Dallas DS1920 Temperature ibutton Datasheet

DS1920
Temperature iButton
www.dalsemi.com
SPECIAL FEATURES
§ 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-WireTM
protocol ensure compatibility with iButton
family
§ Button shape is self-aligning with cupshaped 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)
§ 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’s complement of °C value
§ 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 + 48bit serial number + 8-bit CRC tester) assures
absolute traceability because no two parts are
alike
F3 MICROCANTM
F5 MICROCANTM
5.89
3.10
0.36
0.36
0.51
c 1993
0.51
c 1993
16.25
YYWW REGISTERED RR
A0
10
16.25
YYWW REGISTERED RR
20
17.35
000000FBC52B
10
17.35
000000FBD8B3
DATA
DATA
GROUND
GROUND
All dimensions are shown in millimeters
1 of 22
081699
DS1920
ORDERING INFORMATION
EXAMPLES OF ACCESSORIES
DS1920-F3
DS1920-F5
DS9096P
DS9101
DS9093RA
DS9093F
DS9092
F3 MicroCan
F5 MicroCan
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’s scratchpad memory, and may be read by issuing a
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.
2 of 22
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.
3 of 22
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’s complement reading. Table 1 describes the exact
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
1
1
0
0
= -25°C
4 of 22
1
1
1
LSB
0
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
5 of 22
DS1920
OPERATION - ALARM SIGNALING
After the DS1920 has performed a temperature conversion, the temperature value is compared to the
trigger values stored in TH and TL. Since these registers are 8 bits only, the 0.5°C bit is ignored for
comparison. The most significant bit of TH or TL directly corresponds to the sign bit of the 16-bit
temperature register. If the result of a temperature measurement is higher than TH or lower than TL, an
alarm flag inside the device is set. This flag is updated with every temperature measurement. As long as
the alarm flag is set, the DS1920 will respond to the alarm search command. This allows many DS1920s
to be connected in parallel doing simultaneous temperature measurements. If somewhere the temperature
exceeds the limits, the alarming device(s) can be identified and read immediately without having to read
non-alarming devices.
64-BIT LASERED ROM
Each DS1920 contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code
(DS1920 code is 10h). The next 48 bits are a unique serial number. The last 8 bits are a CRC of the first
56 bits. (See Figure 4.) The 64-bit ROM and ROM Function Control section allow the DS1920 to operate
as a 1-Wire device and follow the 1-Wire protocol detailed in the section “1-Wire Bus System.” The
memory and control functions of the DS1920 are not accessible until the ROM function protocol has been
satisfied. This protocol is described in the ROM function protocol flowchart (Figure 5). The 1-Wire bus
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. After a ROM function sequence has been successfully
executed, the functions specific to the DS1920 are accessible and the bus master may then provide any
one of the five memory and control function commands.
CRC GENERATION
The DS1920 has an 8-bit CRC stored in the most significant byte of the 64-bit ROM. The bus master can
compute a CRC value from the first 56 bits of the 64-bit ROM and compare it to the value stored within
the DS1920 to determine if the ROM data has been received error-free by the bus master. The equivalent
polynomial function of this CRC is:
CRC = X8 + X5 + X4 + 1
The DS1920 also generates an 8-bit CRC value using the same polynomial function shown above and
provides this value to the bus master to validate the transfer of data bytes. In each case where a CRC is
used for data transfer validation, the bus master must calculate a CRC value using the polynomial
function given above and compare the calculated value to either the 8-bit CRC value stored in the 64-bit
ROM portion of the DS1920 (for ROM reads) or the 8-bit CRC value computed within the DS1920
(which is read as a 9th byte when the scratchpad is read). The comparison of CRC values and decision to
continue with an operation are determined entirely by the bus master. There is no circuitry inside the
DS1920 that prevents a command sequence from proceeding if the CRC stored in or calculated by the
DS1920 does not match the value generated by the bus master.
The 1-Wire CRC can be generated using a polynomial generator consisting of a shift register and XOR
gates as shown in Figure 6. Additional information about the Dallas 1-Wire Cyclic Redundancy Check is
available in the Book of DS19xx iButton Standards.
The shift register bits are first initialized to 0. For the ROM section, starting with the least significant bit
of the family code, 1 bit at a time is shifted in. After the 8th bit of the family code has been entered, then
the serial number is entered. After the 48th bit of the serial number has been entered, the shift register
contains the CRC value. Shifting in the 8 bits of CRC should return the shift register to all 0s.
