1 of 21 022103FEATURES§ Unique 1-Wire® interface requires only oneport pin for communication§ Each device has a unique 64-bit serial codestored in an onboard ROM§ Multidrop capability simplifies distributedtemperature sensing applications§ Requires no external components§ Can be powered from data line. Power supplyrange is 3.0V to 5.5V§ Measures temperatures from –55°C to+125°C (–67°F to +257°F)§ ±0.5°C accuracy from –10°C to +85°C§ 9-bit thermometer resolution§ Converts temperature in 750ms (max.)§ User-definable nonvolatile (NV) alarmsettings§ Alarm search command identifies andaddresses devices whose temperature isoutside of programmed limits (temperaturealarm condition)§ Applications include thermostatic controls,industrial systems, consumer products,thermometers, or any thermally sensitivesystemPIN ASSIGNMENTPIN DESCRIPTION GND - GroundDQ - Data In/OutVDD- Power Supply VoltageNC - No ConnectDESCRIPTIONThe DS18S20 Digital Thermometer provides 9-bit centigrade temperature measurements and has analarm function with nonvolatile user-programmable upper and lower trigger points. The DS18S20communicates over a 1-Wire bus that by definition requires only one data line (and ground) forcommunication with a central microprocessor. It has an operating temperature range of –55°C to +125°Cand is accurate to ±0.5°C over the range of –10°C to +85°C. In addition, the DS18S20 can derive powerdirectly from the data line (“parasite power”), eliminating the need for an external power supply.Each DS18S20 has a unique 64-bit serial code, which allows multiple DS18S20s to function on the same1-Wire bus; thus, it is simple to use one microprocessor to control many DS18S20s distributed over alarge area. Applications that can benefit from this feature include HVAC environmental controls,temperature monitoring systems inside buildings, equipment or machinery, and process monitoring andcontrol systems.DS18S20High-Precision1-Wire Digital Thermometerwww.maxim-ic.com8-Pin 150mil SO(DS18S20Z)TO-92(DS18S20)1(BOTTOM VIEW)23DALLASDS18201GNDDQVDD23NCNCNCNCGNDDQVDDNC68753124DS18201-Wire is a registered trademark of Dallas Semiconductor.DS18S202 of 21DETAILED PIN DESCRIPTIONS Table 18-PIN SOIC* TO-92 SYMBOL DESCRIPTION5 1 GNDGround.42DQData Input/Output Pin. Open-drain 1-Wire interface pin. Alsoprovides power to the device when used in parasite power mode(see “Parasite Power” section.)33VDDOptional VDD Pin. VDD must be grounded for operation inparasite power mode.*All pins not specified in this table are “No Connect” pins.OVERVIEWFigure 1 shows a block diagram of the DS18S20, and pin descriptions are given in Table 1. The 64-bitROM stores the device’s unique serial code. The scratchpad memory contains the 2-byte temperatureregister that stores the digital output from the temperature sensor. In addition, the scratchpad providesaccess to the 1-byte upper and lower alarm trigger registers (TH and TL). The TH and TL registers arenonvolatile (EEPROM), so they will retain data when the device is powered down.The DS18S20 uses Dallas’ exclusive 1-Wire bus protocol that implements bus communication using onecontrol signal. The control line requires a weak pullup resistor since all devices are linked to the bus via a3-state or open-drain port (the DQ pin in the case of the DS18S20). In this bus system, the microprocessor(the master device) identifies and addresses devices on the bus using each device’s unique 64-bit code.Because each device has a unique code, the number of devices that can be addressed on one bus isvirtually unlimited. The 1-Wire bus protocol, including detailed explanations of the commands and “timeslots,” is covered in the 1-WIRE BUS SYSTEM section of this datasheet.Another feature of the DS18S20 is the ability to operate without an external power supply. Power isinstead supplied through the 1-Wire pullup resistor via the DQ pin when the bus is high. The high bussignal also charges an internal capacitor (CPP), which then supplies power to the device when the bus islow. This method of deriving power from the 1-Wire bus is referred to as “parasite power.” As analternative, the DS18S20 may also be powered by an external supply on VDD.DS18S20 BLOCK DIAGRAM Figure 1VPU4.7KPOWERSUPPLYSENSE64-BIT ROMAND 1-wire PORTDQVDDINTERNAL VDDCPPPARASITE POWERCIRCUITMEMORY CONTROLLOGICSCRATCHPAD8-BIT CRC GENERATORTEMPERATURE SENSORALARM HIGH TRIGGER (TH)REGISTER (EEPROM)ALARM LOW TRIGGER (TL)REGISTER (EEPROM)GNDDS18S20DS18S203 of 21OPERATION — MEASURING TEMPERATUREThe core functionality of the DS18S20 is its direct-to-digital temperature sensor. The temperature sensoroutput has 9-bit resolution, which corresponds to 0.5°C steps. The DS18S20 powers-up in a low-poweridle state; to initiate a temperature measurement and A-to-D conversion, the master must issue a ConvertT [44h] command. Following the conversion, the resulting thermal data is stored in the 2-bytetemperature register in the scratchpad memory and the DS18S20 returns to its idle state. If the DS18S20is powered by an external supply, the master can issue “read-time slots” (see the 1-WIRE BUS SYSTEMsection) after the Convert T command and the DS18S20 will respond by transmitting 0 while thetemperature conversion is in progress and 1 when the conversion is done. If the DS18S20 is powered withparasite power, this notification technique cannot be used since the bus must be pulled high by a strongpullup during the entire temperature conversion. The bus requirements for parasite power are explained indetail in the POWERING THE DS18S20 section of this datasheet.The DS18S20 output data is calibrated in degrees centigrade; for Fahrenheit applications, a lookup tableor conversion routine must be used. The temperature data is stored as a 16-bit sign-extended two’scomplement number in the temperature register (see Figure 2). The sign bits (S) indicate if thetemperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. Table 2gives examples of digital output data and the corresponding temperature reading.Resolutions greater than 9 bits can be calculated using the data from the temperature, COUNT REMAINand COUNT PER °C registers in the scratchpad. Note that the COUNT PER °C register is hard-wired to16 (10h). After reading the scratchpad, the TEMP_READ value is obtained by truncating the 0.5°C bit(bit 0) from the temperature data (see Figure 2). The
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