Embedded Systems

Introduction to Embedded System

 The embedded system is a combination of computer hardware,
software additional electrical & mechanical parts

 A computer is used in such devices primarily as a means to
simplify
the system design and to provide flexibility.

 Embedded systems employ the use of a RTOS (Real-Time
Operating System).

Block Diagram of Embedded System

SOFTWARE

MEMORY

A/D CPU D/A ACUTATORS

SENSOR HUMAN INTERFERENCE

Microcontroller

• A smaller computer.
• On-chip RAM, ROM, I/O ports...
• Example:- Motorola’s 6811, Intel’s 8051 and PIC 16X

CPU RAM ROM
A single chip

I/O Timer
Port Serial
COM
Port

The 8051 Microcontroller :
• The 8051 is the first microcontroller of
the MCS-51 family introduced by
Intel Corporation at the end of the
1970’s.

• The 8051 family characteristics:

The 8051 family characteristics:
 4K Bytes ROM
 128 Bytes RAM
 Two timer/counters (16 bit)
 A serial port
 32 input/output port
 Interrupt controller

IC 8051 Pin Description
Voltage Supply
(+5V)

Ground


Port 0 from
Pin-39 to Pin-32

An 8-bit open drain
bidirectional port.
Used to address both
data and address


Port 1 from
Pin-1 to pin-8

8-bit bidirectional
I/O port with
internal pull-ups


Port 2 from
Pin-21 to pin-28

Used to access
address and I/O


P3.0 : RxD(serial input port)
P3.1 :TxD (serial output port)
P3.2 :INT0 (external interrupt 0)
P3.3 :INT1 (external interrupt 1)
P3.4 :T0 (timer 0 external input)
P3.5 :T1 (timer 1 external input)
P3.6 :WR (external data memory write strobe)
P3.7 :RD (external data memory read strobe)

Port 3 from
Pin-10 to Pin-17


External memory
reset
Add. Latch
enable

Program Store
Enable
crystal

IC 8051 Daily Applications

Digital clock
Moving message display

7 segment display Traffic Light

Counter / Timers
Two 16-bit Counter/Timers: TIMER0, TIMER1
Up counters, can interrupt on overflow.

Counts:
- CPU cycles (crystal/12). “Timer”.
-External input (max. half CPU rate). “Counter”.

TMOD : Counter/Timer Mode Register
Gate C/T M1 M0 Gate C/T M1 M0

Timer 1 Timer 0

GATE : Permits INTx pin to enable/disable
counter.

- C/T : Set for counter operation, reset for
timer operation.

TMX1 TMX1 TMX1 Description
RST0 RST0 RST0 13 bit timer
RST1 RST1 RST1 16 bit timer
RST2 RST2 RST2 8 bit auto reload
RST3 RST3 RST3 Split timer mode

Timer control (TCON)
TF1 TRI TF0 TR0

TF : timer flag
TR : timer run

TF1 is SET if timer exceeds the maximum limit i.e. : FFFF H

TR starts and stops the timer

The following instruction indicates the condition for start n stop respectively

SET B TR0 : starts the timer
CLR B TR0 : stops the timer

IC 8051 Addressing Modes
• Immediate addressing modes
ADD A, #23h (Add immediate data to Acc)

• Register addressing modes
ADD A, R2 (Add register to Accumulator)

• Direct addressing modes
ADD A, 40h (Add data at location 40h to Accumulator)

• Register Indirect addressing modes
ADD A,@R2 (Add indirect RAM to Acc)

Interrupts in 8051

• An interrupt is an external or internal event that interrupts the microcontroller to
inform it that a device needs its service
• The advantage of interrupts is that the microcontroller can serve many devices
(not all at the same time)

Program Program
Interrupt service

time

Interrupts in 8051
Internal Interrupts
EA - ET2 ES ET1 EX1 ET0 EX0

EX0 Enables or disables external interrupt
ET0 Enables or disables timer 0 overflow interrupt
EX1 Enables or disables external interrupt 1
ET1 Enables or disables timer 1 overflow interrupt
ES Enables or disables the serial port interrupt
ET2 Enables or disables timer 2 overflow or capture interrupt
-- Not implemented, reserved for future use
EA Disables all interrupts

