Introduction to Embedded

Systems,8051 Deepak John

SJCET-Pala

Embedded system An embedded system is a special-purpose computer system designed to perform a dedicated

function.

Applications

1. Biomedical Instrumentation – ECG Recorder, Blood cell recorder, patient monitor system.

2. Communication systems – pagers, cellular phones, cable TV terminals, fax and

transreceivers, video games and so on.

3. Peripheral controllers of a computer – Keyboard controller, DRAM controller, DMA

controller, Printer controller, LAN controller, disk drive controller.

4. Industrial Instrumentation – Process controller, DC motor controller, robotic systems,

CNC machine controller, close loop engine controller, industrial moisture recorder cum

controller.

5. Scientific – digital storage system, CRT display controller, spectrum analyser.

Architecture

Layered architecture of an Embedded System

Hardware architecture of an embedded system

Functional Blocks

Central Processing Unit (CPU)

Memory (Read only memory and Random

access memory)

Input Devices

Output Devices

Communication interfaces

Application specific circuitry

Classification of Embedded Systems

Based on functionality and performance requirements, embedded systems are classified as :

i. Stand-alone Embedded Systems

ii. Real-time Embedded Systems

iii. Networked Information Appliances

iv. Mobile Devices

i. Stand Alone Embedded Systems

work in stand-alone mode.

They take inputs, process them and produce the desired output.

Embedded systems used in process control, automobiles, consumer electronic items etc. fall

into this category.

ii. Real-time Embedded Systems

Embedded systems in which some specific work has to be done in a specific time period are

called real-time systems.

Two types Hard and Soft

iii. Networked Information Appliances

Embedded systems that are provided with network interfaces and accessed by networks such

as Local Area Network or the Internet are called networked information appliances.

iv. Mobile Devices

Mobile devices such as mobile phones, Personal Digital Assistants (PDAs), smart phones etc.

are a special category of embedded systems

Microcontrollers

Microcontrollers are small computing systems on a single chip.

A microcontroller will also be referred to as an MCU.

Central Processing Unit (CPU)

Program memory

Random Access Memory (RAM)

EEPROM - Electrically Erasable Programmable Read Only Memory

USARTs, Timer/Counters, ADC, DAC, I/O Ports, CANs, SPIs, etc.

Serial

Port

CPU RAM ROM

I/O

Port Timer A single chip

Microprocessor vs Microcontroller

Microprocessor

CPU is stand-alone, RAM, ROM, I/O,

timer are separate

designer can decide on the amount of

ROM, RAM and I/O ports.

Expansive

general-purpose

Microcontroller

CPU, RAM, ROM, I/O and timer are all on

a single chip.

fix amount of on-chip ROM, RAM, I/O ports

for applications in which cost, power and

space are critical.

single-purpose

Applications of microcontrollers

Automobile

Aeronautics

Space

Rail Transport

Mobile communications

Industrial processing

Remote sensing , Radio and Networking

Robotics

Consumer electronics , music players, Computer applications

Security (e-commerce, smart cards)

Medical electronics (hospital equipment, and mobile monitoring) and

Defense application

8051 Features

8-bit CPU

16-bit Program Counter

8-bit Processor Status Word (PSW)

8-bit Stack Pointer

Internal RAM of 128bytes

Special Function Registers (SFRs) of 128 bytes

32 I/O pins arranged as four 8-bit ports (P0 - P3)

Two 16-bit timer/counters : T0 and T1

Two external and three internal vectored interrupts

One full duplex serial I/O

Block Diagram

CPU

On-chip

RAM

On-chip

ROM for

program

code

4 I/O Ports

Timer 0

Serial

Port OSC

Interrupt

Control

External interrupts

Timer 1

Timer/Counter

Bus

Control

TxD RxD P0 P1 P2 P3

Address/Data

Counter

Inputs

Oscillator Circuit

The 8051 requires an external oscillator circuit.

The oscillator circuit usually runs around 12MHz.

The crystal generates 12M pulses in one second.

The pulse is used to synchronize the system operation in a controlled pace.

An 8051 machine cycle consists of 12 crystal pulses (clock cycle).

Pins XTAL1 & XTAL2 have been used.

Internal Memory

8051 implements a separate memory space for programs (code) and data.

Internal memory consists of on-chip ROM and on-chip data RAM.

On-chip RAM contains general purpose storage, bit addressable storage, register banks, and

special function registers.

