Exam 2 review #

Sequential circuit small design #

• All small and large sequential circuits are made of flip flops and sets of combination circuits
• Contrary to CC (combination circuits), a sequential circuit design has states and transitions from a current state to the next state
• A sequential circuit design problem is typically modeled as a finite state diagram
• FSD consists of circles as states and arcs (arrows) as transitions, which specifies the behavior of a sequential circuit
• FSD is systematically converted into circuit called finite state machine
• Finite state machines designs categorized into Mealy, Moore, or hybrid
• Mealy is a FSM whose output values are determined by its current e state and input.
• Moore machine whose output values are determined solely by its current inputs.

Moore machines

• Output depends only upon present state
• If input changes output does not change
• More number of states required
• There is more hardware requirement
• React slower to inputs (one clock cycle later)
• Synchronous output and state generation
• Output is placed on states
• Easy to design

Mealy machine

• Output depends on present state as well as present input
• If input changes, output also changes
• Less number of states
• Less hardware
• React faster to inputs
• Asynchronous output generation
• Output is placed on transitions
• Difficult to design

Reviewing the design process of a sequential circuit #

Example: design of a Moore FSM that detects overlapping sequence “101”.

Step 1: design the FSD

Step 2: Determine the minimum number of bits required to store the states

\( \text{number of bits } = \log_2 \lceil k \rceil \) , where \( k = \text{ number of states} \)

So we need 2 flip flops to represent the states because \( \log_2 \lceil 4 \rceil = 2 \)

Step 3: From the FSD, create the truth table for the next state generator (NSG) and the output generator (OG)

Step 5: built the circuit

Large design review #