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  • TI Thinks Resolved

DLPC3430: I2C Pulling Down on Power-up

Prodigy 110 points

Replies: 7

Views: 295

Part Number: DLPC3430

On initial power-up, we apply VSYS (3.8V) and then VCC (1.8V).  Before PROJ_ON pin is toggled, i noticed that the I2C0_SCL and I2C_SDA pins are pulled low.  These pins stay in this state until PROJ_ON turns on the DLPA2000.   After that, the I2C lines go High-Z.    After that, we can clear PROJ_ON and the SDA and SCL lines remain High-Z.

Is this behavior expected?  It seems strange that the I2C bus does not power up to High-Z by default. 

Thanks,
Gerald

  • Expert 4475 points
    Hi Gerald,

    I2C communication should not start until the HOST_IRQ signal is driven low, which should happen after the DLPA2000 is turned on. Any activity on the I2C line while HOST_IRQ is high can prevent the DLPC3430 from booting.

    However, I'm not sure if the DLPC3430 drives the I2C lines low during startup. Let me confirm this and get back to you.

    What value of pullup resistors are you using on the I2C lines? Is this behavior affecting your system design?

    Regards
    Azad
  • In reply to Azad:

    In our main system, we have 2.2k resistors.

    On one board, i removed everything besides the DLPC and DLPA and put weak (100k) pull ups and i see the same behavior.  I2C is pulled low (by the DLPC since that is the only device on the bus) and the pull down is removed as soon as PROJ_ON is set high.   

    Toggling the HOST_IRQ did not clear the pull down.  

  • Expert 4475 points

    In reply to Gerald:

    Hi Gerald,

    Which voltage source are the resistors pulled up to? The external VCC (1.8V) or the 1.8V LS_OUT of the DLPA2000?

    Regards
    Azad

  • In reply to Azad:

    This is pulled to VDD_INTF which is the input to the LS_1P8 rail.   This rail is our system 1.8V rail which is always on when our system is powered. 

  • Expert 4475 points

    In reply to Gerald:

    Hi Gerald,

    Does this VDD_INTF also serve as the input to VCC_INTF of the DLPC3430?

    Regards
    Azad
  • In reply to Azad:

    Yes

  • Expert 4475 points

    In reply to Gerald:

    Hi Gerald,

    I was able to confirm that this is the expected behavior of the chip.

    If the concern is about using the I2C bus while the DLPC3430 is powered off, please add a buffer with an enable pin to connect the DLPC with the bus.

    Regards
    Azad

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