LECTURE 6

 

PROGRAMMABLE LOGIC DEVICES (PLDS). INTRODUCTION TO MICRO-PROCESSORS AND MICRO-CONTROLLERS

 

6.1. INTRODUCTION

 

Programmable Logic Devices (PLD) are programmable systems and are generally used in manufacturing automation  to perform different control functions, according to the programs written in its memory, using low level languages of commands. There are following three types of PLDs are being employed in mechatronics systems:

1. Microprocessor (it is a digital integrated circuit which carries out necessary digital functions to process the information obtained from measurement system);

2. Microcomputer (it uses microprocessor as its central processing unit and contains all functions of a computer);

3. Programmable Logic Controller (PLC). It is used to control the operations of electro-mechanical devices especially in tough and hazardous industrial environments.

 

 

Fig. 6.1.1. Components of a programmable logic device

A typical programmable machine has basic three components as shown in Figure 6.1.1:

1. Processor, which processes the information collected from measurement system and takes logical decisions based on the information. Then it sends this information to actuators or output devices.

2. Memory, it stores:

a.            the input data collected from sensors

b.            the programs to process the information and to take necessary decisions or actions. Program is a set of instructions written for the processor to perform a task. A group of programs is called software

3. Input/output devices: these are used to communicate with the outside world/operator.

 

Microprocessor

It is a multi-purpose, programmable device that reads binary instructions from a storage device called memory, processes the data according to the instructions, and then provides results as output. In common practice it is also known as CPU (central processing unit). CPU can be referred as complete computational engine on a single chip. First Microcontroller, Intel 4004 was launched in 1971. It was able to process just 4 bits. It started a new era in electronics engineering. Microprocessor chip was one of the important inventions of the 20^th century.

Applications of microprocessors are classified primarily in two categories:

1.  Reprogrammable Systems : Micro computers;

2.  Embedded Systems : photocopying machine, Digital camera.

Microprocessor works or operates in binary digits i.e. 0 and 1, bits. These bits are nothing but electrical voltages in the machine, generally 0 – low voltage level, and 1 -high voltage level. A group of bits form a ‘word’. In general, the word length is about 8 bits. This is called as a «byte». A word with a length of 4 bits is called as a «Nibble» Microprocessor processes the «commands in binary form» to accomplish a task. These are called as «instructions». Instructions are generally entered through input devices and can be stored in a storage device called memory.

 

 

Fig. 6.1.2. Schematic of configuration of a micro processor

Figure 6.1.2 and 6.1.3 show the configuration and basic blocks of a microprocessor. The functions of each element are as follows.

 

 

Fig. 6.1.3. Working of a microprocessor

 

1.  ALU: ALU stands for Arithmetical Logical Unit. As name indicates it has two parts:

a.  Arithmetical unit which is responsible for mathematical operations like addition, subtraction, multiplication and division;

b.  Logical unit which is dedicated to take logical decisions like greater than, less than, equal to, not equal to etc. (Basically AND/OR/NOT Operations).

2.  Register Array: Registers are small storage devices that are available to CPU or processors. They act as temporary storage for processing of intermediate data by mathematical or logical operations.

3.  Control: This part of CPU is dedicated to coordinate data flow and signal flow through various types of buses i.e. Data Bus, Control Bus, and Address Bus etc. It directs data flow between CPU and storage and I/O devices.

4.  Memory: There are two different types of memory segments being used by the CPU. First is the ROM which stands for Read Only Memory while other is R/W which stands for Read and Write Memory or Random Access Memory (RAM).

a)  ROM: From this memory unit, CPU can only read the stored data. No writing operations can be done in this part of memory. Thus it is used to store the programs that need no alteration or changes like Monitor Program or Keyboard driver etc.

b)  R/W: As name indicates it is opposite to ROM and used for both reading and writing operations. In general User’s program and instruction are stored in this segment of memory unit.

5.  Input Devices: Input devices are used to enter input data to microprocessor from Keyboard or from ADC which receives data from sensors/signal conditioning systems.

6.  Output Devices: These devices display the results/conclusions coming out from ALUs either in soft copy (Monitor) or in Hard Copy (Printer).

 

6.2. FUNCTIONS OF MICROPROCESSOR

 

Various functions of microprocessor are as follows:

·          Microprocessor performs a variety of logical and mathematical operations using its ALU;

·          It controls data flow in a system and hence can transfer data from one location to another based on the instructions given to it;

·          A microprocessor can take necessary decisions and jump to a new set of instructions based on those decisions.

 

Elements of microprocessor

A simple microprocessor consists of following basic elements (see Figure 6.1.3):

·          Data Bus;

·          Through data bus, the data flow between various storage units;

·          ALU and memory units;

·          Address Bus: It controls the flow of memory addresses between ALU and memory unit;

·          RD (read) and WR (write) lines set or obtain the addressed locations in the memory;

·          Clock line transfers the clock pulse sequence to the processor;

·          Reset Line is used to restart execution and reset the processor to zero;

·          Address Latch is a register which stores the addresses in the memory;

·          Program Counter: It is a register which can increment its value by 1 and keeps the record of number of instructions executed. It can be set to zero when instructed;

·          Test Register: It is a register which stores intermediate or in-process data of ALU operations. For example it is required to hold the «carry» while ALU is performing «addition» operation. It also stores the data which can be accessed by Instruction decoder to make any decision;

·          3-State Buffers: These are tri-state buffers. A tri-state buffer can go to a third state in addition to the states of 1 and 0;

·          The instruction register and instruction decoder are responsible for controlling the operations of all other components of a microprocessor.

There are following control lines present in a microprocessor, which are used to communicate instructions and data with the instruction decoder:

·          Instruct the A register to latch the value currently on the data bus;

·          Instruct the B register to latch the value currently on the data bus;

·          Instruct the C register to latch the value currently output by the ALU;

·          Instruct the program counter register to latch the value currently on the data bus;

·          Instruct the address register to latch the value currently on the data bus;

·          Instruct the instruction register to latch the value currently on the data bus;

·          Instruct the program counter to increment;

·          Instruct the program counter to reset to zero;

·          Activate any of the six tri-state buffers (six separate lines);

·          Instruct the ALU what operation to perform;

·          Instruct the test register to latch the ALU's test bits;

·          Activate the RD line;

·          Activate the WR line.