DS1920
Temperature iButton™
www.dalsemi.com
SPECIAL FEATURES
§
Digital thermometer measures temperatures
from -55°C to +100°C in typically 0.2
seconds
§
Zero standby power
§
0.5°C resolution, digital temperature reading
is two’ complement of °C value
s
§
Access to internal counters allows increased
resolution through interpolation
§
Reduces control, address, data, and power to
a single data contact
§
8-bit device-generated CRC for data integrity
§
8-bit family code specifies DS1920
communications requirements to reader
§
Special command set allows user to skip
ROM section and do temperature
measurements simultaneously for all devices
on the bus
§
2 bytes of EEPROM to be used either as
alarm triggers or user memory
§
Alarm search directly indicates which device
senses alarming temperatures
COMMON iButton FEATURES
§
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 MicroLAN
§
Digital identification and information by
momentary contact
§
Chip-based data carrier compactly stores
information
§
Data can be accessed while affixed to object
§
Economically communicates to bus master
with a single digital signal at 16.3k bits per
second
§
Standard 16 mm diameter and 1-Wire
TM
protocol ensure compatibility with iButton
family
§
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)
F3 MICROCAN
TM
3.10
0.36
0.51
c
1993
F5 MICROCAN
TM
5.89
0.36
16.25
0.51
c
1993
16.25
YYWW REGISTERED RR
YYWW REGISTERED RR
A0
10
17.35
20
10
17.35
000000FBC52B
000000FBD8B3
DATA
GROUND
DATA
GROUND
All dimensions are shown in millimeters
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081699
DS1920
ORDERING INFORMATION
DS1920-F3
DS1920-F5
F3 MicroCan
F5 MicroCan
EXAMPLES OF ACCESSORIES
DS9096P
DS9101
DS9093RA
DS9093F
DS9092
Self-Stick Adhesive Pad
Multi-Purpose Clip
Mounting Lock Ring
Snap-In Fob
iButton Probe
iButton DESCRIPTION
The DS1920 Temperature iButton provides 9-bit temperature readings which indicate the temperature of
the device. Information is sent to/from the DS1920 over a 1-Wire interface. Power for reading, writing,
and performing temperature conversions is derived from the data line itself. Because each DS1920
contains a unique silicon serial number, multiple DS1920s can exist on the same 1-Wire bus. This allows
for placing temperature sensors in many different places. Applications where this feature is useful include
HVAC environmental controls, sensing temperatures inside buildings, equipment or machinery, and in
process monitoring and control.
OVERVIEW
The block diagram of Figure 1 shows the major components of the DS1920. The DS1920 has three main
data components: 1) 64-bit lasered ROM, 2) temperature sensor, and 3) nonvolatile temperature alarm
triggers TH and TL. The device derives its power from the 1-Wire communication line by storing energy
on an internal capacitor during periods of time when the signal line is high and continues to operate off
this power source during the low times of the 1-Wire line until it returns high to replenish the parasite
(capacitor) supply.
Communication to the DS1920 is via a 1-Wire port. With the 1-Wire port, the memory and control
functions will not be available before the ROM function protocol has been established. The master must
first provide one of five ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, 4)
Skip ROM, or 5) Alarm Search. These commands operate on the 64-bit lasered ROM portion of each
device and can single out a specific device if many are present on the 1-Wire line as well as indicate to
the bus master how many and what types of devices are present. After a ROM function sequence has been
successfully executed, the memory and control functions are accessible and the master may then provide
any one of the five memory and control function commands.
One control function command instructs the DS1920 to perform a temperature measurement. The result
of this measurement will be placed in the DS1920’ scratchpad memory, and may be read by issuing a
s
memory function command which reads the contents of the scratchpad memory. The temperature alarm
triggers TH and TL consist of 1 byte of EEPROM each. If the alarm search command is not applied to the
DS1920, these registers may be used as general purpose user memory. Writing TH and TL is done using a
memory function command. Read access to these registers is through the scratchpad. All data is read and
written least significant bit first.
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DS1920
DS1920 BLOCK DIAGRAM
Figure 1
PARASITE POWER
The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power
whenever the data contact is high. Data will provide sufficient power as long as the specified timing and
voltage requirements are met (see the section titled “1-Wire Bus System”). The advantage of parasite
power is that no local power source is needed for remote sensing of temperature.
