Counters.pptx,,,,,......................

COUNTERS

1. Counters Two types: Asynchronous – no common clock Synchronous – master clock used

2. Asynchronous Counters Subsequent flip flop get clock from preceding one also called ripple counter (the clocking/trigger signal ripples though the FF) Can be constructed to do natural counting Can count up/down (read Q/inverted-Q o/p); while up counting is happening on Q o/p, down counting is on inverted Q o/p Can count up to a certain value Use digital logic circuitry to curtail the counting at desired value Decade counter can go up to ten

3. Operation of Asynchronous Counters The external clock feeds only the first flip flop All FFs in toggle mode Therefore at any clock transition, FF o/p toggles The output (inverted Q in this case) acts as clock for next FF The clock signal “ripples” through the counter

4. Disadvantages of Asynchronous Counters They are slow – the output awaits the They create glitches which result in functional hazards

5. Glitches on Asynchronous Counters For the 3-bit counter consider the transition 011 to 100 due to the flip flop propagation delay , each flip flop trigger is delayed the ultimate effect is the transition will be: 011 010 000 100 this is illustrated on next slight

6. Glitches.....cont’d The transient states would generate undesired o/p in any cct that would have to decode the state of the counter Asynchronous counters are only good used as frequency dividers For other purposes, synchronous counters should be used

7. Synchronous Counters Master (external) clock used on each individual FF Consists of : counter memory combinational cct to generate next state For N bit counter D Flip flop; next state generator has N i /p & N o/p JK flip flop; next state generator has N i /p + 2N o/p

8. Design using D-type flip flop Construct present/next state table Formulate each next state as a ftn of present state bits Simplify the equations Construct the circuit

9. Example Design a 3 bit counter using D-type FF 1. Present/Next State table

10. Example... 2. Formulate next state equations...and simplify using Karnaugh maps

11. Example.... 3. Construct final cct

12. Exercise Design the same counter, now using J-K flip flops (check Floyd for explanation)