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</o:shapelayout></xml><![endif]--></head><body lang=FR-CA link=blue vlink=purple><div class=WordSection1><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>In my quest for cheap SPI pressure sensors I found this tiny thing<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>based on Bosch BMP280 I2C/SPI sensor which is now obsolete but still available<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>(over 10,000 available) at aliexpress for $0.45CAD each !<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:#1F497D;mso-fareast-language:FR-CA'><img width=416 height=406 id="Image_x0020_1" src="cid:image001.png@01D72096.3A408890"></span><span lang=EN-CA style='color:#1F497D'><o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:#1F497D;mso-fareast-language:FR-CA'><img width=458 height=252 id="Image_x0020_2" src="cid:image003.jpg@01D7209A.87F87B20"></span><span lang=EN-CA style='color:#1F497D'><o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>My question to the DIY forum is:<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>Since there will be around 90 of these boards used in my project<br>read by the same PIC micro there will be a need for buffering<br>on each MOSI (SDI) & SCK lines. That’s ok. CS lines will be managed by 8 multiplexers (74HC154).<br>That’s ok too.<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>The above schematic shows the board parts.<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>R1,2,3,4 will be removed on each board to avoid heavy loading using 90 devices<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>(down to 97 ohms if each internal 70k pull-up resistors are calculated).<br>Only one of the 90 PCB will keep these resistors (the last one of the row).<o:p></o:p></span></p><p class=MsoPlainText><b><i><span lang=EN-CA style='color:#1F497D'>My concern is about the MISO (SDO) line.<o:p></o:p></span></i></b></p><p class=MsoPlainText><b><i><span lang=EN-CA style='color:#1F497D'>Even tristated each BMP280 will show some kind of capacitance on the MISO bus<br>that will be multiplied by 90 in my case.<o:p></o:p></span></i></b></p><p class=MsoPlainText><b><i><u><span lang=EN-CA style='color:#1F497D'>Should this be taken in account ?</span></u></i></b><b><i><span lang=EN-CA style='color:#1F497D'><br>The SPI clock will be around 1Mhz.</span></i></b><span lang=EN-CA style='color:#1F497D'><br>No other types of SPI devices will be used here.<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'> <o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'>Thanks.<br>JP<o:p></o:p></span></p><p class=MsoPlainText><span lang=EN-CA style='color:#1F497D'><br><br><o:p></o:p></span></p></div></body></html>