DISCUSSION CSE 140L 3 rd November 2010. Vikram Murali. Things we will cover. Finite State Machines -- Mealy and Moore models. -- State Encoding. -- Simple Problem. VHDL Basics -- The Structure -- What we need !. FSM – Very Briefly.
Finite State Machines
-- Mealy and Moore models.
-- State Encoding.
-- Simple Problem.
-- The Structure
-- What we need !
Mathematical abstraction used to design digital logic
Behavior model composed of a finite number of states, transitions between those states, and actions, similar to a flow graph.
Actually realising a problem statement or modelling it in digital logic.
Logic Flows when certain conditions are met.
States – Represent a state of the circuit (or the problem statement modelled). Characterised by the inputs that caused it to happen and the outputs it results in.
Inputs – Decide the flow between states.
Outputs 1 when sequence is 1011 ! Seq. Detector
Mealy model : Output depends on both the current state and the input.
Moore : It depends only on the current state.
Imagine : In a Moore machine, the output is associated with the state. In a mealy machine, the output is associated with the transitions.
Typically, Moore has more states, but is easier to implement. Mealy machines have less states, and also is fasterin clocked systems (the output occurs right away when the inputs are detected). Moore has to wait the clock cycle to transition to the next state before the output is changed. => Mealy can give rise to race conditions. !!
States named as S0 , S1 … Sn are fine with us.
But implementation in Digital Logic ?
Each state must be encoded in 0s and 1s
Each bit has a certain state transition logic
-- We need to derive this to design our circuit
that implements our model/requirement
Eg : 8 states => 2^3 = 8 => 3 bits to encode.
111 DOWNTO 000
Major parts of a VHDL program
-- Entity Declaration
-- Architecture Body
-- Package Declaration
-- Package Body
States in VHDL
-- Building FSMs
-- State Transitions and Outputs