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Counters and Registers

Counters and Registers. eng.almeshal@gmail.com. 7-4 Asynchronous (Ripple) down counter. 2 3 =8 ---> MOD-8 Down Counter. 7-5 Propagation Delay in Ripple Counters. Ripple counters are the simplest type of counters.

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Counters and Registers

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  1. Counters and Registers eng.almeshal@gmail.com

  2. 7-4 Asynchronous (Ripple) down counter 23=8 ---> MOD-8 Down Counter

  3. 7-5 Propagation Delay in Ripple Counters • Ripple counters are the simplest type of counters. • Although they are simple, they have a major drawback which is the propagation delay caused by their operation principle.

  4. 7-5 Propagation Delay in Ripple Counters

  5. 7-5 Propagation Delay in Ripple Counters

  6. 7-5 Propagation Delay in Ripple Counters • To avoid this problem, we should make sure of the following: • N = number of FFs • Tpd = propagation delay time

  7. Example • A 4-bit Ripple counter with tpHL= 16 nS and tpLH= 24 nS, using 74LS112 J-K FF. find the maximum frequency for a proper operation of the counter. Assume a worst case scenario • For 6-bit Ripple counter that has 6 FFs.

  8. 7-6 SYNCHRONOUS (PARALLEL) COUNTERS • The synchronous counters have all of the FF’s triggered simultaneously. That is, all the CLK inputs are connected together • Synchronous counters require more circuitry than the asynchronous counters. • Synchronous counters needs extra logic gates to be added.

  9. 7-6 SYNCHRONOUS (PARALLEL) COUNTERS Each FF should have its j and k inputs connected such that they are HIGH only when the outputs of ALL lower-order FFs are in the HIGH state

  10. 7-6 SYNCHRONOUS (PARALLEL) COUNTERS • Each FF is clocked by the NGT of the clock input signal so that all the FF transitions occur at the same time. • Only A has its J-K inputs permanently at the HIGH level. • A changes at each NGT clock (A FF toggles) • B changes when A=1 and a NGT clock occurs • C changes when A=B=1 and a NGT clock occurs • D changes when A=B=C=1 and a NGT clock occurs

  11. Advantage of synchronous Counters over Asynchronous • In a parallel counter, all FF will change simultaneously. • Propagation delay of FF do not add together to produce the overall delay. Total delay = FF tpd + AND tpd Fmax = FF Fmax+AND Fmax • The total delay is the same no matter how many FF are used. • A synchronous counter can operate at much higher frequency, but the circuitry is more complex than that of the asynchronous counter.

  12. Example • Determine fmax for the synchronous MOD-16 counter if tpd=50ns for each FF and tpd=20ns for each AND gate. • Determine the equivalent fmaxfor asynchronous MOD-16 counter

  13. Unchanged Example • What must be done to convert this counter to MOD-32 parallel counter? • A Fifth Flip-Flop must be added “25=32” • Determine fmax for the MOD-32 parallel (synchronous) counter • For a MOD-32 ripple counter

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