Motion Sensor SPI / I2C minor issue

[Andy Green]

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Hi folks -

This isn't our current problem but a something I noticed that could make
different mischief if not watched out for.

(Chip data is here:
http://www.st.com/stonline/products/literature/ds/12726.pdf )

The chip supports I2C and SPI on the same pins, they use the SPI CS to
select I2C mode when it is disabled for SPI.

We wired two chips up on the same SPI interface and used the CS pin to
enable one or the other.  The problem is that the "disabled" one is not
unexpectedly NOT disabled, it is in I2C mode.

It's possible, but unlikely, to trash the registers in the "disabled"
one when you talk to the other, depending on what data you are sending
out, because it is interpreting what you send on SPI as valid I2C data.

Maybe it's a bit more possible to get the "disabled" one addressed on
I2C accidentally and in "read" mode, where it starts driving the same
SPI data line the other chip or the CPU is driving.

There's no workaround by talking to them in I2C instead either because
they both have the same SCL and SDA hooked up and share the same I2C
address.  If we use CS to push one into SPI mode it's the same kind of
problem that we started with.

Not the problem we are seeing at the moment, nor is it likely to make
much trouble, but still a bit of a funny.

- -Andy

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