
(1st Pass); CRC16 of Data
(Subsequent Passes)
Master TX
Reset
N
Y
N
Y
Y
N
Decision made
by DS1921
Decision made
by Master
Master TX
TA1 (T7:T0), TA2 (T15:T8)
DS1921 sets
EMCLR = 0
From Figure 10
2nd Part
To Figure 10
2nd Part
To Figure 10
4th Part
From Figure 10
4th Part
N
N
Y
Master RX
00 Byte