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Introduction to Sequential Circuits

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Clock period, ON, and OFF periods. Propagation delay ... In clocked SR latch, outputs respond at specific instances. Uses a clock signal. Latches (cont. ... – PowerPoint PPT presentation

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Title: Introduction to Sequential Circuits


1
Introduction to Sequential Circuits
  • Output depends on current as well as past inputs
  • Depends on the history
  • Have memory property
  • Sequential circuit consists of
  • Combinational circuit
  • Feedback circuit
  • Past input is encoded into a set of state
    variables
  • Uses feedback (to feed the state variables)
  • Simple feedback
  • Uses flip flops

2
Introduction (cont.)
  • Main components of a sequential circuit

3
Clock Signal
4
Clock Signal (cont.)
  • Clock serves two distinct purposes
  • Synchronization point
  • Start of a cycle
  • End of a cycle
  • Intermediate point at which the clock signal
    changes levels
  • Timing information
  • Clock period, ON, and OFF periods
  • Propagation delay
  • Time required for the output to react to changes
    in the inputs

5
Clock Signal (cont.)
6
Latches
  • Can remember a bit
  • Level-sensitive (not edge-sensitive)
  • A NOR gate implementation of SR latch

7
Latches (cont.)
  • SR latch outputs follow inputs
  • In clocked SR latch, outputs respond at specific
    instances
  • Uses a clock signal

8
Latches (cont.)
  • D Latch
  • Avoids the SR 11 state

9
Flip-Flops
  • Edge-sensitive devices
  • Changes occur either at positive or negative
    edges
  • Positive edge-triggered D flip-flop

10
Flip-Flops (cont.)
  • Notation
  • Not strictly followed in the literature
  • We follow the following notation for latches and
    flip-flops

Latches
Flip-flops
Low level High level
Positive edge Negative edge
11
Flip-Flops (cont.)
12
Example Sequential Circuits (cont.)
74164 shift Register chip
13
Memory Design with D Flip Flops
Require separate data in and out lines
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