Just replace the 4047 multi vibrator with its RC connection with your Atmel
I've tested this circuit and it works it gives me 185 V RMS , but unfortanately i stoped further enhancements of this circuit but generally it is working well.
i have designed a full function ups based on 89c2051 and using it on commercial basis so i would not give codes n shematics here pl contact me in private.