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Step 1 of 1 8.030E The circuit of the 4-bit ripple down counter using four D flip-flops is shown in Figure 1. D D D D Figure 1 As we cascade more and more flip-flop to form higher modulus counters, the cumulative effect of the propagation delay becomes more. The approximate maximum frequency of a ripple counter due to the accumulation of propagation delays can be determined by using the following formula. Here, N is the number of flip-flops, is the average propagation delay of each flip-flop fmax is the maximum counting speed or frequency. Propagation delay for 74HCT D flip-flop: Since, the propagation delay for 74HCT D flip-flop is 16 The number of flip flops needed to design 4- bit ripple down counter are 4. Calculate the maximum counting speed (frequency) for 74HCT. = Therefore, the maximum counting speed (frequency) for 74HCT is 15.625 MHz Propagation delay for 74AHCT D flip-flop: Since, the propagation delay for 74HCT D flip-flop is 5 The number of flip flops needed to design 4-bit ripple down counter are 4. Calculate the maximum counting speed (frequency) for 74AHCT. Therefore, the maximum counting speed (frequency) for 74AHCT is 50 MHz Propagation delay for 74LS74 D flip-flop: Since, the propagation delay for 74LS74 D flip-flop is 40 ns The number of flip flops needed to design 4- bit ripple down counter are 4. Calculate the maximum counting speed (frequency) for 74LS74. Therefore, the maximum counting speed (frequency) for 74LS74 is 6.25 MHz