I have one questoin about I2C standard.

Ho Cheol Song1

Dear TIer,

I have one question about I2C standard. I2C standrad has the below information for the fall time of SDA and SCL at FAST-Mode(400Khz).

According to the below, the minimum value of fall time is 20ns (ignore capacitance). But in many kinds of real I2C application, actual fall time is under 10ns.

Is it out of spec? Is it right for me to understand it? Please let me know your advise.

Thanks & Regards.

over 11 years ago

0 Ho Cheol Song1 over 11 years ago

Prodigy 120 points

Is there anybody who can reply my question?

0 Ho Cheol Song1 over 11 years ago in reply to Ho Cheol Song1

Prodigy 120 points

Is there any response?

0 Chris Kraft over 11 years ago in reply to Ho Cheol Song1

TI__Intellectual 2020 points

Hi Ho,

I apologize, I had typed up an answer to this question I must have forgot to hit post.

The I2C spec has changed since that datasheet was written, the spec for fall time is now 20 * (VDD / 5.5).

There is normally some level of slope control included on the open drain pull down on I2C devices but you are correct if for some reason the device pulls faster than the defined fall time the device will be out of I2C spec. Keep in mind that this does not mean that the device would not function, even if the fall time is faster than the spec as long as the timing parameters (data valid time, setup time, etc) are still followed, the correct data will be transferred.

Regards,
Chris Kraft

0 Kevin%20Gingerich over 11 years ago in reply to Chris Kraft

Prodigy 20 points

Chris,

I would say a non-compliant output transition time MAY function but cannot be assured in all cases. Otherwise, its specification is superfluous. A minimum transition time is usually imposed for electromagnetic-interference or transmission-line considerations.

Regards,

/Kevin/

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I have one questoin about I2C standard.