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BQ27621: I2C Rise Time

JoFu10
Intellectual 2385 points

Hi,

My customer had a few questions regarding the I2C lines on the BQ27621.

1) It states that the max rise time of the I2C line is 300ns, will a larger rise time lead to potential bit errors?

2) Is turning the clock speed down to 25kHz acceptable for this device? Doing this solved some issues with write fails, but they would like to double check that this would not cause unforseen issues. The DS only specifies 100 and 400 kHz.

2a) Would the I2C block actually perform better with a slower clock speed assuming the rise time wont change by slowing the clock?

Thanks,

Josh

  • 0
    TI__Guru 61126 points
    1. If the rise time is slower than that we can't guarantee correct operation.
    2. This should be ok.
    3. It won't perform better - if you saw problems with 100kHz or 400kHz then one of the timing requirements must have been violated.
  • 0 in reply to Dominik Hartl11
    TI__Intellectual 2385 points

    Hi Dominik,

    Thank you for your quick response. Could you tell me how the rise time is defined for this part? We've seen it defined a few ways (e.g. the time it takes the signal to rise from 10% to 90% of the voltage).

    The interesting thing is, the VIH for this part is actually Vpu x 0.7. So if my customer is using 3.3V as their pull voltage, the VIH will be 2.31V. Based on their measurements, they are reaching 2.31V just around 300ns. Could they just put in a parallel resistor to bring the rise time below 300ns and have the system not violate any specs?

    Thanks,

    Josh

  • 0 in reply to JoFu10
    TI__Guru 61126 points
    It's the time it takes to change from 10% to 90%. It's very likely that the part will work reliably with your customer's timing without additional resistors etc.
    The max. spec is guaranteed. It does not mean that it will not work with slower timing (esp. if it is that close to the spec).