[sdiy] Multiple SPI devices on the same bus.

[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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