External Interrupts

P3.2 INT0 (external interrupt 0)
P3.3 INT1 (external interrupt 1)

General Purpose
Microprocessor
 CPU for Computers
 No RAM, ROM, I/O on CPU chip itself
 Example--Intel’s x86: 8086,8088,80386,80486, Pentium

Data Bus Many chips on mother board
CPU

General Serial
Purpose RAM ROM I/O Timer COM
µP Port Port

Address Bus

General-Purpose Microprocessor System

The 8085 Microprocessor :
 The features of INTEL 8085 are :
 It is an 8 bit processor.
 It is a single chip N-MOS device with
40 pins.
 It has multiplexed address and data
bus.(AD0-AD7).
 It works on 5 Volt dc power supply.
 The maximum clock frequency is 3
MHz while minimum frequency is
500kHz.
 It provides 74 instructions with 5
different addressing modes.

• AD0-AD7 Multiplexed Address and data lines.

• A8-A15 Tri-stated higher order address lines.

• ALE Address latch enable is an output signal. It goes high when
operation is started by processor .

• S0,S1 These are the status signals used to indicate type of operation.
_
• RD Read is active low input signal used to read data from I/O
device or memory.
_
• WR Write is an active low output signal used write data on
memory or an I/O device.

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• HOLD & HLDA HOLD is an input signal .When µP receives HOLD signal
it completes current machine cycle and stops executing
next instruction. In response to HOLD µP generates HLDA
that is HOLD Acknowledge signal.

• RESET IN This is input signal. When RESET IN¯ is low µp restarts and
starts executing from location 0000H.

• SID Serial input data is input pin used to accept serial 1 bit data

• VCC & VSS Power supply VCC=+ -5Volt& VSS=-GND reference.

• X1 & X2 These are clock input signals and are connected to external LC
or RC circuit. These are divide by two so if 6 MHz is
connected to X1&X2, the operating frequency becomes 3
MHz

Arithmetic and Logical Group
Accumulator:
It is 8 bit general purpose register.
It is connected to ALU. So most of the operations are done in Acc.

Temporary register:
All the arithmetic and logical operations are done in the temporary register
but user can’t access it.

Flag:
It is a group of 5 flip flops used to know status of various operations done.

The Flag Register along with Accumulator is called PSW
or Program Status Word.

Arithmetic and Logical Group
Flag is given by :

S Z - AC - P - CY

S: Sign flag is set when result of an operation is negative.

Z: Zero flag is set when result of an operation is 0.

Ac: Auxiliary carry flag is set when there is a carry out of lower nibble or
lower four bits of the operation.

CY: Carry flag is set when there is carry generated by an operation.

P: Parity flag is set when result contains even number of 1’s.
Rest are don’t care flip flops.

Register Group

General purpose There are six general purpose registers in 8085 namely
B,C,D,E,H,L These are used for various data manipulations.

Special purpose: There are two special purpose registers in 8085:

SP : (Stack Pointer) This is a temporary storage memory 16 bit register. Since there
are only 6 general purpose registers, there is a need to reuse them

PC : (Program Counter) It is 16 bit register used to point the location from which the
next instruction is to be fetched.

Temporary registers (W,Z):
These are not available for user. These are loaded only when there is an operation
being performed.

IC 8085 Addressing modes
• Immediate addressing:
Immediate data is transferred to address or register.
Example : MVI A,20H

• Register addressing:
Data is transferred from one register to other.
Example : MOV A, C

• Indirect addressing:
Data is transferred from address pointed by the data in
a register to other register or vice-versa.
Example: MOV A, M

• Implied addressing:
These doesn’t require any operand. The data is specified
in Opcode itself.
Example: RAL: Rotate left with carry.

Interrupts in 8085

• An interrupt is considered to be an emergency signal that may be serviced. The
Microprocessor may respond to it as soon as possible.

