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DS18S20 Datasheet(PDF) 5 Page - Dallas Semiconductor |
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DS18S20 Datasheet(HTML) 5 Page - Dallas Semiconductor |
5 / 27 page DS18S20 5 of 27 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 2 6 2 5 2 4 2 3 2 2 2 1 2 0 2 -1 LSB MSb (unit = °C) LSb S S S SSS S S MSB TEMPERATURE DIGITAL OUTPUT (Binary) DIGITAL OUTPUT (Hex) +85°C 0000 0101 0101 0000 0550h* +125°C 0000 0000 1111 1010 00FAh +25.0°C 0000 0000 0011 0010 0032h +0.5°C 0000 0000 0000 0001 0001h 0°C 0000 0000 0000 0000 0000h -0.5°C 1111 1111 1111 1111 FFFFh -25.0°C 1111 1111 1100 1110 FFCEh -55°C 1111 1111 1001 0010 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: C PER COUNT REMAIN COUNT C PER COUNT READ TEMP E TEMPERATUR _ _ _ _ _ 25 . 0 _ − + − = 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 |
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