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TCA9555: Undershoot issue in I2C lines of TCA9555RGER

Muthulakshmi M
Prodigy 40 points
Part Number: TCA9555

Hi,

 

We have used TCA9555RGER in our design. We have observed the undershoot of I2C clock and data signals. The worst case maximum undershoot observed is -1.21V and the worst case undershoot duration observed is 3.1ns. Attached the scope images for reference. Please clarify the below points,

 

  1. Please confirm whether this undershoot affects the device performance.

 

  1. Please mention the accepted worst case undershoot voltage and duration of undershoot.

 

  1. Suggest solutions to reduce the undershoot value. Please refer to our implementation.

     

  • 0
    Guru 316920 points

    The absolute maximum ratings allow voltages lower than −0.5 V only if the current is limited to less than 20 mA. According to the IBIS model, this current will be exceeded at voltages lower than −0.84 V in the worst case.

    For Fast-mode I²C at 3.3 V, the specification requires a minimum fall time of 12 ns. Your edges are much faster.

    Configure the I²C pins of your microcontroller for a lower drive strength. (I²C never requires more than 3 mA, and your pull-ups actually require less than 1 mA.) If that is not enough, consider adding series resistors (e.g., 100 Ω) at the I²C pins; see section 7.3 of the I²C specification.

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points

    Thanks for the explanation Clemens. I believe drive strength means output driver strength. Initially the drive strength was set to R0/6. As per your statement, I have configured it with R0/7, now also the undershoot value increases to -1.61V. Instead if the drive strength value configured to R0/4 or R0/3, the undershoot value reduces to -400mV. Can you please confirm whether this behavior affects the device performance.

  • 0 in reply to Muthulakshmi M
    TI__Mastermind 47748 points

    Hi Muthulakshmi,

    When you mean R0/7, R0/6, etc. What do these values mean? I second Clemens on his response. Please consider checking the drive strength of your output driver. I am assuming that R0/3 means that the output drive strength has been reduced since undershoots decreased to -400mV. Consider implementing a series resistance to reduce the undershoot voltage. 

    With an undershoot < -0.5V, you are operating the device outside the minimum input output voltage of -0.5V. An undershoot of -1.61V for example could potentially cause permanent damage to the TCA9555. Permanent damage could have lasting effects on device performance or make the device inoperable. 

    Any time you operate this device outside the abs. max conditions, you risk damaging the device. Anytime you operate outside the recommended operating conditions, the values in the electrical characteristics of the datasheet are no longer guaranteed. 

    Regards,

    Tyler

  • 0 in reply to Muthulakshmi M
    Guru 316920 points

    Apparently, values like "R0/7" are used with i.MX processors.

    R0/7 divides the R0 impedance (260 Ω at 3.3 V) by 7, i.e., it is the highest drive strength. To reduce the drive strength, you need a higher impedance, i.e., a lower divider. R0/3 sounds correct.

    When the driver's output impedance is configured correctly, you do not need an additional series resistor.