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TCA6408A-Q1: Inquiries about Timing Spec

MJ Jeon
Genius 5240 points
Part Number: TCA6408A-Q1

Hi, TI Support Team

My client is measuring timing for the TCA6408A-Q1.

I have a question about I2C Interface Timing described in the datasheet.

There is a Min. value for Ticf in Fast Mode.

Of course, Min. values exist for other values as well.

Is the Min. value written in Fast Mode a spec that must be observed?

The Min. value of Ticf is calculated to be about 6ns, but the timing seen through the oscilloscope seems to be ps.

Please check whether the Min. value is a part that must be observed.

Min. value exists even in standard mode, but what I am curious about is the part about Ticf.

Could you please check if the attached waveform is ok?

As you can see from the waveform, the falling edge drops sharply.

The timing at this time is difficult to measure, but it is expected to be ps.

Thanks.

Regards,

MJ

  • 0
    TI__Guru 57810 points

    Hi MJ,

    The table you mention is from the I2C standard and discusses the I2C system level specs. 

    Fall time is kind of a funny subject, its something that most customers care less about and violating it usually isn't going to break the I2C bus/devices on the bus.

    Our TCA6408A-Q1 would still work even if you violated the fall time. I believe the min fall time was included in the spec due to Schmitt triggers used in the I2C inputs which could become unstable if the fall time was too fast but this isn't a problem anymore nowadays.

    If you wanted to try to correct the fall time, you could try to put a small series resistor infront of the device that is driving the I2C line since fall time is a product of the open drain's pull down FET's Ron and the bus cap load. If you make the RC constant larger by putting a series resistor infront of the driver, then the fall time will increase as a result. Keep in mind this would also affect the VoL of the driver though.

    -Bobby