how to embed PI controller code in a simulink model

I am try to design a controller which will measure the output voltage, compare it with a reference value and then according to the error, it will generate appropriate PWM pulses. The controller will be Proportional Integra. I am not interested in code, rather i want to know how to embed this code in a Simulink model. The model contains simpower system components i.e. voltage sources, diodes, IGBTs, transfomers etc.