【正文】 continues to operate off this power source during the low times of the 1–Wire line until it returns high 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 1–Wire port. With the 1–Wire port, the memory and control functions will not be available before the ROM function protocol has been established. The master must first provide one 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 64–bit lasered ROM portion of each device and can single out a specific device if many are present on the 1–Wire 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 mastermay then provide any one of the six memory and control function mands. One control function mand instructs the DS1820 to perform a temperature measurement. The result of this measurement will be placed in the DS1820’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 one byte EEPROM each. If the alarm search mand 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 mand. Read access to these registers is through the scratchpad. All data is read and written least significant bit 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 “1–Wire Bus System”). The advantages of parasite power are two–fold: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 provided 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 pull–up resistor. This problem is particularly acute if several DS1820’s are on the same I/O and attempting to convert 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 pull–up on the I/O linewhenever temperature conversions or copies to the E2 memory are taking place. This may be acplished by using a 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 pull–up 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 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 pull–up is not required on the I/O 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 DS1820’s 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 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 value, is incremented, indicating that the temperature is higher than –55176。C in 176。C resolution. The temperature reading is provided in a 16–bit, sign–extended two’s plement reading. Table 1 describes the exact relationship of output data to measured temperature. The data is transmitted serially over the 1–Wire interface. The DS1820 can measure temperature over the range of –55176。 it will send back a “1” if it is powered from the VDD pin. If the master receives a “0”, it knows that it must supply the strong pull–up on the I/O line during temperature conversions. See “Memory Command Functions” section for more detail on this mand protocol.OPERATION – MEASURING TEMPERATUREThe DS1820 measures temperature through the use of an on–board proprietary temperature measurement technique. A block diagram of the temperature measurement circuitry is shown in Figure 4. The DS1820 measures temperature by counting the number of clock cycles that an oscillator with a low temperature coefficient goes through during a gate period determined by a high temperature coefficient oscillator. The counter is preset with a base count that corresponds to –55176。F to+257176。從設計的選題到資料的搜集直至最后設計的修改的整個過程中，花費了郭老師很多的寶貴時間和精力，在此向?qū)煴硎局孕牡馗兄x！導師嚴謹?shù)闹螌W態(tài)度，開拓進取的精神和高度的責任心都將使學生受益終生！ 還要感謝和我同一設計小組的幾位同學，是你們在我平時設計中和我一起探討問題，并指出我設計上的誤區(qū)，使我能及時的發(fā)現(xiàn)問題把設計順利的進行下去，沒有你們的幫助我不可能這樣順利地結(jié)稿，在此表示深 附　錄 溫度、相對濕度檢測儀的電路原理圖53外文資料原文DS1820FEATURES? Unique 1–WireTM interface requires only one port pinfor munication? Multidrop capability simplifies distributed temperaturesensing applications? Requires no external ponents? Can be powered from data line? Zero standby power required? Measures temperatures from –55176。參考文獻 [1] 謝光忠、蔣亞東等. 2000,19(4):2933[2] :機械工業(yè)出版社,1993[3] 、系統(tǒng)配置與接口技術.[4] 陳寶江,翟涌,.[5] 喻評，：化學工業(yè)出版社，2006[6] ：北京航空航天大學出版社。軟件設計中面臨的主要問題是環(huán)境溫濕度數(shù)據(jù)的采集方法。該檢測儀在濕度的線性化上用的時間可能會稍長一些。 stop()。 write_shj(0xa1)。 write_shj(qjnbdz)。 start()。 response()。 write_shj(qjnbdz)。}//一定不要括錯void write(uchar qjnbdz,uchar date){ start()。 delay(5)。 for(k=0。} sda