1、外文资料原文 DS1820 FEATURES Unique 1WireTM interface requires only one port pinfor communication Multidrop capability simplifies distributed temperaturesensing applications Requires no external components Can be powered from data line Zero standby power required Measures temperatures from 55C to +125C in
2、0.5C increments. Fahrenheit equivalent is 67F to+257F in 0.9F increments Temperature is read as a 9bit digital value. Converts temperature to digital word in 200 ms (typ.) Userdefinable, nonvolatile temperature alarm settings Alarm search command identifies and addressesdevices whose temperature is
3、outside of programmedlimits (temperature alarm condition) Applications include thermostatic controls, industrialsystems, consumer products, thermometers, or anythermally sensitive system DESCRIPTION The DS1820 Digital Thermometer provides 9bit temperature readings which indicate the temperature of t
4、he device. Information is sent to/from the DS1820 over a 1Wire interface, so that only one wire (and ground) needs to be connected from a central microprocessor to a DS1820. Power for reading, writing, and performing temperature conversions can be derived from the data line itself with no need for a
5、n external power source. Because each DS1820 contains a unique silicon serial number, multiple DS1820s can exist on the same 1Wire bus. This allows for placing temperature sensors in many different places.Applications where this feature is useful include HVAC environmental controls, sensing temperat
6、ures inside buildings, equipment or machinery, and in process monitoring and control. DETAILED PIN DESCRIPTION OVERVIEW The block diagram of Figure 1 shows the major componentsof the DS1820. The DS1820 has three main data components: 1) 64bit lasered ROM, 2) temperature and sensor, 3) nonvolatile te
7、mperature alarm triggers TH and TL. The device derives its power from the 1Wire communication line by storing energy on an internal capacitor during periods of time when the signal line is high and continues to operate off this powersource during the low times of the 1Wire line until it returns high
8、 to replenish the parasite (capacitor) supply. As an alternative, the DS1820 may also be powered from an external 5 volts supply. Communication to the DS1820 is via a 1Wire port. With the 1Wire port, the memory and control functions will not be available before the ROM function protocol has been est
9、ablished. The master must first provide one of five ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, 4) Skip ROM, or 5) Alarm Search. These commands operate on the 64bit lasered ROM portion of each device and can single out a specific device if many are present on the 1Wire line as w
10、ell 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 mastermay then provide any one of the six memory and control function commands. One control function
11、 command instructs the DS1820 to perform a temperature measurement. The result of this measurement will be placed in the DS1820s scratchpad memory, and may be read by issuing a memory function command which reads the contents of the scratchpad memory. The temperature alarm triggers TH and TL consist
12、 of one byte EEPROM each. If the alarm search command is not applied to the DS1820, these registers may be used as general purpose user memory. Writing TH and TL is done using a memory function command. Read access to these registers is through the scratchpad. All data is read and written least sign
13、ificant bit first.The block diagram (Figure 1) shows the parasite powered circuitry. This circuitry “steals” power whenever the I/O or VDD pins are high. I/O will provide sufficient power as long as the specified timing and voltage requirements are met (see the section titled “1Wire Bus System”). Th
14、e advantages of parasite power are twofold: 1) by parasiting off this pin, no local power source is needed for remote sensing of temperature, 2) the ROM may be read in absence of normal power. In order for the DS1820 to be able to perform accurate temperature conversions, sufficient power must be pr
15、ovided over the I/O line when a temperature conversion is taking place. Since the operating current of the DS1820 is up to 1 mA, the I/O line will not have sufficient drive due to the 5K pullup resistor. This problem is particularly acute if several DS1820s are on the same I/O and attempting to conv
16、ert simultaneously. There are two ways to assure that the DS1820 has sufficient supply current during its active conversion cycle. The first is to provide a strong pullup on the I/O linewhenever temperature conversions or copies to the E2 memory are taking place. This may be accomplished by using a
17、MOSFET to pull the I/O line directly to the power supply as shown in Figure 2. The I/O line must be switched over to the strong pullup within 10 ms maximum after issuing any protocol that involves copying to the E2 memory or initiates temperature conversions. When using the parasite power mode, the
18、VDD pin must be tied to ground. Another method of supplying current to the DS1820 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 I/O line, and the bus master need not be tied up holding
19、that line high during temperature conversions. This allows other data traffic on the 1Wire bus during the conversion time. In addition, any number of DS1820s 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 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 100C, since it
