Foreclosing on a carryback balloon mortgage

We sold a vacant piece of land for $100,000. Buyer put 50,000 down and we carried back the rest for 12 months at buyers request. The buyer now cannot pay as the balloon final payment was due 3/28/14. We have given a month to come up with the cash, but has failed to do so. Do I have to wait 90 days from 3/28/14 before filling the notice of default?