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TCA9546A: Fall time definition in different I2C version

Hideki Hayashi1
Expert 6000 points
Part Number: TCA9546A

Hi team,

It seems that 7.6 I 2C Interface Timing Requirements describes the same as I2C characteristics written in UM10204 at the specific revision.

But currently there are updates here, for example, minimum fall time is spec as 20 × (VDD / 5.5 V). Which revisions should customer satisfy?

https://www.nxp.com/docs/en/user-guide/UM10204.pdf

I saw some E2E thread mentioned minimum Tr, Tf is not so much important, but please leave a comment about a whole guideline of I2C revisions.

Best regards,

Hideki

  • 0
    TI__Genius 15345 points

    Hi Hideki-san,

    When it comes to the minimum fall time this was a standard set by NXP to mitigate EMI. It essentially ensures that you have some bus capacitance. This is not a hard rule that you have to follow and our devices will work below this minimum fall time. However, a hard rule that you do have to follow is the maximum rise and fall time of the device.

    In terms of the rise time and fall time even NXP says that they only care about the maximum rise time and fall times of their device:

    "In applications where only Fast-mode Plus parts are present, the high drive strength and tolerance for slow rise and fall times allow the use of larger bus capacitance as long as set-up, minimum LOW time and minimum HIGH time for Fast-mode Plus are all satisfied and the fall time and rise time do not exceed the 300 ns tf and 1 μs tr specifications of Standard-mode."

    Here they are saying that the minimum LOW time and minimum HIGH time matter. This is the total amount of time that clock and data signal stay high or low. For the rise and fall times it is important that they do not exceed maximum time because this will determine the maximum speed your system can run at. The minimum speed is not as important.

    The only reason that Fast mode plus has this minimum fall time rule is so that it can be backwards compatible with fast mode.

    Best,

    Chris