Interrupts can also be classified into:
• Maskable Interrupts (Can be delayed or Rejected)
• Non-Maskable Interrupts (Can not be delayed or Rejected)

The 8085 has 5 interrupt inputs :

The INTR input INTR is mask-able using the EI/DI instruction pair.
RST 5.5, 6.5, 7.5 They are all mask-able.
TRAP Is the only non-mask-able interrupt in the 8085

Interrupts in 8085
The 8085 recognizes 8 RESTART instructions: RST0 - RST7 . Each of these would send
the execution to a predetermined hard-wired memory location:

Restart Instruction Equivalent to

RST0 CALL 0000H
RST1 CALL 0008H
RST2 CALL 0010H
RST3 CALL 0018H
RST4 CALL 0020H
RST5 CALL 0028H
RST6 CALL 0030H
RST7 CALL 0038H

Timing and State Diagram

• The µP operates with reference to clock signal. The rise and fall of the pulse of the
clock gives one clock cycle.
• Each clock cycle is called a T state and a collection of several T states gives a
machine cycle.
• Important machine cycles are :

I. Op-code fetch.
II. Memory read.
III. Memory write.
IV. I/Op-read.
V. I/O write.

Timing and State Diagram
Op-code fetch cycle :

Seven Segment Display
Seven segments are electronic
components that can be used to
displaying alphanumeric characters, 7
LED are used (as shown in figure) and is
designed from a dot-point with the size.

<< pin configuration of seven segment

7 pins of the microcontroller are used to shape the
character of your LED display

Microcontroller

Port Relations Microcontroller with pin >>
7 Segment


ble 1. Configuring the Character 7 Segment Data

8051
Microcontroller

Design Softwares

Software’s
Used

Keil Top View Express
µVision Simulator PCB

Keil µVision IDE
 The µVision IDE from Keil
combines project management,
make facilities, source code
editing, program debugging, and
complete simulation in one
powerful environment. The
µVision development platform is
easy-to-use and helping you
quickly create embedded
programs that work. The µVision
editor and debugger are
integrated in a single application
that provides a seamless
embedded project development
environment.

Top View Simulator
• Topview Simulator gives an
excellent simulation environment
for MCS 51 Microcontroller.

A beginner can learn about 8051
based embedded solutions
without any hardware. An
experienced designer, you may
find most of the required facilities
built in the simulator that
enabling you to complete your
next project without waiting for
the target hardware.

Top View Simulator

A Top View
Simulator
Program

Express PCB
Express PCB allows you to design
schematics and PCB's. It is ideal
for those that are looking to
design anywhere from a two to
four layer circuit board.
A drop and drag design allows
users to pick the components of
the circuit board and drop it
onto the circuit board in the
software program. The designer
can then place various pins in the
circuit board to show where
things need to be connected.
It also has the ability of the
program to show the designer
where there are flaws in their
design.

Express PCB

This way we can include resistors

This way we can include different
kind of IC’s

DC Motor interfacing with
Microcontrollers
DC motors are always preferred over stepper motors.

There are many things which we can do with our DC motor when interfaced with
microcontroller.

For example:
• we can control the speed of motor,
• we can control the direction of rotation,
• we can also do encoding of the rotation made by DC motor i.e. keeping track of
how many turns are made by our motors etc.

Usually H-bridge is preffered way of interfacing a DC motor.
L293D is most used H-Bridge driver IC.

Working of H-Bridge
The name "H-Bridge" is derived from the actual shape of the switching circuit which
control the motion of the motor. It is also known as "Full Bridge". Basically there are
four switching elements in the H-Bridge as shown

Working of H-Bridge

Truth Table
High Left High Right Low Left Low Right Description

Motor runs
On Off Off On
clockwise

Motor runs anti-
Off On On Off
clockwise

Motor stops or
On On Off Off
decelerates

Motor stops or
Off Off On On
decelerates

Truth Table For H-Bridge

Stepper Motor
Of all motors, step motor is the easiest to control. It's
handling simplicity is really hard to deny - all there
is to do is to bring the sequence of rectangle
impulses to one input of step controller and
direction information to another input. Direction
information is very simple and comes down to
"left" for logical one on that pin and "right" for
logical zero.
Motor control is also very simple - every impulse
makes the motor operating for one step and if
there is no impulse the motor won't start. Pause
between impulses can be shorter or longer and it
defines revolution rate. This rate cannot be infinite
because the motor won't be able to "catch up"
with all the impulses (documentation on specific
motor should contain such information)..

Interfacing of Stepper motor with 8051

Clock-wise Rotation of the Coils

Truth table

1 2 3 4
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1

Embedded Systems

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Editor's Notes