How to design a synchronous counter that counts the sequence 0, 2, 4, 6, 8, 10, 12, 11, 9, 7, 5, 3, 1, 0,2,… and uses JK, SR, T and D flip-flop for each bit - Quora
Asynchronous 3-bit up down counter| Electronics Engineering Study Center
4 bits Synchronous Counter with J K Flip Flop - YouSpice