Файл: Microcontroller based applied digital control (D. Ibrahim, 2006).pdf

102 PROGRAMMING PIC MICROCONTROLLERS IN C

When these symbols are used in the program their values are substituted to where the symbols are. For example,

if(result > MAX)

is changed by the pre-processor into

if(result > 1000)

#define commands can be complex, such as:

In the last example, the pre-processor will replace every occurrence of SECONDS by the string (60* 60* 24).

Other examples of #define are:

where SQ(x) will be replaced with ((x) * (x)). For example, SQ(a + b) will be replaced with ((a + b) * (a + b)).

#define can also be used to define macros. A macro is frequently used to replace function calls by in-line code, which is more efficient. An example is given below:

#define MIN(x, y) (((x) < (y)) ? (x): (y))

which compares x and y and if x is less than y, the result is x, otherwise the result is y. After this definition, an expression such as:

k = MIN(u, v);

gets expanded by the pre-processor to:

k = (((u) < (v)) ? (u): (v));

Example 4.3

An integer consists of two bytes. Write a macro to extract the upper and the lower bytes of an integer. Call the upper byte ubyte and the lower byte lbyte.

Solution

We can shift right eight times to extract the upper byte:

The lower byte can be extracted by masking the integer with hexadecimal number 0xFF.

Notice that because the x is an integer, the result is converted into a character by preceding it with ‘(unsigned char)’. This is called casting in C.

Example 4.4

Write a macro to set or clear a bit of a variable.

Solution

If we name the macro to set a bit as bit set,

#define bit set(var, bitno) ((var) = (1 (bitno)))

and the macro to clear a bit as bit reset,

#define bit reset(var, bitno) ((var) &= (1 (bitno)))

we can now set bit 3 of the variable x to 1 with the following statement:

bit set(x, 3);

Similarly, bit 6 of the variable q can be cleared to 0 with the following statement:

bit set(q, 6);

4.18.2 #include

This command causes the pre-processor to replace the line with a copy of the contents of the named file. For example,

#include <myproject.h>

or

#include ‘myproject.h’

causes the contents of file myproject.h to be included at the line where the command is issued. When developing programs with the PICC Lite compiler, the following line must be included

at the beginning of all programs:

#include <pic.h>

The file pic.h includes all the PIC microcontroller register definitions.

4.18.3 #asm and #endasm

These pre-processor commands are used to include assembler instructions in C programs. The

#asm command identifies the beginning of the assembly code, and #endasm identifies the end of the assembly code. For example,

...

...

i = 10;

#asm

movlw 10h

#endasm

...

...

104 PROGRAMMING PIC MICROCONTROLLERS IN C

4.19 ACCESSING THE EEPROM MEMORY

Some PIC microcontrollers (e.g. PIC16F84) have EEPROM memories which can be used to store nonvolatile data. PICC Lite provides instructions for reading and writing to this memory. The EEPROM WRITE command is used to write a data byte to the EEPROM memory. For example, the following command writes 0x2E to address 2 of the EEPROM memory:

EEPROM WRITE(2,0 × 2E);

Similarly, the EEPROM READ command is used to read a data byte from the EEPROM memory. For example, the following command reads a byte from address 5 of the EEPROM memory and loads it into the variable k:

k = EEPROM READ(5);

4.20 INTERUPTS IN C PROGRAMS

Interrupts are very important in real-time programs. Most PIC microcontrollers offer internal and external interrupt facilities. Internal interrupts are usually timer-generated interrupts, while the external interrupts are triggered by activities on the I/O pins. When an interrupt occurs the processor jumps to the interrupt service routine where the interrupt is serviced. Medium-range microcontrollers (such as the PIC16F84) have only one ISR, which is at address 0x04 of the program memory. If interrupts from multiple sources are expected then the interrupting device can be found by inspecting the INTCON register bits.

Interrupts can be used in PICC Lite programs in the form of special functions. When an interrupt occurs the program jumps to this special function where the interrupt is handled. ISR function must have the name interrupt followed by a user-selected function name, and the function must not return any value and must not have any arguments. For example, an ISR function named my int is given below:

void interrupt my int(void)

{

statement;

statement;

...

statement;

}

Interrupts must be enabled before they can be recognized by the microcontroller. The ei() and di() instructions enable and disable global interrupts, respectively. Additionally, the interrupt control bit of each interrupt source must be enabled in the appropriate SFR register (e.g. T0IE bit in INTCON must be set to 1 to enable timer interrupts).

The PIC microcontroller only saves the program counter on stack when an interrupt occurs. The PICC Lite compiler determines which registers and objects are used by the interrupt function and saves these appropriately.

106 PROGRAMMING PIC MICROCONTROLLERS IN C

Also, the file (in the compiler’s include directory) should be included in all PICC Lite programs. An example is given below:

//Author: D. Ibrahim

//Date: February 2005

//File: AVERAGE.C

//This program calculates the average of three numbers 3, 5, and 9

//**********************************************************

#include <pic.h>

//Main Program

//************

//

main(void)

{

float average;

average = (3 + 5 + 9)/3;

}

When functions are used in programs, they should normally be included before the main program. An example is given below.

Example 4.5

Write a program to calculate the square of a number. The square should be calculated using a function.

Solution

//Author: D. Ibrahim

//Date: February 2005

//File: SQUARE.C

//This program calculates the square of an integer number

//**********************************************************

#include <pic.h>

//Functions

// ********

int square of number(int x)

{

int k;

k = x * x; return(k);

}

//Main Program

//************

main(void)

{

int p, z;

p = 5;

z = square of number(p);

}

4.23 PIC MICROCONTROLLER INPUT–OUTPUT INTERFACE

A microcontroller communicates with the outside world using its input–output ports. These ports can be analog or digital. Analog ports are used to read data from analog devices, e.g. the voltage across a resistor. Digital ports are used to read data from digital devices, e.g. the state of a switch. Microcontroller outputs are usually connected to LEDs, LCDs, buzzers, sevensegment displays and similar devices. The input can be a push-button switch, a keyboard or a similar device.

PIC microcontroller output ports can source and sink 25 mA of current. When an output port is sourcing current, the current flows out of the port pin. Similarly, when an output port is sinking current, the current flows towards the output pin. Devices such as LEDs, LCDs and other small devices which operate with up to 25 mA can be connected to the microcontroller output ports. Devices which require larger currents can be connected to the microcontroller ports by using a transistor switch to increase the current capability of the port pin.

4.23.1 Connecting an LED

Most LEDs operate with about 2V and they draw about 10 mA from the power supply. An LED can be connected to the output port of the PIC microcontroller by using a series current limiting resistor, as shown in Figure 4.3.

Assuming the output voltage Vo of the port pin is +5 V when it is at logic 1, the value of the resistor can be calculated as

Example 4.6

An LED is connected to bit 0 of port B (i.e. pin RB0) of a PIC16F84 microcontroller. Write a program to flash the LED at 100 ms intervals. The hardware set-up is shown in Figure 4.4, where the microcontroller is operated with a 4 MHz crystal.

108 PROGRAMMING PIC MICROCONTROLLERS IN C

2V

Output port

LED 10mA

PIC

Microcontroller

Figure 4.3 Connecting an LED

PIC 16F84

Figure 4.4 Hardware setup for the Example 4.6

Solution

The required program is given in Figure 4.5. At the beginning of the program port B is configured as an output port. The LED is then turned on by sending a logic 1 to the port pin (RB0 = 1). After 100 ms delay the LED is turned off (RB = 0) and this cycle is repeated forever.