Chapter 5

PIC PROGRAMMING IN C

Chapter Objective

• Understand C programming for embedded

system

• Understand input output (I/O) programming in C

• Understand programming timer

Why program the PIC18 in C

• Compilers produce hex file downloaded into the

ROM of the microcontroller

• The size of the hex file is the concern because

of:

• limited size of on-chip ROM

• limited code space for PIC18 (2Mbyte)

C Vs. Assembly

C Language Assembly Language

Easy to write the program Harder to write the program

Less time consuming Time consuming

Easier to modified and update Tedious to modified

Code available in function library Must know /write every instruction

Code is portable to other

microcontroller ( less modification)Not portable to other microcontroller

Bigger HEX file Smaller HEX file

The structure of C program for PIC18

Functions

• void main (void):

It represents that "main" function does not return any

value as well as does not accept any

parameter/arguments.

In old compilers it was mandate to specify void in

parenthesis in case main is not going to accept any

parameters. but these days you may need to use like

below only:

void main()

• its just simply means that a main function

has no any return value or any passed

variable

• if we write only main in our program then

the compiler has show some error

because it could not find any return type of

the main function

C data types for the PIC18

Data Type Bit Size Data Range

unsigned char 8-bit 0 to 255

char 8-bit -128 to +127

unsigned int 16-bit 0 to 65535

int 16-bit -32768 to

+32767

*PIC18 has a limited

number of registers and

data RAM locations

Example 1 (unsigned numbers)

#include<htc.h>

void main()

{

unsigned int z;

TRISB = 0;

for (z=0; z<=50000; z++)

{

PORTB = 0x55;

PORTB = 0xAA;

}

while(1);

}

Toogle all bits at

PortB for 50,000

times

Example 2

#include<htc.h>

void main()

{

unsigned short long z;

unsigned int x;

TRISB = 0;

for (z=0; z<=100000; z++)

{

PORTB = 0x55;

PORTB = 0xAA;

}

while(1); //stay here forever

}

Toogle all bits at

PortB for 100,000

times

Time delay

• Two way of creating a time delay in PIC18;

i. using a simple loop program

ii.using PIC18 timer function (discuss later)

Time delay using loop program

• Two factor that effect the delay accuracy:

– Crystal frequency

– The compiler used.

# Assembly lang. can control exact instruction

in delay subroutine.

# In C programs, different compilers produces

different-size HEX code.

• instructions are implemented in one

instruction cycle(except branching, they

are implemented in two instruction cycle).

• We can calculate instruction cycle by

dividing oscillator frequency by 4.

Example

• Let's say we have 4MHz oscillator

connected to our microcontroller. Then,

one instruction cycle will take

4/4Mhz = 1us(micro second).

• Write a C18 program to toggle all bits of

Port B ports continuously with a 250 ms

delay. Assume that the system is

PIC184550 with XTAL=10MHz

Example

Answer#include<htc.h>

__CONFIG(0x3F3A);

void delay(unsigned int);

void main(void)

{

TRISB=0;

PORTB=0;

while(1)

{

PORTB = 0X55;

delay(250);

PORTB = 0xAA;

delay(250);

}

}

void delay(unsigned int tempoh)

{

unsigned int i; unsigned char j;

for (i=0;i<tempoh;i++)

for(j=0;j<165;j++);

}

Exercise

• Assume that a system is PIC18F4550 with

XTAL=20MHz.

• The I/O port preserve be access using 2 method:

a. byte addressable , or

b. bit addressable

Ports PORTA-PORTD are byte accessible

Input and Output (I/O)

Programming in C

Write a C program to get a byte of data

from PORTB, wait 0.5 second, and then

send it to PORTC. Assume XTAL = 4MHz.

