Direct Memory Access

Direct memory access (DMA) is a technique for transferring blocks of data directly between two hardware devices. In the absence of DMA, the processor must read the data from one device and write it to the other, one byte or word at a time. If the amount of data to be transferred is large, or the frequency of transfers is high, the rest of the software might never get a chance to run. However, if a DMA controller is present it is possible to have it perform the entire transfer, with little assistance from the processor. 

Here's how DMA works. When a block of data needs to be transferred, the processor provides the DMA controller with the source and destination addresses and the total number of bytes. The DMA controller then transfers the data from the source to the destination automatically. After each byte is copied, each address is incremented and the number of bytes remaining is reduced by one. When the number of bytes remaining reaches zero, the block transfer ends and the DMA controller sends an interrupt to the processor. 

In a typical DMA scenario, the block of data is transferred directly to or from memory. For example, a network controller might want to place an incoming network packet into memory as it arrives, but only notify the processor once the entire packet has been received. By using DMA, the processor can spend more time processing the data once it arrives and less time transferring it between devices. The processor and DMA controller must share the address and data buses during this time, but this is handled automatically by the hardware and the processor is otherwise uninvolved with the actual transfer

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What is Cross compiler?

An embedded systems developer writes and compiles programs on a larger computer which can support a C development environment. The compiler used does not translate to the machine language of the development computer, it produces a version of the program in the machine language of the 8 bit microcontroller. A compiler that runs on one type of computer and provides a translation for a different type of computer is called a cross-platform compiler or cross-compiler.

The object code formats generated by a cross-compiler are based on the target device. For example, a compiler for the Motorola MC68HC705C8 could generate an S-record file for its object code.

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What is Simulator?

A simulator is a software program which allows a developer to run a program designed for one type of machine (the target machine) on another (the development machine). The simulator simulates the running conditions of the target machine on the development machine. Using a simulator you can step through your code while the program is running.

You can change parts of your code in order to experiment with different solutions to a programming problem. Simulators do not support real interrupts or devices. An in-circuit simulator includes a hardware device which can be connected to your development system to behave like the target microcontroller. The incircuit simulator has all the functionality of the software simulator while also supporting the emulation of the microcontroller’s I/O capabilities.

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What is The Linker?

When programs were written in the past often the development computer was not powerful enough to hold the entire program being developed in memory at one time. Historically, programs had to be divided into separate modules where each module would be compiled into object code and a linker would link the object modules together. Our development machines today are very powerful and the use of a linker is no longer absolutely necessary.

Many implementations of C provide function libraries which have been precompiled for a particular computer. These functions serve common program needs such as serial port support, input/output, and description of the destination computer. Functions within libraries are usually either linked with modules which use them or included directly by the compiler if the compiler supports library function inclusion.

When your program has been pre-processed, compiled and linked, the destination computer will be able to read and execute your program.

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What is The Compiler?

The compiler translates a program into an intermediate form containing both machine code and information about the program’s contents. The compiler is the second component to handle your program. The compiler has the most important job: digesting and translating the program into a language readable by the destination computer.

Many compilers operate in different passes through the code. There are often passes specifically to handle optimizations of code which will reduce the size of the machine code generated.

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What are the Buses in Microcontroller?

A bus carries information in the form of signals. There are three main buses: address, data, and control.

1) The address bus is unidirectional and carries the addresses of memory locations indicating where the data is stored. The number of wires in the address bus determines the total number of memory locations. With a 13 bit address bus, for example, there would be 213 or 8192 memory locations.

2) The data bus is bi-directional and carries information between the CPU and memory or I/O devices. Computers are often classified according to the size of their data bus. The term “8-bit microcontroller” refers to a microcontroller with 8 lines on its data bus. The number of wires in the data bus determines the number of bits that can be stored in each memory location.

3) The control bus carries data which controls system activity. Often this data includes timing signals which synchronize the movement of other information.

