Dear Himanshu,
There is no such constraint. You can use one of your past project ideas. However, the idea has to include at least 1 TI MCU in it to be eligible for selection. Please go aheada nd register your self at the URL below:
http://www.timcucontest.com/submission.aspx
The last date for registration is January 31st, 2012. We wish yo all the best!
Warm Regards,
TI MCU Design Contest Team.