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DAC121C085 - Timing Requirement of I2C

Guru 19775 points
Other Parts Discussed in Thread: DAC121C085

Hi Team,

Please let me ask you about the timing requirement for setup and hold time for I2C.

Our customer will use 100KHz standard mode I2C.

For standard mode, I understand the minimum value is listed in the "A.C and Timing Characteristics" below. However, can you design setup and hold time specification  based on 400kHz fast mode I2C timing ?

Another question is that how and when does the device recognize standard / fast / High speed mode ?

Best Regards,
Kawai

  • Hello Kawai,

    I am not sure I understand the first art of your post: "design set-up and hold specification..." What do you mean by that?

    The device conforms to the I2C spcification as published by NXP - search for document UM10204 on the web. All the answers pertaining to the timing are there.

    As for switching between the Standard/Fast/HS modes:

    From user's point of view difference between Standard and Fast it is just a matter of SCL rate. No other action or circuit change is necessary.

    HS mode requires a pre-amble to be sent by the Master to indicate to the slave that the rest of the transaction will occur at 3.4MHz rate. This is outlined in the datasheet on pg 17, and in the above mentioned NXP document.

    Sincerely,

    tom 

  • Hi Tom-san,

    Thanks for your support and sorry for the confusion.

    I understand that DAC121C085 meets I2C standard. This may be a question about I2C specification, but, my question was that whether it still meets the I2C spec or the device could operate normally, if the device I2C timing of tHD;STA and tSU;STO is designed based on Fast mode when using at Standard mode.

    For example,
    The minimum spec for Standard mode is  tHD;STA= 4.0us (MIN).
    The minimum spec for Fast mode tHD;STA = 0.6us (MIN).

    Our customer will use in Standard mode but their tHD;STA is like 2us. Same thing for tSU;STO.

    Can you say it is okay to use in the above situation ?

    When you look at some other vendor device which supports Standard and Fast mode, the timing specification of Fast mode were only described in the electrical characteristics.

    Best Regards,
    Kawai

  • Hello Tom-san,

    Is my explanation not clear for you ?

    As our customer is waiting for the answer. Your quick reply would be greatly appreciated.

    Best Regards,
    Kawai

  • Hi Kawai-san,

    The I2C bus is defined such that the FAST slave will always work in the system with the Standard Speed Master.

    So yes, ADC121C085 will work in your custtomer's system.

    The datasheet timing reflects the I2C bus spec, and as specs go this one was evolving as well. Our device i conforms to the "tighter" specification i.e. FAST mode, which by default makes it compatible with the earlier specification.

    NOTE: I am not discussing HS-mode as it is a completely different story.

    Sincerely,

    tom

  • Hi Tom-san,

    Thank you very much for the answer !!

    Best Regards,
    Kawai

  • You are welcome Kawai.

    Good luck.

    Sincerely,

    tom