Example ( BYTE ADDRESSABLE )

#include<htc.h>

int a;

void main()

{

TRISB = 0xFF;

PORTB = 0x00;

TRISC=0x00;

while(1)

{

a = PORTB;

PORTC = a;

}

}

Answer

Write a C program to get a byte from

PORTC. If it is less than 100, send it to

PORTB; otherwise, send it to PORTD

Example ( BYTE ADDRESSABLE )

#include<htc.h>

void main(void)

{

unsigned char z;

TRISC = 0xFF;

TRISB = 0x00;

TRISD = 0x00;;

while(1)

{

z = PORTC;

if (z < 100)

PORTB = z;

else

PORTD = z;

}

}

Answer

Bit-addressable I/O

programming

• The l/O ports of PIC 18 are bit-addressable. We can

access a single bit without disturbing the rest of the port.

We use PORTxbits. Rxy to access a single bit of Portx,

where x is the port A, B, C, or D, and y is the bit (0-7)

of that port.

• For example, PORTBbits.RB7 indicates PORTB.7. We

access the TRISx registers in the same way where

TRISBbits.TRISB7 indicates the D7 of the TRISB.

Example ( BiT ADDRESSABLE )

Logic Operation and data

conversion in C

• One of the most important and powerful features of the C

language is its ability to perform bit manipulation.

• While every C programmer is familiar with the logical

operators AND (&&), OR (II), and NOT (!), many C

programmers are less familiar with the bitwise operators

AND (&), OR (I), EX-OR (^), inverter (~), shift right

(>>), and shift left(<<)).

Logic Operation and data conversion in C

• These bit-wise operators are widely used

in software engineering for embedded

systems and control; consequently, their

understanding and mastery are critical in

microprocessor-based system design and

interfacing.

Operation Symbol Example

Equal to == if(a == 0) b=b+5;

Not equal to != if(a != 1) b=b+4;

Greater than > if(a > 2) b=b+3;

Less than < if(a < 3) b=b+2;

Greater than or equal to >= if(a >= 4) b=b+1;

Less than or equal to <= if(a <= 5) b=b+0;

Logic Operation and data conversion in C

OPERATION OPERATOR DESCRIPTION SOURCE CODE EXAMPLE RESULT

Increment ++ Add one to integer result = num1++; 0000 0000 0000 00001

Decrement -- Subtract one from integer result = num1--; 1111 1111 1111 1110

Complement ~ Invert all bits of integer result = ~num1; 0000 0000 1111 1111

Arithmetic Operation

Add + Integer or Float result = num1 + num2; 0000 1010

+ 0000 0011

0000 1101

Subtract - Integer or Float result = num1 - num2; 0000 1010

- 0000 0011

0000 0111

Multiply * Integer or Float result = num1 * num2; 0000 1010

*0000 0011

0001 1110

Divide / Integer or Float result = num1 / num2; 0000 1100

/ 0000 0011

0000 0100

Logical Operation

Logical AND & Integer Bitwise result = num1 & num2; 1001 0011

& 0111 0001 0001 0001

Logical OR | Integer Bitwise result = num1 | num2; 1001 0011

|0111 0001

1111 0011

Logical OR ^ Integer Bitwise result = num1 ^ num2; 1001 0011

^ 0111 0001

1110 0010

Data conversion in C

• Recall that BCD numbers were discussed earlier. As

stated there, many newer microcontrollers have a real-

time clock (RTC) where the time and date are kept even

when the power is off.

• Very often the RTC provides the time and date in packed

BCD. To display them, however, it must convert them to

ASCII. In this section we show the application of logic

and rotate instructions in the conversion of BCD and

ASCII.

ASCII numbers

On ASCII keyboards, when the key "0" is activated, "0 II

0000" (30H) is provided to the computer. Similarly, 31H

(0110001) is provided for the key "1", and so on.

• The PICI8 has two to five timers depending on the family

member. They are referred to as Timers 0, I, 2, 3, and 4.

They can be used either as timers to generate a time

delay or as counters to count events happening outside

the microcontroller.

Programming Timer

• Every timer needs a clock pulse to tick. The clock source

can be internal or external. If we use the internal clock

source, then 1/4th of the frequency of the crystal

oscillator on the OSCI and OSC2 pins (Fosc/4) is fed

into the timer.

