hi,
I have generated different PWM thru 8051 controller for the range of frequncies in 20 Hz to 200Hz

and
The requirment is that the user can regulate the voltage from 0 to 120V using keypad.
The steps should be 1 Volts.

Maybe I am totally wrong but it is not possible. Why?
Via triac you may regulate only Power, not Voltage of AC. To regulate Voltage of AC you need with transformer. Triac may be only opened/closed and so what you may do is only regulation of the time while current goes through it.
Indeed for DC it is possible to regulate Voltage with usage of PWM with triac and inductiver or capacitor as power accumulator (even with such scheme, you still need with loop-back to control real output voltage).
As about AC so drop this idea. I do not see a way how your scheme may do it with simple PWM and triac. What you may obtain is only the regulation of Max. peak value of AC using look-up tables. But in this case it will not produce original sine wave output.

Regards,
Oleg