6 of 22
DS1920
64-BIT LASERED ROM Figure 4
8-BIT CRC CODE
MSB
48-BIT SERIAL NUMBER
8-BIT FAMILY CODE (10H)
LSB MSB
LSB MSB
LSB
ROM FUNCTIONS FLOW CHART Figure 5
7 of 22
DS1920
1-WIRE CRC CODE Figure 6
MEMORY
The DS1920’s memory is organized as shown in Figure 7. The memory consists of a scratchpad and 2
bytes of EEPROM which store the high and low temperature triggers TH and TL. The scratchpad helps
insure data integrity when communicating over the 1-Wire bus. Data is first written to the scratchpad
where it can be read back. After the data has been verified, a copy scratchpad command will transfer the
data to the EEPROM. This process insures data integrity when modifying the memory.
The scratchpad is organized as 8 bytes of memory. The first 2 bytes contain the measured temperature
information. The 3rd and 4th bytes are volatile copies of TH and TL and are refreshed with every power-on
reset. The next 2 bytes are not used; upon reading back, however, they will appear as all logic 1s. The 7th
and 8th bytes are count registers, which may be used in obtaining higher temperature resolution (see
“Operation-Measuring Temperature” section).
There is a 9th byte which may be read with a Read Scratchpad command. This byte is a cyclic redundancy
check (CRC) over all of the 8 previous bytes. This CRC is implemented as described in the section titled
“CRC Generation.”
DS1920 MEMORY MAP Figure 7
SCRATCHPAD
BYTE
EEPROM
TEMPERATURE LSB
0
TEMPERATURE MSB
1
TH/USER BYTE 1
2
TH/USER BYTE 1
TL/USER BYTE 2
3
TL/USER BYTE 2
RESERVED
4
RESERVED
5
COUNT REMAIN
6
COUNT PER °C
7
CRC
8
1-WIRE BUS SYSTEM
The 1-Wire bus is a system which has a single bus master and one or more slaves. The DS1920 behaves
as a slave. The discussion of this bus system is broken down into three topics: hardware configuration,
transaction sequence, and 1-Wire signaling (signal types and timing).
8 of 22
DS1920
HARDWARE CONFIGURATION
The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to
drive it at the appropriate time. To facilitate this, each device attached to the 1-Wire bus must have open
drain or 3-state outputs. The 1-Wire port of the DS1920 (data contact) is open drain with an internal
circuit equivalent to that shown in Figure 8. A multidrop bus consists of a 1-Wire bus with multiple
slaves attached. The 1-Wire bus requires a pull up resistor of approximately 5 kΩ . The idle state for the 1Wire bus is high. If for any reason a transaction needs to be suspended, the bus MUST be left in the idle
state if the transaction is to resume. If this does not occur and the bus is left low for more than 120 ms,
one or more of the devices on the bus will be reset.
HARDWARE CONFIGURATION Figure 8
TRANSACTION SEQUENCE
The protocol for accessing the DS1920 via the 1-Wire port is as follows:
§ Initialization
§ ROM Function Command
§ Memory/Control Function Command
§ Transaction/Data
INITIALIZATION
All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence
consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the
slave(s).
The presence pulse lets the bus master know that the DS1920 is on the bus and is ready to operate. For
more details, see the “1-Wire Signaling” section.
ROM FUNCTION COMMANDS
Once the bus master has detected a presence pulse, it can issue one of the five ROM function commands.
All ROM function commands are eight bits long. A list of these commands follows (refer to flowchart in
Figure 5):
9 of 22
DS1920
Read ROM [33h]
This command allows the bus master to read the DS1920’s 8-bit family code, unique 48-bit serial
number, and 8-bit CRC. This command can only be used if there is a single DS1920 on the bus. If more
than one slave is present on the bus, a data collision will occur when all slaves try to transmit at the same
time (open drain will produce a wired AND result).
Match ROM [55h]
The match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a
specific DS1920 on a multidrop bus. Only the DS1920 that exactly matches the 64-bit ROM sequence
will respond to the subsequent memory function command. All slaves that do not match the 64-bit ROM
sequence will wait for a reset pulse. This command can be used with a single or multiple devices on the
bus.
Skip ROM [CCh]
This command can save time in a single drop bus system by allowing the bus master to access the
memory functions without providing the 64-bit ROM code. If more than one slave is present on the bus
and a read command is issued following the Skip ROM command, data collision will occur on the bus as
multiple slaves transmit simultaneously (open drain pulldowns will produce a wired AND result). The
Skip ROM command is useful to address all DS1920s on the bus to do a temperature conversion. Since
the DS1920 uses a special command set, other device types will not respond to these commands.
