I have generated a simple square wave frequency generator with 0-5v amplitude using uC, but i need the amplitude range from 0.8v-to-5v how can i do that?
It depends on what your load is. The simplest solution is to add a small value resistor in series with your square wave and at its out end, add another resistor to pull the level toward 5V slightly. The voltage can't go above 5V but you can control the low level part by adjusting the resistor values.