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ADS7830: I2C of ADS7830 SDA is pull down to zero

Frank Xiao
Genius 12055 points
Part Number: ADS7830

   Hi team,

My customer is using the Ads7830 now, the schematic is below. The I2C bus have three devices. When power up, we find the ADS7830 i2c SDA is pulled down to low.  We have checked the device, isn't damaged. The problem will be reproduced when power on.(before power off, the master will reach hte ADS7830, when power on we will not communicate with the device right now. When the problem happens, SDA will be low, SCL will be high. We pull the SCL to low, the SDA will be recove to high. 

Could you please help check the schematic and help check why the SDA is pulled to low?

thanks.

 

 

  • 0
    TI__Expert 4405 points
    Frank,

    It looks like the customer is trying to use the FETs to achieve a level translation between a 1.8V IO level on the host side and the 5V IO the slave (ADS7830) side.

    But SDA is an bi-directional signal as per the I2C standard. With the FETs between the host and slave SDA connected as they are in the circuit diagram, it is not apparent how data would flow in both directions.

    I may be missing the full picture, but could you please clarify this?

    Thanks.

    Regards,
    Sandeep
  • 0 in reply to Sandeep Kasargod
    TI__Genius 12055 points
    Hi Sandeep,

    i have modified the question, could you please help check it? Thanks.
  • 0 in reply to Frank Xiao
    TI__Expert 4405 points
    Hi,

    This thread was taken offline, debugged and a solution was offered to the customer. A short summary is posted here to close this thread:

    The ADS7830 is capable of stretching the SCL when waiting for the result of a data conversion requested by the host, as described in Figure 16 of the datasheet.

    Debug revealed that this issue occurs when the Master wakes up from a power cycle and finds the SCL held by the ADS7830 from a previous request and is unable to initiate any new transactions. Ideally, the power cycle should have reset the ADS7830 as well. But in this particular case the master and slave operate from different power supplies and the master is able to recover from its power cycle, without resetting the ADS7830’s 5V supply. Hence this issue.

    In order to prevent this issue, it was recommended that the 5V supply be power cycled whenever the master wakes up from reset.

    Regards,
    Sandeep