GATE 2015 MOD - 5 Asynchronous Counter using JK flip flops - YouTube
Design a Synchronous Counter Using D Flip Flops - YouTube
Counter Circuits
How to design a synchronous counter 4 bit using JK flip flop that can count up even numbers from 0 to 14, and count down odd numbers from 15 to 0 in 1 system - Quora