combinational pld-based state machines

COMBINATIONAL PLD-BASED STATE MACHINES

The combinational logic requires for the realization of a state machines can be done using a ROM,PLA or PAL.

Such systems have a capability of changing the state machines output by changing the program of PLD.

The state machines realized using PLDs will require less number of chips.


In some applications, the output sequence of state machines need modification. This requires a change in IFL so as to control a new sequence. We may use PLDs for implementing IFLs in such cases.

It is possible to program a PLD in order to create the correct signals which drive the state flip flops.


If the sequence of state machine is to be changed then it can be achieved simply by replacing the techniques would not be easy.

This concept of changeable ROM cartiges has become very popular for state machines as well as microprocessor control.



PLD-(Programmable Logic Device)

• PLD are special type of ICs which can be programmed by the users as per their requirements.

• Therefore it is possible to implement a combinational or sequential circuit using the PLD Ics.


PLA-(Programmable Logic Array) There are two types of PLDs namely the PLA

i.e. Programmable logic arrays and PAL i.e. The programmable array logic .Due to the use of AND matrix followed by OR matrix, we can use them for the implementation of logic function in the SOP form.

The output are connected to the device pins through inverting buffers and flip flops.

PAL-(Programmable Array Logic)

• PAL is the most commonly used type of PLD. It is a programmable array of logic gates.

• The special feature of PAL is that it has a programmable AND array and a fixed OR array.


ROM-(Read Only Memory) can hold data permanently. It can be programmes only once by the

manufacturer as per the requirements of the user.

◦ Originally not thought of as a programmable device at all – simply a memory for holding machine specific information, such as the control store operation.


The IFL can be changed very quickly.

The same EPROM can be erased and reprogrammed any number of times.

It is possible to debug and modify the system under software control.

COMBINATIONAL PLD-BASED STATE MACHINES ADVANTAGES OF ‘ROM’

Values stored in registers represent the state of the circuit

Combinational logic computes:◦ Next state

Function of current state and inputs◦ Outputs

Function of current state and inputs (Mealy machine) Function of current state only (Moore machine)

State Machine Model

Inputs

Outputs

Next State

Current State

outputlogic

next statelogic

States: S1, S2, ..., Sk Inputs: I1, I2, ..., Im Outputs: O1, O2, ..., On Transition function: Fs(Si, Ij) Output function: Fo(Si) or Fo(Si, Ij)

State Machine Model

Inputs

Outputs

Next State

Current State

outputlogic

next statelogic

Clock

Next State

State

0 1 2 3 4 5

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combinational pld-based state machines

Engineering