19-4887; Rev 3/12
SPECIAL FEATURES
DS1921H/DS1921Z
High-Resolution Thermochron® iButton®
Range H: +15°C to +46°C, Z: -5°C to +26°C
COMMON iButton 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
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
5.89
0.51
16.25
3B2000FBC52B
Thermochron
®
D6
21
17.35
1-Wire
IO
GND
All dimensions are shown in millimeters.
ORDERING INFORMATION
PART
DS1921H-F5#
DS1921Z-F5#
TEMP RANGE
+15°C to +46°C
-5°C to +26°C
PIN-PACKAGE
F5 iButton
F5 iButton
#Denotes a RoHS-compliant device that may include lead(Pb) that is
exempt under the RoHS requirements.
EXAMPLES OF ACCESSORIES
DS9096P
DS9101
DS9093RA
DS9093A
DS9092
Self-Stick Adhesive Pad
Multi-Purpose Clip
Mounting Lock Ring
Snap-In Fob
iButton Probe
Thermochron, iButton, and 1-Wire are registered trademarks of Maxim Integrated Products, Inc.
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DS1921H/Z
iButton DESCRIPTION
The DS1921H/Z Thermochron iButtons 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.
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 +15C to +46°C. The DS1921Z has a fixed range of -5C 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).
APPLICATION
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.
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DS1921H/Z BLOCK DIAGRAM
Figure 1
1-Wire
Port
IO
ROM
Function
Control
64-Bit
Lasered
ROM
Parasite
Powered
Circuitry
DS1921H/Z
Memory
Function
Control
256-Bit
Scratchpad
General-Purpose
SRAM
32.768kHz
Oscillator
Internal
Timekeeping &
Control Reg. &
Counters
Register Page
Alarm Time Stamp
and Duration
Logging Memory
Temperature
Sensor
3V Lithium
Control
Logic
Histogram Memory
Datalog
Memory
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 X
8
+ X
5
+ X
4
+ 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.
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DS1921H/Z
HIERARCHICAL STRUCTURE FOR 1-Wire PROTOCOL
Figure 2
Bus
Master
1-Wire net
Other
Devices
DS1921
Command
Level:
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
Cmd. Data Field
Codes: Affected:
33h
55h
F0h
CCh
3Ch
69h
ECh
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
1-Wire ROM Function
Commands
DS1921-Specific
Memory/Control
Function Commands
0Fh
AAh
55h
F0h
A5h
3Ch
Convert Temperature
44h
64-BIT LASERED ROM
Figure 3
MSB
8-Bit
CRC Code
MSB
DEVICE
DS1921H-F5
DS1921Z-F5
LSB
12-Bit Temperature
Range Code
MSB
TEMP.
RANGE (°C)
+15 to +46
-5 to +26
LSB
36-Bit Serial Number
MSB
LSB
TEMP. RANGE CODE
0100
0011
1111
1011
0010
0010
LSB
8-Bit Family
Code (21h)
MSB
LSB
RESOLUTION
(°C)
0.125
0.125
HEX.
EQUIVALENT
4F2
3B2
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1-Wire CRC GENERATOR
Figure 4
Polynomial = X + X + X + 1
8
5
4
DS1921H/Z
1
STAGE
st
2
STAGE
nd
3
STAGE
rd
4
STAGE
th
5
STAGE
th
6
STAGE
th
7
STAGE
th
8
STAGE
th
X
0
X
1
X
2
X
3
X
4
X
5
X
6
X
7
X
8
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
0200h to
021Fh
0220h to
027Fh
0280h to
07FFh
0800h to
087Fh
0880h to
0FFFh
1000h to
17FFh
1800h to
1FFFh
General-Purpose SRAM (16 Pages)
32-Byte Register Page
Alarm Time Stamps and Durations
(Reserved for Future Extensions)
Temperature Histogram Memory
(Reserved for Future Extensions)
Datalog Memory (64 Pages)
(Reserved for Future Extensions)
Pages 0 to 15
Page 16
Pages 17 to 19
Pages 20 to 63
Pages 64 to 67
Pages 68 to 127
Pages 128 to 191
Pages 192 to 255
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