PID controller is not working properly in matlab simulink with TMS320F28335 microcontroller.

Iam designing buck converter in closed loop system using discrete PID controller.PID controller is not working properly in matlab simulink with TMS320F28335 microcontroller. Buck Converter output voltage should be 12V but it is coming around 9.6V. When I run the buck converter in the open-loop system, it's perfectly working fine.Now the problem is coming when I am running this system in closed-loop. when I am running the Simulink model without DSP it's working fine, I have Tuned a P and I values in discrete form.What should i do to improve my model when working in closed loop system using TMS320F28335 microcontroller.