Search ROM [F0h]
When a system is initially brought up, the bus master might not know the number of devices on the 1Wire bus or their 64-bit ROM codes. The search ROM command allows the bus master to use a process
of elimination to identify the 64-bit ROM codes of all slave devices on the bus. The ROM search process
is the repetition of a simple, three-step routine: read a bit, read the complement of the bit, then write the
desired value of that bit. The bus master performs this simple, three-step routine on each bit of the ROM.
After one complete pass, the bus master knows the contents of the ROM in one device. The remaining
number of devices and their ROM codes may be identified by additional passes. See Chapter 5 of the
Book of DS19xx iButton Standards for a comprehensive discussion of a ROM Search, including an actual
example.
Alarm Search [ECh]
The flowchart of this command is identical to the Search ROM command; however, the DS1920 will
respond to this command only if an alarm condition has been encountered at the last temperature
measurement. An alarm condition is defined as a temperature higher than TH or lower than TL. The
alarm condition remains set as long as the DS1920 is powered up or until another temperature
measurement reveals a non-alarming value. For alarming, the trigger values stored in EEPROM are taken
into account. If an alarm condition exists and the TH or TL settings are changed, another temperature
conversion should be done to validate any alarm conditions.
MEMORY AND CONTROL FUNCTION COMMANDS
The following command protocols are summarized in Table 2, and by the flowchart of Figure 9.
Write Scratchpad [4Eh]
This command writes to the scratchpad of the DS1920, starting at address 2. The next 2 bytes written will
be saved in scratchpad memory, at address locations 2 and 3. Writing may be terminated at any point by
issuing a reset. However, if a reset occurs before both bytes have been completely sent, the contents of
these bytes will be indeterminate. Bytes 2 and 3 can be read and written; all other bytes are read only.
10 of 22
DS1920
Read Scratchpad [BEh]
This command reads the complete scratchpad. After the last byte of the scratchpad is read, the bus master
will receive an 8-bit CRC of all scratchpad bytes. If not all locations are to be read, the master may issue
a reset to terminate reading at any time.
Copy Scratchpad [48h]
This command copies from the scratchpad into the EEPROM of the DS1920, storing the temperature
trigger bytes in nonvolatile memory. The bus master has to enable a strong pullup for at least 10 ms
immediately after issuing this command.
Convert Temperature [44h]
This command begins a temperature conversion. No further data is required. The bus master has to enable
a strong pullup for 0.5 seconds immediately after issuing this command.
Recall [B8h]
This command recalls the temperature trigger values stored in EEPROM to the scratchpad. This recall
operation happens automatically upon power-up to the DS1920 as well, so valid data is available in the
scratchpad as soon as the device has power applied.
11 of 22
DS1920
MEMORY AND CONTROL FUNCTIONS FLOW CHART Figure 9
12 of 22
DS1920
13 of 22
DS1920
MEMORY AND CONTROL FUNCTIONS FLOW CHART (cont’d) Figure 9
FROM FIGURE 9
FIRST PART
TO FIGURE 9
THIRD PART
14 of 22
DS1920
MEMORY AND CONTROL FUNCTIONS FLOW CHART (cont’d) Figure 9
1-WIRE SIGNALING
The DS1920 requires strict protocols to ensure data integrity. The protocol consists of five types of
signaling on one line: Reset Sequence with Reset Pulse and Presence Pulse, Write 0, Write 1, Read Data
and Strong Pullup. All these signals except Presence Pulse are initiated by the bus master. The
initialization sequence required to begin any communication with the DS1920 is shown in Figure 10. A
Reset Pulse followed by a Presence Pulse indicates the DS1920 is ready to accept a ROM command. The
bus master transmits (TX) a Reset Pulse (tRSTL, minimum 480 µs). The bus master then releases the line
and goes into receive mode (RX). The 1-Wire bus is pulled to a high state via the pullup resistor. After
detecting the rising edge on the 1-Wire line, the DS1920 waits (tPDH, 15-60 µs) and then transmits the
Presence Pulse (tPDL, 60-240 µs).
15 of 22
DS1920
READ/WRITE TIME SLOTS
The definitions of write and read time slots are illustrated in Figure 11. All time slots are initiated by the
master driving the data line low. The falling edge of the data line synchronizes the DS1920 to the master
by triggering a delay circuit in the DS1920. During write time slots, the delay circuit determines when the
DS1920 will sample the data line. For a read data time slot, if a 0 is to be transmitted, the delay circuit
determines how long the DS1920 will hold the data line low overriding the 1 generated by the master. If
the data bit is a 1, the DS1920 will leave the read data time slot unchanged.
