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I use LIS3DH accelerometer. For some reason pin 8 left open ( not connected) I use I2C protocol, and it works. any exploitation how it works ( pin 8 should be tied high for I2C), Pin 8 : CS : "1" - I2C , "0" - SPI Thanks

OWigd.1
Associate II
 
6 REPLIES 6
Eleon BORLINI
ST Employee

Hi @OWigd.1​ , it's not a totally safe condition leaving Pin8 unconnected, since it is by default an input high impedance node. Is it working I2C only sometimes or always? Regards

Hi
Its working I2C all the time.
I have lots of such boards in the field , and I need
A good excuse not to recall !!!
Thanks
OWigd.1
Associate II

Hi Eleon,

Thanks for your answer.

Yes, i know it not "totally safe"

But is there an explanation why it works well for me ( I2C) and never wakes up SPI mode ?

Are you familiar with the HW design of pin8 (CS) input ?

Thanks

Hi Eleon,

Thanks for your answer.

Yes, i know it not "totally safe"

But is there an explanation why it works well for me ( I2C) and never wakes up SPI mode ?

Are you familiar with the HW design of pin8 (CS) input ?

Thanks

Hi @OWigd.1​ , what I understood from our experts is that the CS is high impedance to be properly driven but the device sets internally a status (i.e. like an internal pull-up) to prevent a wrong or unknown startup status in the case the pin is not connected. Btw, I confirm again that this pin should be connected at least to the application processor to be able to manage it in a known status. Regards

OWigd.1
Associate II

Thanks Eleon,

This is important information for us.

So, to my understanding , If Pin8 ( CS) left open (I know I should be tied high for I2C)

LIS3DH shall establish I2C communication regardless ( this is what we actually see)

Could you elaborate more about : "but the device sets internally a status (i.e. like an internal pull-up) to prevent a wrong or unknown startup status in the case the pin is not connected"

A more technical / design explanation ( from the experts , chip designer) would be appreciated .

Regards and many thanks

Oded