[sdiy] Multiple SPI devices on the same bus.

[René Schmitz]

HI Jean-Pierre,

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.

No multiplexers, no capacitive loading on the processor pins, and 
simpler wiring.

Best,

  René

Am 24.03.2021 um 15:43 schrieb Jean-Pierre Desrochers:
>
> In my  quest for cheap SPI pressure sensors I found this tiny thing
>
> based on Bosch BMP280  I2C/SPI sensor which is now obsolete but still 
> available
>
> (over 10,000 available) at aliexpress for $0.45CAD each !
>
> My question to the DIY forum is:
>
> Since there will be around 90 of these boards used in my project
> read by the same PIC micro there will be a need for buffering
> on each MOSI (SDI) & SCK lines. That’s ok. CS lines will be managed by 
> 8 multiplexers (74HC154).
> That’s ok too.
>
> The above schematic shows the board parts.
>
> R1,2,3,4 will be removed on each board to avoid heavy loading using 90 
> devices
>
> (down to 97 ohms if each internal 70k pull-up resistors are calculated).
> Only one of the 90 PCB will keep these resistors (the last one of the 
> row).
>
> */My concern is about the MISO (SDO) line./*
>
> */Even tristated each BMP280 will show some kind of capacitance on the 
> MISO bus
> that will be multiplied by 90 in my case./*
>
> */_Should this be taken in account ?_/**/
> The SPI clock will be around 1Mhz./*
> No other types of SPI devices will be used here.
>
> Thanks.
> JP
>
>
>
>
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