【文章內(nèi)容簡介】 of five ROM function mands: 1) Read ROM, 2) Match ROM, 3) Search ROM, 4) Skip ROM, or 5) Alarm Search. These mands operate on the 64bit laser ROM portion of each device and can single out a specific device if many are present on the 1Wire 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 six memory and control function mands. One control function mand instructs the DS18B20 to perform a temperature measurement. The result of this measurement will be placed in the DS18B20’s scratchpad memory, and may be read by issuing a memory function mand which reads the contents of the scratchpad memory. The temperature alarm triggers TH and TL consist of 1 byte EEPROM each. If the alarm search mand is not applied to the DS18B20, these registers may be used as general purpose user memory. The scratchpad also contains a configuration byte to set the desired resolution of the temperature to digital conversion. Writing TH, TL, and the configuration byte is done using a memory function mand. Read access to these registers is through the scratchpad. All data is read and written least significant bit first.2 PARASITE POWERThe block diagram (Figure 1) shows the parasitepowered 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 “1Wire 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 DS18B20 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 DS18B20 is up to mA, the DQ line will not have sufficient drive due to the 5k pull up resistor. This problem is particularly acute if several DS18B20s are on the same DQ and attempting to convert simultaneously.There are two ways to assure that the DS18B20 has sufficient supply current during its active conversion cycle. The first is to provide a strong pull up on the DQ line whenever temperature conversions or copies to the E2 memory are taking place. This may be acplished 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 us 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 DS18B20 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 pull up 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 1Wire bus during the conversion time. In addition, any number of DS18B20s may be placed on the 1Wire bus, and if they all use external power, they may all simultaneously perform temperature conversions by issuing the Skip ROM mand and then issuing the Convert T mand. 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 remended above 100176。C, since it may not be able to sustain munications given the higher leakage currents the DS18B20 exhibits at these temperatures. For applications in which such temperatures are likely, it is strongly remended that VDD be applied to the DS18B20. For situations where the bus master does not know whether the DS18B20s on the bus are parasite powered o