STRONG PULLUP
To provide energy for a temperature conversion or for copying data from the scratchpad to the EEPROM,
a low impedance pullup of the 1-Wire bus to 5V is required just after the corresponding command has
been sent by the master. During temperature conversion or copying the scratchpad, the bus master
controls the transition from a state where the data line is idling high via the pullup resistor to a state where
the data line is actively driven to 5 volts, providing a minimum of 1 mA of current for each DS1920
doing temperature conversion. This low impedance pullup should be active for 0.5 seconds for
temperature conversion or at least 10 ms for copying to the scratchpad. After that, the data line returns to
an idle high state controlled by the pullup resistor. The low-impedance pullup does not affect other
devices on the 1-Wire bus. Therefore it is possible to multidrop other 1-Wire devices with the DS1920.
INITIALIZATION PROCEDURE “RESET AND PRESENCE PULSES” Figure 10
RESISTOR
MASTER
DS1920
*
480 µs ≤tRSTL < ∞ ∗
480 µs ≤tRSTH < ∞ (includes recovery time)
15 µs ≤tPDH < 60 µs
60 µs ≤tPDL < 240 µs
In order not to mask interrupt signaling by other devices on the 1-Wire bus, tRSTL + tR should always
be less than 960 µs.
16 of 22
DS1920
DS1920 MEMORY AND CONTROL FUNCTION COMMANDS Table 2
INSTRUCTION
Convert
Temperature
Read Scratchpad
Write Scratchpad
Copy Scratchpad
Recall
1-WIRE BUS
AFTER ISSUING
DESCRIPTION
PROTOCOL PROTOCOL
TEMPERATURE CONVERSION COMMANDS
Initiates temperature
44H
strong pullup
conversion
MEMORY COMMANDS
Reads bytes from scratchpad
BEH
<read up to 9 data
and reads CRC byte.
bytes>
4EH
<write data into 2
Writes bytes into scratchpad
bytes at addr. 2 and
at addresses 2 and 3 (TH and
addr. 3>
TL temperature triggers).
48H
strong pullup
Copies Scratchpad into
nonvolatile memory
(addresses 2 and 3 only).
Recalls values stored in
B8H
idle
nonvolatile memory into
scratchpad (temperature
triggers).
NOTES
1
2
NOTES:
1. Temperature conversion takes up to 0.5 seconds. After receiving the Convert Temperature command,
the data line for the DS1920 must be held high for at least 0.5 seconds to provide power during the
conversion process. As such, no other activity may take place on the 1-Wire bus for at least this
period after a Convert Temperature command has been issued.
2. After receiving the Copy Scratchpad command, the data line for the DS1920 must be held high for at
least 10 ms to provide power during the copy process. As such, no other activity may take place on
the 1-Wire bus for at least this period after a Copy Scratchpad command has been issued.
READ/WRITE TIMING DIAGRAM Figure 11
Write-1 Time Slot
RESISTOR
MASTER
60 µs ≤tSLOT < 120 µs
1 µs ≤tLOW1 < 15 µs
1 µs ≤tREC < ∞
17 of 22
DS1920
READ/WRITE TIMING DIAGRAM (cont’d) Figure 11
Write-0 Time Slot
60 µs ≤tLOW0 < tSLOT < 120 µs
1 µs ≤tREC < ∞
Read-Data Time Slot
RESISTOR
MASTER
DS1920
60 µs ≤tSLOT < 120 µs
1 µs ≤tLOWR < 15 µs
0 ≤tRELEASE < 45 µs
1 µs ≤tREC < ∞
tRDV = 15 µs
tSU < 1 µs
18 of 22
DS1920
MEMORY FUNCTION EXAMPLE Table 3
Example: Bus Master initiates temperature conversion, then reads temperature.
MASTER MODE
TX
RX
TX
TX
TX
TX
DATA (LSB FIRST)
Reset
Presence
55H
<64-bit ROM code>
44H
<DATA LINE HIGH>
TX
RX
TX
TX
TX
RX
Reset
Presence
55H
<64-bit ROM code>
BEH
<9 data bytes>
TX
RX
Reset
Presence
COMMENTS
Reset pulse( 480-960 µs)
Presence pulse
Issue "Match ROM" command
Issue address for DS1920
Issue "Convert Temperature" command
Data line is held high for at least 0.5 seconds by bus
master to allow conversion to complete.
Reset pulse
Presence pulse
Issue "Match ROM" command
Issue address for DS1920
Issue "Read Scratchpad" command.
Read entire scratchpad plus CRC; the master now
recalculates the CRC of the eight data bytes received
from the scratchpad, compares the CRC calculated and
the CRC read. If they match, the master continues; if
not, this read operation is repeated.
