Main Home and Rental Income

We lived in our home for about half of last year then rented it out for the other half. When I go through the prompts in TurboTax, it ends up showing we are deducting more than the total mortgage interest paid and less than the total real estate taxes paid. That's not right, is it? What are the "rules" on deducting mortgage interest on a rental that was also your primary home during a given year?