【正文】 output data from the DS18B20 is valid for 15 us after the falling edge of the read time slot. The host therefore must stop driving the DQ pin low in order to read its state 15 us from the start of the read slot (see Figure 12). By the end of the read time slot, the DQ pin will pull back high via the external pull up resistor. All read time slots must be a minimum of 60 us in duration with a minimum of a 1us recovery time between individual read slots.9。C.The scratchpad is organized as eight bytes of memory. The first 2 bytes contain the LSB and the MSB of the measured temperature information, respectively. The third and fourth bytes are volatile copies of TH and TL and are refreshed with every poweron reset. The fifth byte is a volatile copy of the configuration register and is refreshed with every poweron reset. The configuration register will be explained in more detail later in this section of the datasheet. The sixth, seventh, and eighth bytes are used for internal putations, and thus will not read out any predictable pattern. It is imperative that one writes TH, TL, and config in succession。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 or supplied with external VDD, a provision is made in the DS18B20 to signal the power supply scheme used. The bus master can determine if any DS18B20 are on the bus which require the strong pull up by sending a Skip ROM protocol, then issuing the read power supply mand. After this mand is issued, the master then issues read time slots. The DS18B20 will send back “0” on the 1Wire bus if it is parasite powered。通過(guò)使TINRT和TRC盡可能小且把主機(jī)采樣時(shí)間定在15us期間的末尾，系統(tǒng)時(shí)序關(guān)系就有最大的余地。所有讀時(shí)間片的最短持續(xù)期限為60us，各個(gè)讀時(shí)間片之間必須有最短為1us的恢復(fù)時(shí)間。因此，為了讀出從讀時(shí)間片開(kāi)始算起15us的狀態(tài)主機(jī)必須停止把I/O引腳驅(qū)動(dòng)至低電平。 Time Slots讀時(shí)隙 當(dāng)從DS1820 讀數(shù)據(jù)時(shí)，主機(jī)產(chǎn)生讀時(shí)間片段。對(duì)于主機(jī)產(chǎn)生寫(xiě)1時(shí)間片的情況，數(shù)據(jù)線必須先被拉至邏輯低電平，然后就被釋放，使數(shù)據(jù)線在寫(xiě)時(shí)間片開(kāi)始之后的15 us之內(nèi)拉至高電平。如果線為高電平，寫(xiě)1就發(fā)生。有兩種類型的寫(xiě)時(shí)間片：寫(xiě)1時(shí)間片和寫(xiě)0時(shí)間片，所有時(shí)間片必須有最短為60 us的持續(xù)期，在各寫(xiě)周期之間必須有最短為1us的恢復(fù)時(shí)間。還有第九個(gè)字節(jié)它可用Read Scratchpad（讀暫存存貯器）命令讀出，該字節(jié)包含一個(gè)循環(huán)冗余校驗(yàn)（CRC）字節(jié)，它是前面所有8個(gè)字節(jié)的CRC值，此CRC值以（CRC產(chǎn)生）一節(jié)中所述的方式產(chǎn)生。接著的兩個(gè)字節(jié)沒(méi)有使用，但是在讀回時(shí)，它們呈現(xiàn)為邏輯全1。這一過(guò)程確保了更改存貯器的時(shí)候保持?jǐn)?shù)據(jù)的完整性。暫存存貯器有助于在單線通信時(shí)確保數(shù)據(jù)的完整性，數(shù)據(jù)首先寫(xiě)入暫存存貯器，在那里它可以被讀回。移入CRC的8 位應(yīng)該使移位寄存器返回至全零。當(dāng)產(chǎn)品系列編碼的8 位移入以后，接著移入序列號(hào)?？偩€CRC 可以使用一個(gè)移位寄存器和“異或”（XOR）門組成的多項(xiàng)式產(chǎn)生器來(lái)產(chǎn)生，其它有關(guān)Dallas 公司單線循環(huán)冗余校驗(yàn)的信息可參見(jiàn)標(biāo)題為“理解和使用Dallas 半導(dǎo)體公司接觸式存貯器產(chǎn)品”的應(yīng)用注釋移。CRC 值的比較和是否繼續(xù)操作都由總線主機(jī)來(lái)決定，當(dāng)存貯在DS1820 內(nèi)或由DS1820 計(jì)算得到的CRC 值與總線主機(jī)產(chǎn)生的值不相符合時(shí)，在DS1820 內(nèi)沒(méi)有電路來(lái)阻止命令序列的繼續(xù)執(zhí)行?？偩€上的主機(jī)可以根據(jù)64 位ROM 的前56 位計(jì)算機(jī)CRC 的值并把它與存貯在DS1820 內(nèi)的值進(jìn)行比較以決定ROM 的數(shù)據(jù)是否已被主機(jī)正確地接收。在成功地執(zhí)行了ROM操作序列之后，DS1820 特定的功能便可訪問(wèn)，然后總線上主機(jī)可提供六個(gè)存貯器和控制功能命令之一。4 64BIT LASER4 44444464位激光ROMEach DS18B20 contains a unique ROM code that is 64bits ，開(kāi)始的8位是單線產(chǎn)品系列編碼（DS1820編碼是10h），接著的48位是唯一的系列號(hào)，最后的8位是開(kāi)始56位CRC，64位ROM和ROM 操作控制部分允許DS1820 作為一個(gè)單線器件工作并遵循“單線總線系統(tǒng)”的單線協(xié)議，直到ROM 操作協(xié)議被滿足，DS1820 控制部分的功能是不可訪問(wèn)的。每次溫度測(cè)量將更新告警標(biāo)志，只要告警標(biāo)志置位，DS1820 將對(duì)告警搜索命令做出響應(yīng)。3 OPERATION ALARM SIGNAL3運(yùn)算報(bào)警信號(hào)After the DS18B20 has performed a temperature conversion, the temperature value is pared to the trigger values stored in TH and 完成溫度變換之后，溫度值與貯存在TH和TL內(nèi)的觸發(fā)值相比較，因?yàn)檫@些寄存器僅僅是8位。如果是寄生供電，DS1820 將在單線總線上送回（0）；如果由VDD 引腳供電，它將送回（1）?？偩€上主機(jī)通過(guò)發(fā)出跳過(guò)（Skip）ROM 的操作約定，然后發(fā)出讀電源命令，可以決定是否有需要在DS1820 的總線上放置上拉電阻。注意只要外部電源處于工作狀態(tài)，GND（地）引腳不可懸空。Another method of supplying current to the DS18B20 is through the use of an external power supply tied to the V DD pin, as shown in Figure 供電的另外一種方法是通過(guò)使用連接到VDD 引腳的外部電源，這種方法的優(yōu)點(diǎn)是在I/O 線上不要求強(qiáng)的上拉電阻，總線上主機(jī)不需向上連接便在溫度變換期間使線保持高電平，這就允許在變換時(shí)間內(nèi)其它數(shù)據(jù)在單線上傳送。There are two ways to assure that the DS18B20 has sufficient supply current during its active conversion 。因?yàn)镈S1820的工作電流高達(dá)1mA，5K 的上拉電阻將使I/O線沒(méi)有足夠的驅(qū)動(dòng)能力。In order for the DS18B20 to be able to perform accurate temperature conversions, sufficient power must be provided over the DQ line when a temp