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Hot!PIC16LF18326 I2C/SMBus powering through SDA/SCL

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teranz
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2019/09/19 16:59:11 (permalink)
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PIC16LF18326 I2C/SMBus powering through SDA/SCL

I spent a day tracking down this issue- I am using a PIC16LF18326 to implement a smart battery interface to a laptop computer.  Overall the project is working well.  The 16LF18328 (I2C master mode) works with a MAX17261 battery gas gauge via I2C channel 1, and the computer SMBus via I2C channel 2 (I2C slave mode).  Both the 16LF18326 and MAX17261 can detect a Power-On Reset (POR).   I discovered that when the computer is connected to its external power supply and the SMBus is still operating internally that disconnecting the common power (+3.3v) to the 16LF18326 and MAX17261 did not cause the 16LF18326 to stop running or the MAX17261 to report a POR when re-applying power.  The system is all 3.3 volt based and both parts draw very low power.  It appears that the SMBus SCL and SDA lines continue clocking high while communicating with the 16LF18326... those lines are pulsing approximately 1.5-2.0 volts on the 16LF18326 Vdd pin keeping the parts active.  I`ve looked at every power path to the two parts and it is removing the I2C channel 2 SCL & SDA that resolves the  strange voltage at Vdd and shuts it down.  Bizarre.  So, I am looking for a simple solution to this problem.  I can get around the issue with the 16LF18326 by enabling the BOR at the high volts setting.  Ideally the battery (+12v) that supplies the volts (regulated 3.3) that go to the 16LF18326 and MAX17261 would never disconnect in normal operation with the computer also being powered but I want the circuit to be able to handle all possibilities.
 
Thanks-
#1

4 Replies Related Threads

    NKurzman
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    Re: PIC16LF18326 I2C/SMBus powering through SDA/SCL 2019/09/19 17:18:49 (permalink)
    +1 (1)
    The Chip is Powered via its own ESD protection Diodes.  So nothing on that live Bus can be off.You can try an I2C switch or mux to disconnect.  Or and I2C isolator, or a relay, or leave them on and put them to sleep.
    #2
    Jerry Messina
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    Re: PIC16LF18326 I2C/SMBus powering through SDA/SCL 2019/09/20 02:42:16 (permalink)
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    In situations like this I've had decent success using some of the hot-swap buffers like the TI TCA4311.
     
    #3
    ric
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    Re: PIC16LF18326 I2C/SMBus powering through SDA/SCL 2019/09/20 03:32:20 (permalink)
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    I guess every engineer has to learn by hard experience that the pins on a powered down chip are NOT high impedance.
    It seems a common (but sadly incorrect) assumption.
     

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    #4
    teranz
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    Re: PIC16LF18326 I2C/SMBus powering through SDA/SCL 2019/09/21 08:37:59 (permalink)
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    Thanks everyone... I simply forgot about the protection diodes.
    #5
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