A sequence generator is a sequential circuit which generates a prescribed sequence at its output. The output sequence is in synchronication with the clock input. It is possible to design a sequence generator using counters or using the shift registers. Sequence Generator using Shift Register : The sequence generator is a circuit which generates a desired sequence bf bits at its output in synchronization with the clock. Some of the applications of the sequence generator are as follows :
Figure shows the basic structure of a sequence generator.
The outputs of an N bit shift register (Q0 through QNâ€"1) are applied as inputs to a combinational circuit called â€œNext state decoderâ€. And the output (Y) of the next state decoder is applied to the serial input of the shift register. The â€œNext state decoderâ€ is designed according to the required sequence. Sequence Generator using Counters : â€¢ The general block diagram of a sequence generator using counter is shown in Figure below.
The next state decoder is a combinational circuit. The input to it are obtained from the flip-flop outputs and its outputs are applied to the inputs of the flip-flops. Deciding the number of flip-flops : We will be given the required sequence of 1â€™s and 0â€™s that is to be generated such as 1011011 â€¦.
Based on the given sequence we have to decide the number of flip-flops as follows. 1. Count the number of 1â€™s and 0â€™s in the given sequence. 2. Choose the higher number of the two. Let this number be N. 3. The number of FFs (n) is then calculated as, N ï‚£ 2n â€" 1Let us apply this to the given sequence, i.e. 1011011 â€¦ Number of 1â€™s = 5, number of 0â€™s = 2. So select higher one of them i.e. 5. So N = 5. But N = 2n â€" 1 Hence 5 ï‚£ 2n â€" 1 ïœ n = 4 â€¦ so four FFs will be required.