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PDF DS18S20 Data sheet ( Hoja de datos )
| Número de pieza | DS18S20 | |
| Descripción | High Precision 1-Wire Digital Thermometer | |
| Fabricantes | Dallas Semiconductor | |
| Logotipo |  |
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FEATURES
Unique 1–Wire interface requires only one
port pin for communication
Multidrop capability simplifies distributed
temperature sensing applications
Requires no external components
Can be powered from data line. Power supply
range is 3.0V to 5.5V
Zero standby power required
Measures temperatures from –55°C to
+125°C. Fahrenheit equivalent is –67°F to
+257°F
±0.5°C accuracy from –10°C to +85°C
Temperature is read as a 9–bit digital value
Converts temperature to digital word in
750 ms (max.)
User–definable, nonvolatile temperature
alarm settings
Alarm search command identifies and
addresses devices whose temperature is
outside of programmed limits (temperature
alarm condition)
Functionally compatible with DS1820 1-Wire
digital thermometer
Applications include thermostatic controls,
industrial systems, consumer products,
thermometers, or any thermally sensitive
system
PRELIMINARY
DS18S20
High Precision
1-Wire® Digital Thermometer
PIN ASSIGNMENT
DALLAS
DS18S20
123
BOTTOM VIEW
123
DS18S20 To-92
Package
NC 1 8 NC
NC 2 7 NC
VDD 3 6 NC
DQ 4 5 GND
DS18S20Z
8-Pin SOIC (150 mil)
PIN DESCRIPTION
GND - Ground
DQ - Data In/Out
VDD - Power Supply Voltage
NC - No Connect
DESCRIPTION
The DS18S20 Digital Thermometer provides 9–bit temperature readings which indicate the temperature
of the device.
Information is sent to/from the DS18S20 over a 1–Wire interface, so that only one wire (and ground)
needs to be connected from a central microprocessor to a DS18S20. Power for reading, writing, and
performing temperature conversions can be derived from the data line itself with no need for an external
power source.
Because each DS18S20 contains a unique silicon serial number, multiple DS18S20s 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 process monitoring and control.
1 of 27
050400
Free Datasheet http://www.datasheet4u.com/
1 page  
DS18S20
OPERATION – MEASURING TEMPERATURE
The core functionality of the DS18S20 is its direct-to-digital temperature sensor. The direct readout of
temperature data is 9 bits wide, equating to a resolution of 0.5°C; if higher resolution is desired, an
algorithm is described later in this section to achieve that need. Following the issuance of the Convert T
[44h] command, a temperature conversion is performed and the thermal data is stored in the scratchpad
memory in a 16-bit, sign-extended two’s complement format. The temperature information can be
retrieved over the 1-Wire interface by issuing a Read Scratchpad [BEh] command once the conversion
has been performed. The data is transferred over the 1-Wire bus, LSB first. The MSB of the temperature
register contains the “sign” (S) bit, denoting whether the temperature is positive or negative.
Table 2 describes the exact relationship of output data to measured. For Fahrenheit usage, a lookup table
or conversion routine must be used.
Temperature/Data Relationships Table 2
26 25 24 23 22 21 20 2-1 LSB
MSb
(unit = °C)
LSb
S S S S S S S S MSB
TEMPERATURE DIGITAL OUTPUT
(Binary)
+85°C
+125°C
+25.0°C
+0.5°C
0°C
-0.5°C
-25.0°C
-55°C
0000 0101 0101 0000
0000 0000 1111 1010
0000 0000 0011 0010
0000 0000 0000 0001
0000 0000 0000 0000
1111 1111 1111 1111
1111 1111 1100 1110
1111 1111 1001 0010
DIGITAL
OUTPUT
(Hex)
0550h*
00FAh
0032h
0001h
0000h
FFFFh
FFCEh
FF92h
*The power on reset register value is +85°C
Higher resolutions may be obtained by the following procedure. First, read the temperature,
COUNT_REMAIN, and COUNT_PER_C registers from the scratchpad. Truncate the 0.5°C bit (the
LSB) from the temperature value. This value is TEMP_READ. The actual temperature may then be
calculated by using the following:
TEMPERATURE = TEMP _ READ − 0.25 + COUNT _ PER _ C − COUNT _ REMAIN
COUNT _ PER _ C
OPERATION – ALARM SIGNALING
After the DS18S20 has performed a temperature conversion, the temperature value is compared to the
trigger values stored in TH and TL. Since these registers are 8–bit 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
5 of 27
5 Page 
DS18S20
Example of a ROM Search
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.
The following example of the ROM search process assumes four different devices are connected to the
same 1–Wire bus. The ROM data of the four devices is as shown:
ROM1
ROM2
ROM3
ROM4
00110101...
10101010...
11110101...
00010001...
The search process is as follows:
1. The bus master begins the initialization sequence by issuing a reset pulse. The slave devices respond
by issuing simultaneous presence pulses.
2. The bus master will then issue the Search ROM command on the 1–Wire bus.
3. The bus master reads a bit from the 1–Wire bus. Each device will respond by placing the value of the
first bit of their respective ROM data onto the 1–Wire bus. ROM1 and ROM4 will place a 0 onto the
1–Wire bus, i.e., pull it low. ROM2 and ROM3 will place a 1 onto the 1–Wire bus by allowing the
line to stay high. The result is the logical AND of all devices on the line, therefore the bus master
sees a 0. The bus master reads another bit. Since the Search ROM data command is being executed,
all of the devices on the 1–Wire bus respond to this second read by placing the complement of the
first bit of their respective ROM data onto the 1–Wire bus. ROM1 and ROM4 will place a 1 onto the
1–Wire, allowing the line to stay high. ROM2 and ROM3 will place a 0 onto the 1–Wire, thus it will
be pulled low. The bus master again observes a 0 for the complement of the first ROM data bit. The
bus master has determined that there are some devices on the 1–Wire bus that have a 0 in the first
position and others that have a 1.
The data obtained from the two reads of the three–step routine have the following interpretations:
00 There are still devices attached which have conflicting bits in this position.
01 All devices still coupled have a 0–bit in this bit position.
10 All devices still coupled have a 1–bit in this bit position.
11 There are no devices attached to the 1–Wire bus.
4. The bus master writes a 0. This deselects ROM2 and ROM3 for the remainder of this search pass,
leaving only ROM1 and ROM4 connected to the 1–Wire bus.
5. The bus master performs two more reads and receives a 0–bit followed by a 1–bit. This indicates that
all devices still coupled to the bus have 0s as their second ROM data bit.
6. The bus master then writes a 0 to keep both ROM1 and ROM4 coupled.
7. The bus master executes two reads and receives two 0–bits. This indicates that both 1–bits and 0–bits
exist as the third bit of the ROM data of the attached devices.
11 of 27
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