Internal RAM

8051 CPU Registers

Register banks in the 8051 Microcontroller

Special Function Registers

8051 has 21 special function registers (SFRs) at the top of internal RAM from address 80H

to FFH.

Some SFR’s are both bit-addressable and byte addressable, depending on the instruction

accessing the register.

All 8051 CPU registers, I/O ports, timers and other architecture components are accessible

in 8051 C through SFRs

PSW (Program Status word) / Flag Register

I/O Ports

four bidirectional I/O ports. Each port has an 8-bit latch in the SFR space

To reduce the overall package pin count, the 8051 employs multiple functions for each port.

Each port also has an output drive and an input buffer.

These ports can be used to general purpose I/O, as an address and data lines.

The four 8-bit I/O ports P0, P1, P2 and P3 each uses 8 pins

Port 0

is 8-bit bidirectional I/O port.

is also the multiplexed low-order address and data bus during accesses to external program

and data memory.

pins no. from 32 to 39.

When used as an output the pin latches are programmed to 0.

When used as an input the pin latches are programmed to 1.

Port 1

is an 8-bit bidirectional I/0 port.

pins no. from 1 to 9.

no dual functions.

When used as an output the pin latches are programmed to 0.

When used as an input the pin latches are programmed to 1.

Port 2

is an 8-bit bidirectional I/O port.

When used as an output the pin latches are programmed to 0.

When used as an input the pin latches are programmed to 1.

pins no. from 21 to 28.

Port 3

is an 8-bit bi-directional I/0 port.

pins no. from 10 to 17.

RXD (P3.0): Serial input port,TXD (P3.1): Serial output port,INT0 (P3.2): External

interrupt,INT1 (P3.3): External interrupt,T0 (P3.4): Timer 0 external input,T1 (P3.5): Timer 1

external input, WR (P3.6): External data memory write strobe, RD (P3.7): External data

memory read strobe,

Timers and Counters

Many microcontroller applications require the counting of external events, such as frequency

of a pulse , or the generation of precise internal time delays between actions.

The 8051 has two 16-bit registers that can be used as either timers or counters.

These two up counters are named T0 and T1 and are provided for general use of the

programmer.

Each counter may be programmed to count internal clock pulses, act as a timer, or

programmed to count external events as a counter.

Both Timer/Counters have four operating modes, which Modes 0, 1 , 2 and 3

Modes are selected by bit pairs (M1, M0) in TMOD SFR

The counters are divided into two 8-bit registers called the timer low (TL0, TL1) and timer

high (TH0, TH1) bytes.

TCON (Timer/Counter Control Register)

Interrupts

Whenever any device needs its service, the device

notifies the microcontroller by sending it an interrupt

signal.

There are total 5 interrupt sources in 8051

Microprocessor as follows.

1. Timer Flag 0

2. Timer Flag 1

3. INT 0

4. INT 1, (INT 0 & INT 1 are external interrupts).

5. Serial Port Interrupt (RI or TI).

Interrupts Priorities

Pin Description of 8051 The 8051 is a 40 pin device, but out

of these 40 pins, 32 are used for I/O.

24 of these are dual purpose, i.e. they

can operate as I/O or a control line or

as part of address or data bus.

IMPORTANT PINS (I/O Ports)

Port 0 （pins 32-39）：P0（P0.0～P0.7）

8-bit R/W - General Purpose I/O

Or acts as a multiplexed low byte address and data bus for external memory design

Port 1 （pins 1-8） ：P1（P1.0～P1.7）

Only 8-bit R/W - General Purpose I/O

Port 2 （pins 21-28）：P2（P2.0～P2.7）

8-bit R/W - General Purpose I/O

Or high byte of the address bus for external memory design

Port 3 （pins 10-17）：P3（P3.0～P3.7）

General Purpose I/O

if not using any of the internal peripherals (timers) or external interrupts.

Each port can be used as input or output (bi-direction)

PSEN (out) pin 29: Program Store Enable, the read signal for external program memory

(active low).

ALE (out) pin 30: Address Latch Enable, to latch address outputs at Port0 and Port2.

EA (in) pin 31: External Access Enable, active low to access external program memory

locations 0 to 4K .

RXD,TXD: UART (pins10,11) for serial I/O on Port 3.

XTAL1 & XTAL2 (pins 19,18): Crystal inputs for internal oscillator.

Vcc（pin 40）：provides supply voltage(+5) to the chip.

GND（pin 20）：ground

Instruction set Arithmetic Instructions

Logic Instructions

Data Transfers