• Therefore, it is used for time delay generation and for

that reason is called a timer. By choosing the external

clock option, we feed pulses through one of the PICI8‘s

pins: this is called a counter.

Programming Timer

• PIC18 has two to five timers

– Depending on the family number

• These timers can be used as

– Timers to generate a time delay

– Counters to count events happening outside

the uC

Programming timers 0 and 1

• Every timer needs a clock pulse to tick

• Clock source can be

– Internal 1/4th of the frequency of the crystal oscillator

on OSC1 and OSC2 pins (Fosc/4) is fed into timer

– External: pulses are fed through one of the PIC18’s pins

Counter

• Timers are 16-bit wide

– Can be accessed as two separate reg. (TMRxL & TMRxH)

– Each timer has TCON (timer Control) reg.

T0CON (Timer0 control) register

• Each timer has a control register, called TCON, to set

the various timer operation modes. T0CON is an 8-bit

register used for control of Timer0.

Example

TMR0IF flag bit

• Notice that the TMR0IF bit (Timer0 interrupt flag) is part

of the INTCON (interrupt control) register.

• when the timer reaches its maximum value of FFFFH, it

rolls over to 0000, and TMR0IF is set to I.

16-bit timer programming

The following are the characteristics and operations of 16-bit

mode:

1. It is a 16-bit timer; therefore, it allows values of 0000 to

FFFFH to be loaded into the registers TMR0H and TMR0L.

2. After TMR0H and TMR0L are loaded with a 16-bit initial value,

the timer must be started. This is done by "BSF T0CON,

TMR0ON" for Timer0.

3. After the timer is started, it starts to count up. It counts up until

it reaches its limit of FFFFH. When it rolls over from FFFFH to

0000, it sets HIGH a flag bit called TMR0IF (timer interrupt

flag, which is part of the INTCON register). This timer flag

can be monitored. When this timer flag is raised, one option

would be to stop the timer.

4. After the timer reaches its limit and rolls over, in order to

repeat the process, the registers TMR0H and TMR0L

must be reloaded with the original value, and the

TMR0IF flag must be reset to 0 for the next round.

Steps to program Timer0 in 16-bit mode

To generate a time delay using the Timer0 mode 16, the

following steps are taken:

1. Load the value into the T0CON register indicating which mode

(8-bit or 16- bit) is to be used and the selected prescaler

option.

2. Load register TMR0H followed by register TMR0L with initial

count values.

3. Start the timer with the instruction "BSF T0CON, TMR0ON".

4. Keep monitoring the timer flag (TMR0IF) to see if it is raised.

Get out of the loop when TMR0IF becomes high.

5. Stop the timer with the instruction "BCF T0CON, TMR0ON".

6. Clear the TMR0IF flag for the next round.

7. Go back to Step 2 to load TMR0H and TMR0L again.

COUNTER PROGRAMMING

• We used the timers of the PICI8 to generate time delays.

These timers can also be used as counters to count

events happening outside the PIC 18.

• When it is used as a counter, however, it is a pulse

outside the PIC 18 that increments the TH, TL registers.

In counter mode, notice that registers such as T0CON,

TMR0H, and TMR0L are the same as for the timer

discussed in the last section; they even have the same

names.

T0CS bit in T0CON register

• Recall from the last section that the T0CS bit (Timer0 clock

source) in the T0CON register decides the source of the clock

for the timer. If T0CS = 0, the timer gets pulses from the

crystal oscillator connected to the OSC I and OSC2 pins

(Fosc/4).

• In contrast, when T0CS = I, the timer is used as a counter and

gets its pulses from outside the PIC 18. Therefore, when

T0CS = I, the counter counts up as pulses are fed from pin

RA4 (PORTA.4). The pin is called T0CKI (Timer0 clock input).

Notice that the pin belongs to Port A.

• In the case of Timer0, when T0CS = I, pin RA4 (PORTA.4)

provides the clock pulse and the counter counts up for each

clock pulse coming from that pin. Similarly, for Timer I, when

TMRI CS = I, each clock pulse coming in from pin RC0

(PORTC.0) makes the counter count up.