Dear Gaston,
I am not a Simulink user, so I can't give you direct help. However, our
product, VisSim, can read in Simulink diagrams directly and supports both MSP430
and F28xx I2C with no problem. We have used VisSim to generate MSP430 code with
I2C comms, and also generate the F28xx side with I2C comms and it works well.
You can download a free trial of VisSim from www.vissim.com/downloads/demos.html
and look at the example that reads and writes EEPROM on the F28335 eZdsp. The
example is under VisSim/DSP > Examples > F283x > eepromReadWriteF28335.vsm
You can generate code and compare it to Simulink to see what might be the
difference, or you can import the Simulink diagram to VisSim and try your
algorithm in VisSim.