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<p>HI Jean-Pierre,<br>
</p>
<p>You should check if you can daisy-chain them, SDO to SDI of the
next chip, and then treat them as one very long shift register. <br>
</p>
<p>No multiplexers, no capacitive loading on the processor pins, and
simpler wiring.</p>
<p>Best,</p>
<p> René<br>
</p>
<div class="moz-cite-prefix">Am 24.03.2021 um 15:43 schrieb
Jean-Pierre Desrochers:<br>
</div>
<blockquote type="cite"
cite="mid:005301d720bc$0f71dda0$2e5598e0$@oricom.ca">
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<div class="WordSection1">
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA">In
my quest for cheap SPI pressure sensors I found this tiny
thing<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA">based
on Bosch BMP280 I2C/SPI sensor which is now obsolete but
still available<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA">(over
10,000 available) at aliexpress for $0.45CAD each !<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span
style="color:#1F497D;mso-fareast-language:FR-CA"><img
id="Image_x0020_1"
src="cid:part1.D610EC79.5A8AD79A@schmitzbits.de" class=""
width="416" height="406"></span><span
style="color:#1F497D" lang="EN-CA"><o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span
style="color:#1F497D;mso-fareast-language:FR-CA"><img
id="Image_x0020_2"
src="cid:part2.04E7B40C.F6DBA320@schmitzbits.de" class=""
width="458" height="252"></span><span
style="color:#1F497D" lang="EN-CA"><o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA">My
question to the DIY forum is:<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA">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 style="color:#1F497D" lang="EN-CA">The
above schematic shows the board parts.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA">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 style="color:#1F497D" lang="EN-CA">(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 style="color:#1F497D"
lang="EN-CA">My concern is about the MISO (SDO) line.<o:p></o:p></span></i></b></p>
<p class="MsoPlainText"><b><i><span style="color:#1F497D"
lang="EN-CA">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 style="color:#1F497D"
lang="EN-CA">Should this be taken in account ?</span></u></i></b><b><i><span
style="color:#1F497D" lang="EN-CA"><br>
The SPI clock will be around 1Mhz.</span></i></b><span
style="color:#1F497D" lang="EN-CA"><br>
No other types of SPI devices will be used here.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA">
<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA">Thanks.<br>
JP<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:#1F497D" lang="EN-CA"><br>
<br>
<o:p></o:p></span></p>
</div>
<br>
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