4-bit Asynchronous Counter (Propagation Delay Problem)
Given Data
- Number of flip-flops (n) = 4
- Propagation delay of each D flip-flop = 10 ns
- Counter type = Asynchronous (Ripple) counter
Step 1: Understand the Concept
In an asynchronous counter, each flip-flop is triggered one after another. So, propagation delays are added cumulatively.
Step 2: Total Propagation Delay
Total delay = number of flip-flops × propagation delay
t_total = n × t_pd
t_total = 4 × 10 ns = 40 ns
Step 3: Minimum Clock Period
To avoid errors, clock period must be at least equal to total propagation delay:
T_min = 40 ns
Step 4: Maximum Frequency Calculation
Frequency formula:
f_max = 1 / T_min
f_max = 1 / (40 × 10⁻⁹)
f_max = 25 × 10⁶ Hz
Final Answer
Maximum allowed frequency = 25 MHz
Important Note
- Ripple counters suffer from cumulative delay
- More flip-flops → more delay → lower speed
- Synchronous counters are faster because all flip-flops are clocked together