§Embedded ‘C’ Programming Notes – Part1

Here I am trying to summarize some embedded programming concepts. If you are new in the field of Microcontroller and embedded system then you should read this once before going to start programming. 

Requirements: Knowledge of ‘C’ programming, Basics of Digital-Electronics and most important thing is your INTEREST** :)

1.    Data Types : Standardize data types are defined in the header file  stdint.h

typedef signed char int8_t;               Range : -127  to  127

typedef unsigned char uint8_t;         Range : 0  to  255

typedef signed char int16_t;             Range : -32768  to  32767

typedef unsigned short uint16_t;      Range : 0  t0  65535

      Ex: uint8_t x=0;

2.    Number System :

Decimal Number: ex: 85; 

Binary Number:  ex: 0b01010101;    ‘0b’ represents binary number

Hexadecimal Number:  ex: 0x55;     ‘0x’ represents hexadecimal number

3.     Shift Operator:

<< -- Left shift Operator

>> -- Right Shift Operator

Suppose: x=01010101;    or     (x=0x55 ;  x=85;)

 

x=(x<<3);   means shift ‘x’ 3 times left

 

Now x=10101000;    or     (x=0xA8; x=200;)

X=(x>>2); means shift ‘x’ two times right

Now x=00101010;    or    ( x=0x2A; x=200;)

 

4.    Logical Operation with bits :

      | - Bitwise OR

     & - Bitwise AND

     ~ - Bitwise NOT or One’s Complement

     ^ - Ex-OR or Toggle Operator

     ! – Logical NOT

Suppose: X=00001001; and Y=00000111;

a. Z=(X|Y)=00001111;   

b. Z=(X&Y)=00000001;

c. Z=~X;      Z=~Y;

 

d. Z=(X^Y)=00001110;

 

Now you are thinking why we call this “Toggle Operator”?? Let’s see an example:

Suppose x=1;     while (1) {x=x^1;}    then  x toggles infinite times

 

e. Z=!X

         !(00001001)=0;

Note:‘~’ and ‘!’ are unary operator.

'!' is logical operator it returns ‘1’ if value is zero otherwise returns ‘0’.

  !(1)=0;

  !(236)=0;

  !(NON-ZERO)=0;

  !(0)=1;

5.    Defining Macros: Macros are very useful during programming. We have to define macro at the beginning of the program.

ex: #define SIZE 5

ex: #define LCD_PORT PORTD

Suppose we connect LCD port at port D. In another circuit we connect LCD port at port B then we only have to make change in one line i.e. in its macro definition.

ex: #define BEEP PORTB=0X01;\

        _delay_ms(500);\

        PORTB=0X00;\

We don’t have to write these lines again and again just write BEEP and we done.

Note: ‘\’ is preprocessor new line character

Ex: For(i=0;i<SIZE;i++);

Whenever we use Macro definition in our program at compile time it is replaced by its body. (Expanded source code)

6.    AVR I/O Registers: AVR microcontroller has 8-bit ports and it deals with 8-bit data. To access any port or pins of AVR microcontroller there is 3 AVR I/O port access register is defined, these are-

DDRX: Data Direction Register of port X

PORTX:  Data Register on port X

PINX: Input data register at port X

X is port name it can be A,B,C or D depends upon Microcontroller.

7.    Write to Register: Before write data to Data Register we have to define its Data Direction Register.

1     – Data out

0     -  Data In

Suppose we want to write on port A then we have to write two lines –

DDRA = 0b11111111;

PORTA=0b00000001;

or

PORTA=(1<<0);

or

PORTA=(1<<PA0);  //PA0 is a macro and it is defined in i/o.h

Ex: Write a program to LIT a LED connected at PA0

 

  

#define <avr/io.h>

void main()

 {

   DDRA=0XFF; //all pins of port A are output pins

    PORTA=0X01;

}

//---------------------or----------------------------

#define <avr/io.h>

void main()

 {

   DDRA=0X01; //PA0 is output pin

    PORTA=0X01;

}

//----------------------or-------------------------

//This is the Better Way:

#define <avr/io.h>

void main()

 {

   DDRA=(1<<PA0); //PA0 is output pin

    PORTA=(1<<PA0);

}

//-------------------------------------------------

Note: SET=1(HIGH)   and  RESET=0(LOW)

Ex: PORTA=0X03;    //set bit 0 and 1, rest is low

Or we can write—

PORTA=(1<<PA0)|(1<<PA1);

“|” operator is used to add bits.

 

8.    SET and RESET a bit without affecting remaining bits – only the state of the specified bit is changed, the previous state of other bit is preserved.

Suppose we write 7 in port A: 

PORTA=0B00000111;

Bit 0,1 and 2 is SET rest is RESET. Now if you want to set bit 3 without changing in previous bits then if you write-

PORTA=(1<<PA3);

Q. what will be the content of register A??

Answer: PORTA=0B00001000;

Because you are not adding this bit with port A you are just overlapping the bits of port A.

The correct way to do this is –

PORTA=PORTA | (1<<PA3);

or

PORTA|=(1<<PA3);

Now PORTA becomes PORTA=0B00001111;

In general: To set a bit in register X

 

Reset a bit: suppose PORTA=0B00001111; and we want to reset bit-2.

Now you are definitely thinking that to reset a bit you will write-

PORTA=(0<<PA2);

THIS IS WRONG. PORTA=(0<<PA2);  we cant shift zero. Shifting must be performed with a non-zero quantity.

NOTE: We can shift any number except ZERO.

Then how can we do this?? We can do this indirectly using ‘~’ and ‘&’ operator. See ex:

 

Or we can write this in one line:  PORTA &=~(1 << PA2);

In general: To Reset a bit in port X.

 

to be continued…

                                                                                                                 ~Pratyush Gehlot