15-SequentialDesign

15-SequentialDesign - Sequential circuit design :,...

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Sequential circuit design 1 Sequential circuit design Now let’s reverse the process: In  sequential circuit design , we turn some description into a  working circuit. We first make a state table or diagram to express the computation. Then we can turn that table or diagram into a sequential circuit.
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Sequential circuit design 2 Sequence recognizers sequence recognizer  is a special kind of sequential circuit that looks for a special bit pattern in  some input. The recognizer circuit has only one input, X. One bit of input is supplied on every clock cycle. For example, it would take 20 cycles to  scan a 20-bit input. This is an easy way to permit arbitrarily long input sequences. There is one output, Z, which is 1 when the desired pattern is found. Our example will detect the bit pattern “1001”: Inputs: 1 1 1 0 0 1 1 0 1 0 0 1 0 0 1 1 0 …  Outputs: 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 0 0 …  Here, one input and one output bit appear every clock cycle. This requires a sequential circuit because the circuit has to “remember” the inputs from previous  clock cycles, in order to determine whether or not a match was found.
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Sequential circuit design 3 A basic state diagram What state do we need for the sequence recognizer? We have to “remember” inputs from previous clock cycles. For example, if the previous three inputs were 100 and the current input is 1, then the  output should be 1. In general, we will have to remember occurrences of parts of the desired pattern—in this  case, 1, 10, and 100. We’ll start with a basic state diagram (edges are labeled input/output): A B C D 1/0 0/0 0/0 S t at e Me aning A None  of  t h e  d e s ir e d  pat t e r n (10 0 1)  h as  b e e n input  ye t . B W e ’ve  alr e ad y s e e n t h e  f ir s t  b it  (1)  of  t h e  d e s ir e d  pat t e r n. C W e ’ve  alr e ad y s e e n t h e  f ir s t  t wo b it s  (10 )  of  t h e  d e s ir e d  pat t e r n. D W e ’ve  alr e ad y s e e n t h e  f ir s t  t h r e e  b it s  (10 0 )  of  t h e  d e s ir e d  pat t e r n.
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Sequential circuit design 4 Step 1: Making a state table The first thing you have to figure out is precisely how the use of state will help you solve the  given problem. Make a state table based on the problem statement. The table should show the present  states, inputs, next states and outputs.
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This note was uploaded on 12/10/2011 for the course ECE 200 taught by Professor Nasis during the Spring '08 term at Drexel.

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15-SequentialDesign - Sequential circuit design :,...

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