Reset pulse
Presence pulse, done.
19 of 22
DS1920
ABSOLUTE MAXIMUM RATINGS*
Voltage on Any Pin Relative to Ground
Operating Temperature
Storage Temperature
-0.5V to +7.0V
-55°C to +100°C
-55°C to +100°C
∗ This is a stress rating only and functional operation of the device at these or any other conditions
above those indicated in the operation sections of this specification is not implied. Exposure to
absolute maximum rating conditions for extended periods of time may affect reliability.
DC ELECTRICAL CONDITIONS
PARAMETER
Pull up Voltage
SYMBOL
VPUP
Logic 1
Logic 0
(-55°C to +100°C)
CONDITION
I/O Functions
MIN
2.8
+½°C Accurate
Temperature
Conversions
4.3
VIH
VIL
2.2
-0.3
DC ELECTRICAL CHARACTERISTICS
PARAMETER
Thermometer
Error
SYMBOL
tERR
Active Current
Input Load
Current
Output Logic
Low @ 4 mA
TYP
5.0
CONDITION
0°C to + 70 °C
MAX
6.0
UNITS
V
6.0
V
+0.8
V
V
2
2, 10
(-55°C to +100°C; VPUP=4.3V to 6.0V)
MIN
-55°C to +0°C
and + 70 °C to
+100 °C
TYP
MAX
+½
UNITS
°C
SEE TYPICAL CURVE
IDD
IL
1000
1500
NOTES
11
11
µA
3,4
µA
5
VOL
0.4
V
CAPACITANCE
PARAMETER
I/O (1- Wire)
NOTES
1,2
2
(TA =25°C)
SYMBOL
CIN/OUT
MIN
TYP
MAX
800
UNITS
pF
NOTES
9
AC ELECTRICAL CHARACTERISTICS; TEMPERATURE CONVERSION AND
COPY SCRATCHPAD
(-55°C to +100°C; VPUP =4.3V to 6.0V)
PARAMETER
Temperature Conversion
Copy Scratchpad
SYMBOL
tCONV
tCOPY
MIN
TYP
0.2
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MAX
0.5
10
UNITS
seconds
ms
NOTES
5
DS1920
AC ELECTRICAL CHARACTERISTICS:
1-WIRE INTERFACE
PARAMETER
Time Slot
Write 1 Low Time
Write 0 Low Time
Read Data Valid
Release Time
Read Data Setup
Recovery Time
Reset Time High
Reset Time Low
Presence Detect High
Presence Detect Low
SYMBOL
tSLOT
tLOW1
tLOW0
tRDV
tRELEASE
tSU
tREC
tRSTH
tRSTL
tPDHIGH
tPDLOW
MIN
60
1
60
(-55°C to +100°C; VPUP=2.8V to 6.0V)
TYP
exactly 15
15
0
1
480
480
15
60
MAX
120
15
120
45
1
4800
60
240
UNITS
µs
µs
µs
µs
µs
µs
µs
µs
µs
µs
µs
NOTES
8
6,7
NOTES:
1. Temperature conversion will work with ±2°C accuracy down to VPUP = 3.4V.
2. All voltages are referenced to ground.
3. IDD specified with low impedance pull up to 5.0V.
4. Active current refers to temperature conversion.
5. Writing to EEPROM consumes approximately 200 µA.
6. tRSTL may be up to 4800 µs. With longer times, the result of temperature conversion may get lost.
7. The reset low time should be restricted to a maximum of 960 ms, to allow interrupt signaling,
otherwise it could mask or conceal interrupt pulses.
8. Read data setup time refers to the time the host must pull the 1-Wire bus low to read a bit. Data is
guaranteed to be valid within 1 µs of this falling edge and will remain valid for 14 µs minimum. (15
µs total from falling edge on 1-Wire bus.)
9. Capacitance on the data contact could be 800 pF when power is first applied. If a 5kΩ resistor is used
to pull up the data line to VCC , 5 µs after power has been applied, the parasite capacitance will not
affect normal communications.
10. Under certain low voltage conditions VILMAX may have to be reduced to as much as 0.5V to always
guarantee a presence pulse.
11. See Typical Curve for specification limits outside the 0°C to 70°C range. Thermometer error reflects
sensor accuracy as tested during calibration.
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DS1920
TYPICAL PERFORMANCE CURVE
DS1920 TEMPERATURE iButton
TRUE TEMPERATURE (°C)
Error = Reading - True Temperature
When cold, the true temperature is typically colder than the temperature reading
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