I2C Fast Mode Rise/Fall Time Lower Limit

MiKK — Tue, 16 Sep 2025 09:27:59 GMT

The I2C standard (UM10204) specifies a lower limit for the rise/fall time in fast mode.
Is this specified to manage undershoot and prevent EMS?
If these are not issues, is it not necessary to adhere to the lower limit?
※This is the opinion of another manufacturer that handles I2C.

Re: I2C Fast Mode Rise/Fall Time Lower Limit

TomasVaverka — Tue, 16 Sep 2025 10:25:19 GMT

Hi Keitaro,

The lower limit is defined not only to manage electromagnetic susceptibility and undershoot, but also to ensure robust signal integrity and interoperability across I²C-compliant devices.

Here are the key reasons for this specification:

- Signal Integrity: Excessively fast transitions can cause reflections, ringing, and timing violations, especially in systems with longer traces or higher capacitance. The lower limit helps maintain stable communication.

- EMS and Undershoot: Fast edges increase high-frequency emissions and can lead to undershoot due to inductive effects. The timing limits help mitigate these risks.

- Device Compatibility: The I²C bus is designed for multi-device, multi-vendor environments. Adhering to timing specifications ensures reliable operation across all compliant devices.

While it may be technically feasible to operate outside the lower limit in controlled environments where EMS and undershoot are not concerns, doing so would mean deviating from the I²C standard. This could compromise compatibility and long-term reliability.

In summary, even if EMS and undershoot are not immediate issues, I recommend adhering to the specified timing limits to maintain compliance and ensure consistent performance.

BRs, Tomas

Re: I2C Fast Mode Rise/Fall Time Lower Limit

MiKK — Wed, 24 Sep 2025 09:55:24 GMT

Hi Tomas,

Thank you for your quick response.
This is very helpful.

However, if that's the case, why is there no minimum limit on rise time in Fast-mode Plus, even though it is even faster?

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