Motion Sensor SPI / I2C minor issue

pHilipp Zabel philipp.zabel at
Wed Jan 23 14:46:29 CET 2008

On Jan 23, 2008 2:04 PM, Andy Green <andy at> wrote:
> Hash: SHA1
> 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:
> )
> 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.

Funny. I wonder why the SDO pin wasn't used to tell them apart in I²C mode.


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