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Harvard Architecture of Microcontroller

Harvard architecture computers have separate memory areas for program instructions and data. There are two or more internal data buses which allow simultaneous access to both instructions and data. The CPU fetches instructions on the program memory bus. If the fetched instruction requires an operation on data memory, the CPU can fetch the next program instruction while it uses the data bus for its data operation. This speeds up execution time at the cost of more hardware complexity.

Since Harvard machines assume that only instructions are stored in program memory space, how do you write and access data stored in program memory space? For example, a data value declared as a C constant must be stored in ROM as a constant value. Different microcontrollers have different solutions to this problem. A good C compiler automatically generates the code to suit the target hardware’s requirements. Some chips have special instructions allowing the retrieval of information from program memory space. These instructions are always more complex or expensive than the equivalent instructions for fetching data from data memory.

Typically these chips have a register analogous to the program counter (PC) which refers to addresses in program space. Also, some chips support the use of any 16 bit value contained in data space as a pointer into the program address space. These chips have special instructions to use these data pointers.

NOTE

It is important that you understand how your Harvard architecture part deals with data in program space. It is possible to generate more efficient code using symbolic constants declared with #define directives instead of declared constants. You may also create global variables for constant values.

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Von Neumann Architecture of Microcontroller

Von Neumann architecture has a single, common memory space where both program instructions and data are stored. There is a single data bus which fetches both instructions and data. Each time the CPU fetches a program instruction it may have to perform one or more read/write operations to data memory space. It must wait until these subsequent operations are complete before it can fetch and decode the next program instruction. The advantage to this architecture lies in its simplicity and economy.

NOTE

On some Von Neumann machines the program can read from and write to CPU registers, including the program counter. This can be dangerous as you can point the PC at memory blocks outside program memory space. Careless PC manipulation can cause errors which require a hard reset.

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What are the Registers in microcontroller?

Registers store the state of the CPU. If the contents of microcontroller memory and the contents of these registers are saved it is possible to suspend program operation for an indefinite period of time and restart exactly in the state when the program was suspended.

The number and names of registers varies drastically among microcontrollers. However there are certain registers which are common to most microcontrollers, although the names may vary. These include:

"# The stack pointer

The stack pointer contains the address of the next location on the stack.The address in the stack pointer is decremented when data is pushed on the stack and incremented when data is popped from the stack.

"# The index register

The index register is used to specify an address when certain addressing modes are used. It is also known as the pointer register. The Microchip devices use the name FSR (file select register).

"# The program counter

Perhaps the single most important CPU register is the program counter (PC). The PC holds the address of the next instruction in program memory space. It contains the address of the next instruction the CPU will process.

As each instruction is fetched and processed by the ALU, the CPU increments the PC and thereby steps through the program stored in the program memory space.

"# The accumulator

The accumulator is a register that can hold operands or results of operations as necessary. The Microchip devices use the name W (working) register.

Other registers may reflect results from the instruction just executed, control the options available on the device, and enable access to certain areas of memory.

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The Microcontroller in a System

Microcontrollers do not function in isolation. As their name suggests they are designed to control other devices. The microcontroller can accept inputs from some devices and provide outputs to other devices within any given system. For example, a microcontroller may accept input from a switch and may send output to an LED. If the switch is pressed the microcontroller can be instructed to illuminate the LED.

The microcontroller is often part of a larger system. For example, the switch and LED may be part of a compact disc player in a car stereo system. When a microcontroller is part of a larger system it is often referred to as an embedded controller because it is embedded within the larger system.

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What is RAM?

RAM, random access memory, is used to write and read data values as a program runs. RAM is volatile: if you remove the power supply its contents are lost. Any variables used in a program are allocated from RAM. The time to retrieve information from RAM does not depend upon the location of the information because RAM is not sequential, hence the term random access.

Most small microcontrollers provide very little RAM which forces you to write applications that use RAM wisely. Manipulating large data structures and using pointers, re-entrant or recursive functions use large amounts of RAM and are techniques which are generally avoided on microcontrollers.

