Hi,
I can't see how you want a stepper motor to move.
You just control two outputs: pc0 and pc3.
If one channel is higher, them pc0 is ON and pc3 is OFF.
When the other is higher, the, pc0 is OFF and pc3 is ON.
If both are the same, then it repeats the previous state.
But with this you can't control a stepper. Read on how to control stepper motors.
You could connect pc0 and pc3 to two halfbridges and drive ad dc motor. But with your program the motor never stops...
Klaus