VIDEO TUTORIALS: The 4-Bit JK Flipflop Counter
Modulo16 means the counter goes through sixteen different states and so can count from 0 to 15. For each bit we use one JK flipflop and one AND gate. The reason this counter works is because JK flipflops toggle when both inputs are asserted (i.e. J=1, K=1). So, for example, on every cycle of the clock the least significant bit always toggles. The reason the other bits dont toggle has to do with the AND gates that are in the way. The output of an AND gate is asserted only when all the inputs are asserted. As a consequence for the counter, a bit will only toggle when all the less significant bits that precede it are asserted.