ISI222巴赫曼
ISI222主機在對DS18B20進行寫入操作時，首先應該拉低1- wire總線至少1μs 。然后，如果寫入數據位是 0，則繼續拉低總線至60μs結束（t2）;否則就釋放總線，由上拉電阻把總線拉成高電平。而DS18B20則在15~60μs之間（t1~t2）讀入數據（典型值為15μs）。同樣，在60μs之后（t2），1-wire總線還必須繼續保持高電平至少1μs，作為兩個數據位之間的分隔。
2 實驗電路參數的分析與實驗結果
以1-wire總線的數字溫度集成電路DS18B20與單片機89C2051 為例，其接口電路如圖2所示，時鐘頻率為12MHz 。89C2051的P1.0口與DS18B20的DQ相連。1-wire總線器件的DQ口（數據口線）都為開漏結構，以便可以在1-wire總線上掛接多個DS18B20。因此，在1-wire總線上應該加接1個上拉電阻，文獻推薦值為4.7kΩ。筆者使用2 m長的4芯屏蔽電纜連接89C2051 和DS18B20 ，其中一芯以及外屏蔽單端接地，其余分別為VCC、GND 和DQ 。經測試，DQ線到地的分布電容為131pF/m 。為了能夠測試1-wire總線的驅動能力，在DQ線上加接電容，以便近似地模擬長電纜，等效電路如圖3所示。可以通過選擇上拉電阻的值，合理安排 t0~t1和t0~t2兩個時間段，以提高1-wire總線器件驅動長總線的能力。自然，上拉電阻越小，其拉動1-wire總線的能力就越大，但是要受到 DS18B20的低電平吸入電流的限制。綜合考慮各種因素，選取上拉電阻值為1.5kΩ。
圖2 89C2051與DS18B20接口電路
ISI222
全球價甩賣 YASKAWA PLC SERVO MOTOR CONTROLLER 9200SH INCLUDE
PS-02,
9200SH CPU,
2171F,
9200SVA
LIO-01.
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