Some C instructions which are rarely used on larger platforms are more commonly used in C programs for microcontrollers. One example is the goto instruction reviled by traditional C programmers. While goto is rarely used on larger platforms, in embedded system programming it can sometimes be used to save RAM.

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What is ROM?

ROM, read only memory, is non-volatile memory used for program information and permanent data. The microcontroller uses ROM memory space to store program instructions it will execute when it is started or reset.

Program instructions must be saved in non-volatile memory so that they are not affected by loss of power. The microcontroller usually cannot write data to program memory space.

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What is a Microcontroller?

A microcontroller is a single chip, self-contained computer which incorporates all the basic components of a personal computer on a much smaller scale.

Microcontrollers are often referred to as single chip devices or single chip computers. The main consequence of the microcontroller’s small size is that its resources are far more limited than those of a desktop personal computer.

In functional terms, a microcontroller is a programmable single chip which controls a process or system. Microcontrollers are typically used as embedded controllers where they control part of a larger system such as an appliance, automobile, scientific instrument or a computer peripheral. Microcontrollers are designed to be low cost solutions; therefore using them can drastically reduces part and design costs for a project.

Physically, a microcontroller is an integrated circuit with pins along each side. The pins presented by a microcontroller are used for power, ground, oscillator,I/O ports, interrupt request signals, reset and control. In contrast, the pins exposed by a microprocessor are most often memory bus signals (rather than I/O ports).

NOTE

A microcontroller is not the same as a microprocessor. A microprocessor is a single chip CPU used within other computer systems. A microcontroller is itself a single chip computer system

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Why the clock frequency of embedded system should be as low as possible?

Many developers select an oscillator / resonator frequency that is at or near the maximum value supported by a particular device.

This can be a mistake:

• Many application do not require the levels of performance that a modern 8051 device can provide.

• The electromagnetic interference (EMI) generated by a circuit increases with clock frequency.

• In most modern (CMOS-based) 8051s, there is an almost linear relationship between the oscillator frequency and the power-supply current. As a result, by using the lowest frequency necessary it is possible to reduce the power requirement: this can be useful in many applications.

• When accessing low-speed peripherals (such as slow memory, or LCD displays), programming and hardware design can be greatly simplified - and the cost of peripheral components, such as memory latches, can be reduced - if the chip is operating more slowly

In general, you should operate at the lowest possible oscillator frequency compatible with the performance needs of your application.

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Why we use C programming for embedded systems?

It is a ‘mid-level’, with ‘high-level’ features (such as support for functions and modules), and ‘low-level’ features (such as good access to hardware via pointers);

• It is very efficient;

• It is popular and well understood;

• Even desktop developers who have used only Java or C++ can soon understand C syntax;

• Good, well-proven compilers are available for every embedded processor (8-bit to 32-bit or more);

• Experienced staff are available;

Books, training courses, code samples and WWW sites discussing the use of the language are all widely available.

Overall, C may not be a perfect language for developing embedded systems, but it is a good choice (and is unlikely that a ‘perfect’ language will ever be created).

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Top Tips To Make Your Soldering Experience Awesome

Being an electronics engineer you will have to learn to solder some
time in your life, I am sure some or most of you already know the art..
" ART !!! " you will say, Yes soldering is an art, the more you are
good at it the less time it will take and your circuits will look good
as well, no one likes bad soldering work, nor should you, why to
compromise on good enough when it can be awesome, so here are some tips
form me.

1 ). Good Iron: A good quality soldering iron will be your best friend
while soldering, a 25 to 35 watt iron is recommended, if you live in
India Soldron is my favorite brand, its a little costly but will last
you for years and years..

2 ). Iron Tips : The Tip is the business end of a soldering iron, make
sure to change them regularly, and while using them keep them clean.

3 ). Good Quality Soldering Wire : Yes it matters, buy good quality
soldering wire, the less the diameter of the wire the better, less than
.5 mm is recommended, in this way you will not be able to over feed the
solder and less wastage will occur, good quality rosin wire will also
give of less fumes which are harmful to health.

