Материал: 18S20

DS18S20

DS18S20 BLOCK DIAGRAM Figure 1

PARASITE POWER

The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power whenever the DQ or VDD pins are high. DQ will provide sufficient power as long as the specified timing and voltage requirements are met (see the section titled “1–Wire Bus System”). The advantages of parasite power are twofold: 1) by parasiting off this pin, no local power source is needed for remote sensing of temperature, and 2) the ROM may be read in absence of normal power.

In order for the DS18S20 to be able to perform accurate temperature conversions, sufficient power must be provided over the DQ line when a temperature conversion is taking place. Since the operating current of the DS18S20 is up to 1.5 mA, the DQ line will not have sufficient drive due to the 5K pullup resistor. This problem is particularly acute if several DS18S20s are on the same DQ and attempting to convert simultaneously.

There are two ways to assure that the DS18S20 has sufficient supply current during its active conversion cycle. The first is to provide a strong pullup on the DQ line whenever temperature conversions or copies to the E2 memory are taking place. This may be accomplished by using a MOSFET to pull the DQ line directly to the power supply as shown in Figure 2. The DQ line must be switched over to the strong pull– up within 10 s maximum after issuing any protocol that involves copying to the E2 memory or initiates temperature conversions. When using the parasite power mode, the VDD pin must be tied to ground.

Another method of supplying current to the DS18S20 is through the use of an external power supply tied to the VDD pin, as shown in Figure 3. The advantage to this is that the strong pullup is not required on the DQ line, and the bus master need not be tied up holding that line high during temperature conversions. This allows other data traffic on the 1–Wire bus during the conversion time. In addition, any number of DS18S20s may be placed on the 1–Wire bus, and if they all use external power, they may all simultaneously perform temperature conversions by issuing the Skip ROM command and then issuing the Convert T command. Note that as long as the external power supply is active, the GND pin may not be floating.

The use of parasite power is not recommended above 100° C, since it may not be able to sustain communications given the higher leakage currents the DS18S20 exhibits at these temperatures. For applications in which such temperatures are likely, it is strongly recommended that VDD be applied to the DS18S20.

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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

*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

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