[sdiy] Multiple SPI devices on the same bus.

[John Luciani]

Another option would be to use multiple PICs. You could connect 8, 12, 16
sensors
to a single peripheral PIC. These could be setup to monitor and interrupt
the
main PIC when a change is detected. This should eliminate loading and
muxing and
would free up processing time on the main PIC.

It is more code and some of it is interrupt routines which can get tricky
to debug.


John L


On Wed, Mar 24, 2021 at 10:45 AM Jean-Pierre Desrochers <jpdesroc at oricom.ca>
wrote:

> 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
> busthat 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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