When calculating transfer rate, please be careful of the following points.
1) Calculation of Tr and Tf values
The values the Tr and Tf in the formula for transfer rate shown in the user's manual: hardware need to be recalculated using actual measurements of the SCL clock.
Tr is the time until VIH (VCC x 0.7). Tf is the time from the pull-up voltage of the IIC bus to VIL (VCC x 0.3).
Remeasure the above values and substitute them into the formula to calculate transfer rate.
2) Digital noise filter circuit
The formula for transfer rate shown in the user's manual: hardware applies when the digital noise filter circuit is disabled.
If the digital noise filter is enabled, a delay equivalent to the number of noise filter stages is added to the low width and high width of the SCL clock.
Therefore, for example if the number of noise filter stages is four, you need to set the setting values of the CBRL and ICBRH registers from which four stages' worth have been subtracted.