Hello, Yasser

Yasser >>In this project I must use a DAC. I am going to use hardware PWM as DAC. I know that PIC16F877 has 10 bit PWM module. My compiler is PBP.<<

Ok, that could work.

Yasser >>My program reads an analog signal and converts it to digital. Then does some calculations.
I want to convert the above result to analog with pwm.<<

A AD would work on this. Thus choose a chip with a A/D and PWM.

Yasser >>I am waiting for your helpful guidance.<<

Ok. Why don't you send us some Psuedo code of what you want implemented. and we can go from there...

Something like

ADnumber var byte
ADin porta, ADnumber

calculations...

CCPIN1=New value between 0 and 256.


etc.

Dwayne