Preheat oven to 425 degrees F (220 degrees C). Place one crust in a nine inch pie pan.
Mix together sugar, flour, and cinnamon. Mix lightly through the berries. Pour filling into pastry lined pan, and dot fruit with butter or margarine. Cover with top crust, and cut slits in the top. Seal and flute the edges.
Bake for 35 to 45 minutes, or until the crust is slightly browned.