4 ). Atmosphere : Sit in in a well ventilated place, the fumes that
released by solder are not good for health if possible use a fume
extractor fan.

5 ). Per Heating The Iron : Before starting to solder let the iron heat
up for about 10 to 15 minutes, this gets the iron properly heated.

6 ). Keeping Things Handy : Keep all the things you will need handy,
close to you, so that when you start soldering you don't have to get up
to fetch something or another..

7 ). Be Aware : In your hand you have a iron which is HOT and I mean
extremely HOT about 300 to 350 C, so please be careful, you don't want
to get a burn, I have it hurts allot for DAYS !!!

In the end all I want to say is BE CAREFUL !!! And if I have missed any
point write it down in the comments

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Compare Microprocessor and Microcontroller.

Microprocessor	Microcontroller
Microprocessor contains ALU,general purpose registers,stack pointer, program counter, clock timing circuit and interrupt circuit.	Microcontroller contains the circuitry of microprocessor and in addition it has built- in ROM, RAM, I/O devices, timers and counters.
It has many instructions to move data between memory and CPU.	It has one or two instructions to move data between memory and CPU.
It has one or two bit handling instructions.	It has many bit handling instructions.
Access times for memory and I/O devices are more.	Less access times for built-in memory and I/O devices.
Microprocessor based system requires more hardware.	Microcontroller based system requires less hardware reducing PCB size and increasing the reliability.

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Distinguish between the memories mapped I/O peripheral I/O?

Memory Mapped I/O	Peripheral MappedI/O
16-bit device address	8-bit device address
Data transfer between any general-purpose register and I/O port.	Data is transfer only between accumulator and I.O port
The memory map (64K) is shared between I/O device and system memory.	The I/O map is independent of the memory map; 256 input device and 256 output device can be connected
More hardware is required to decode 16-bit address	Less hardware is required to decode 8-bit address
Arithmetic or logic operation can be directly performed with I/O data	Arithmetic or logical operation cannot be directly performed with I/O data

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DC Motor Speed Regulation with A PWM Feed Back System

DC Motor Speed Regulation with A PWM Feed Back System

Basic PWM
One of the easiest ways of generating an analog voltage from a digital value is by pulse-width modulation ( PWM ). I PWM a high frequency square wave is generated as a digital output. For example, a port bit continuously swiched on and off at a reltive high frequency. The signal is fed to a low pass filter. The voltage at the otuput of filter is equal to ther Roo Mean Squeare ( RMS ) of the squeare wave signal. The RMS of the square wave signal may then be varied by changing the duty cycle of the signal. A cycle is initiated by a low to high transition of the signal and terminates at the next such transition. During one cycle if the time the signal stays high is equal to the time the signal stays low, then the duty cycle is said to be 50 percent.

Figure 1. Duty Cycle 30 %

The following circuit shows a DAC constructed with PWM. The Program controls the speed of a DC motor by pulse widht modulation ( PWM ). Bit P3.0 Drives a switching transistor as shown in the circuit diagram. Motor is swiched on for a period of time, and then off. The fraction of time the motor is on is call the duty cycle.

Figure 2. Pulse widht modulation to Control Motor Speed

This program uses a byte to store the time the motor is on, that is the number of cycles out of 256 cycles while the motor is on. a value of 10 means that the motor is on 10 cycles and off 246 cycles.The duty cycle value is stored in the internal register labeled dCycle. The complement of the duty cycle is similarly stored in register labeled dCycleC. ( Download File : motor.zip )