In order for the DS1920 to be able to perform accurate temperature conversions, sufficient power must be
provided over the data line when a temperature conversion is taking place. The DS1920 requires a current
during conversion of up to 1 mA, therefore, the data line will not have sufficient drive due to the 5 kΩ
pullup resistor. This problem is particularly acute if several DS1920s are on the same data line and
attempting to convert simultaneously.
The way to assure that the DS1920 has sufficient supply current is to provide a strong pullup on the data
line whenever temperature conversion or copying to the EEPROM is taking place. This may be
accomplished by using a MOSFET to connect the data line directly to the power supply as shown in
Figure 2. The data line must be switched over to the strong pullup within 10
µs
maximum after issuing a
command that involves copying to the EEPROM or initiates a temperature conversion.
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DS1920
STRONG PULL-UP FOR SUPPLYING DS1920 DURING TEMPERATURE
CONVERSION
Figure 2
OPERATION - MEASURING TEMPERATURE
The DS1920 measures temperatures through the use of an on-board proprietary temperature measurement
technique. A block diagram of the temperature measurement circuitry is shown in Figure 3.
The DS1920 measures temperature by counting the number of clock cycles that an oscillator with a low
temperature coefficient goes through during a gate period determined by a high temperature coefficient
oscillator. The counter is preset with a base count that corresponds to -55°C. If the counter reaches 0
before the gate period is over, the temperature register, which is also preset to the -55°C value, is
incremented, indicating that the temperature is higher than -55°C.
At the same time, the counter is then preset with a value determined by the slope accumulator circuitry.
The counter is then clocked again until it reaches 0. If the gate period is still not finished, then this
process repeats.
The slope accumulator compensates for the non-linear behavior of the oscillators over temperature,
yielding a high-resolution temperature measurement. This is done by changing the number of counts
necessary for the counter to go through for each incremental degree in temperature. To obtain the desired
resolution, therefore, both the value of the counter and the number of counts per degree C (the value of
the slope accumulator) at a given temperature must be known.
Internally, this calculation is done inside the DS1920 to provide 0.5°C resolution. The temperature
reading is provided in a 16-bit, sign-extended two’ complement reading. Table 1 describes the exact
s
relationship of output data to measured temperature. The data is transmitted serially over the 1-Wire
interface. The DS1920 can measure temperature over the range of -55°C to +100°C in 0.5°C increments.
For Fahrenheit usage, a lookup table or conversion factor must be used.
Note that temperature is represented in the DS1920 in terms of a ½°C LSB, yielding the following 9-bit
format:
MSB
1
= -25°C
LSB
0
1
1
0
0
1
1
1
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DS1920
The most significant (sign) bit is duplicated into all of the bits in the upper MSB of the 2-byte
temperature register in memory. This “sign-extension” yields the 16-bit temperature readings as shown in
Table 1.
Higher resolutions may be obtained by the following procedure. First, read the temperature, and truncate
the 0.5°C bit (the LSB) from the read value. This value is TEMP_READ. The value left in the counter
may then be read. This value is the count remaining (COUNT_REMAIN) after the gate period has
ceased. The last value needed is the number of counts per degree C (COUNT_PER_C) at that
temperature. The actual temperature may be then be calculated by the user using the following formula:
TEMPERATURE = TEMP_READ - 0.25 +
(COUNT_PER_C - COUNT_REMAIN)
COUNT_PER_C
TEMPERATURE MEASURING CIRCUITRY
Figure 3
SET/CLEAR
LSB
TEMPERATURE/DATA RELATIONSHIPS
Table 1
TEMPERATURE
DIGITAL OUTPUT
(BINARY)
DIGITAL OUTPUT
(HEX)
+100°
C
+25°
C
+½°
C
+0°
C
-½°
C
-25°
C
-55°
C
00000000 11001000
00000000 00110010
00000000 00000001
00000000 00000000
11111111 11111111
11111111 11001110
11111111 10010010
00C8H
0032H
0001H
0000H
FFFFH
FFCEH
FF92H
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