dCycle   equ 30h
dCycleC  equ 31h
count    equ 32h
Analog0  equ 33h
Analog1  equ 34h
PWM      bit P3.0
Channel  bit P3.1
PortADC  equ P0
DCLB     equ 000h
DCUB     equ 0F0h
MotorCondition bit 20h
;
         org 0h
         ljmp start
         org 0bh
         ljmp Timer_Interrupt0
start:   call Init_Interrupt_Timer0
         clr PWM ; Turn off motor
;==================
; Main Routine
;==================
Forever: call ADC
         call Update
         sjmp Forever
;
;======================
;Subrutine Timer Interruption
;======================
Timer_Interrupt0:
         JB MotorCondition,MotoroFF
         Setb PWM
         Mov TH0,dCycle
         Setb MotorCondition
         reti 
;
Motoroff:clr PWM
         mov TH0,dcycleC
         clr MotorCondition 
         reti 
;
;=========================
;Subrutine Initialize Timer Interrupt
;timer 0 as counter 8 bit mode 2
;========================
Init_interrupt_Timer0:
         mov TH0,#dCycle
         Mov TMOD,#00000010b ;
         setb ET0 ; Enable Timer 0 Interrupt
         Setb EA ; Master Interrupt Enable 
         setb TR0 ; start rock and roll timer 0
         Setb MotorCondition
         mov count,#50
         mov dCycle,#0
         mov dCycleC,#0FFh
         ret
;
;========================
;Subrutine Update - Update duty cycle
;=========================
Update:  djnz count,upDone
         mov A,analog1; reference voltage
         mov b,analog0; tachometer voltage
         clr c
         subb a,b
         jnz update1
         ret
Update1: jc decrease
         call higherDC
         ret
Decrease:
         call lowerDC
UpDone: 
         ret
;
;====================
;Subrutine lowerDC 
;====================
LowerDC: mov a,dCycle
         cjne a,#DCLB,DecDC
         ret
         DecDC: dec dCycle
         inc dCycleC
         ret
;
;===================
;Subrutine HigherDC 
;===================
HigherDC:
         mov a,dCycle
         cjne a,#DCUB,IncDC
         ret
         IncDC: Inc dCycle
         dec dCycleC
         ret
;
;=============
;Subrutine ADC 
;=============
ADC: Clr channel ; Pick channel X0 from multiplexer 4051
     mov Analog0,PortADC
     Setb channel ;Pick channel X1 from multiplexer 4051
     mov Analog1,PortADC
     ret
; 
     end

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Makes LED blink every 0.5 second C Programming 8051

A basic circuit of the 89C2051 shown here can be made easily using point-to-point soldering with a universal PCB. Use an ordinary 20-pin socket, do not use a circle-pin socket. D1 is a small dot LED. U2 can be either 7805 or 78L05. U3 is optional for correcting any polarity DC adapter. Without the 2051 chip in the socket, checks the connection, then measures +5V between pin 20 and pin 10. Test the LED by shorting P1.7 pin to GND.

Test your board with myfirst.c, a simple c program 
that makes LED blink every 0.5 second. 

/* 










           * myfirst.c 
           * First C program for 2051 experiment 
           * complement P1.7 every 0.5 sec 
           * Copyright (C) 1999 Wichit Sirichote 
           * compiled with Dunfield Micro-C for 8051 Release 3.2
           */ 

#include c:\mc\8051io.h /* include i/o header file */
           #include c:\mc\8051reg.h 

extern register char cputick; 
// cputick was incremented every 10ms            
   register unsigned char sec100,flag1; 

#define n 50 

task1(); // functions declarations 
           task2(); 

main() 
{ 
 flag1 = 0; 
 sec100 = 0; 
 serinit(9600); // set timer0 to be 16 bit counter 
 while(1){ 

 while(cputick == 0) 
 ; 
 cputick = 0; 
 task1(); 
 task2(); 
 } 
} 

task1() // set bit 0 of flag1 every n*10ms 
{ 
 sec100++; // increment sec100 
 if (sec100 >= n) 
 {sec100 = 0; // clear sec100 
 flag1 |= 0x01; // set bit 0 of flag1 
 } 
} 

task2() 
{ 
 if ((flag1 & 0x01) != 0) // execute below if bit 0 of flag1 is set
{ 
  // P1 ^= 0x80; // exclusive or the latch bit 7 with 0x80 
  asm " CPL P1.7"; // complement P1.7 
  flag1 &= ~0x01; // clear bit 0 of flag1 
 } 
} 

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