Interfacing LED with PIC Microcontroller (PIC18F4520)

Aim:

It is necessary to understand basic I/O operations of PIC18F4520 before dealing with its complexities. This article presents a way to take simple output from a PIC microcontroller. This learning would also help in interfacing of external devices with the controller. Here the output from the microcontroller is taken on a set of LEDs which are made to blink in an alternate fashion.

Description:

PIC18F4520 has a total of 35 I/O (input-output) pins which are distributed among 5 Ports. The following table shows the names and numbers of I/O pins of these 5 ports:

Port Name	Number of Pins	Pins
PORTA	7	RA0-RA6
PORTB	8	RB0-RB7
PORTC	8	RC0–RC7
PORTD	8	RD0-RD7
PORTE	4	RE0-RE3

As opposed to a basic 8051 microcontroller like AT89C51 which has most of the port pins serving single function, the port pins of a PIC microcontroller are multiplexed to serve more than one purpose.

The 35 I/O pins of PIC18F4520 are also multiplexed with one or more alternative functions of controller’s various peripherals. Each Port of a PIC microcontroller corresponds to three 8-bit registers which should be configured to use the Port for general I/O purpose. These registers are:

1.            TRISx: This is a data direction register which sets the direction of each port pin as input or output.

2.            PORTx: This register stores the input level of pins (High or Low). When a pin configured as input, the input signal from external source is read from PORTx register.

3.            LATx: This is output latch register. The data which has to be sent to external hardware as output is stored in LATx register.

Port Description:

PORTA:

PortA has 7 pins which can be used as both input as well as output pin. The 7th bit is missing from all the three registers. The input and output given to this port are of 8-bit but the 8th bit is internally masked.

	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
TRISA	–	TRISA6	TRISA5	TRISA4	TRISA3	TRISA2	TRISA1	TRISA0
PORTA	–	RA6	RA5	RA4	RA3	RA2	RA1	RA0
LATA	–	LATA6	LATA5	LATA4	LATA3	LATA2	LATA1	LATA0

PORTB:

PortB has 8 pins which can all be used for both input and output operation.

	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
TRISB	TRISB7	TRISB6	TRISB5	TRISB4	TRISB3	TRISB2	TRISB1	TRISB0
PORTB	RB7	RB6	RB5	RB4	RB3	RB2	RB1	RB0
LATB	LATB7	LATB6	LATB5	LATB4	LATB3	LATB2	LATB1	LATB0

PORTC:

PortC has 7 I/O pins. In PortC, Bit 3 is missing in hardware and Pins 4 & 5 can only be used as input pins. There are no 4th & 5th latch bits in LATC register, so these bits are internally masked during 8-bit write operation on PortC.

	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
TRISC	TRISC7	TRISC6	–	–	–	TRISC2	TRISC1	TRISC0
PORTC	RC7	RC6	RC5	RC4	–	RC2	RC1	RC0
LATC	LATC7	LATC6	–	–	–	LATC2	LATC1	LATC0

PORTD:

PortD has 8 pins which can all be used for both input and output operation.

	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
TRISD	TRISD7	TRISD6	TRISD5	TRISD4	TRISD3	TRISD2	TRISD1	TRISD0
PORTD	RD7	RD6	RD5	RD4	RD3	RD2	RD1	RD0
LATD	LATD7	LATD6	LATD5	LATD4	LATD3	LATD2	LATD1	LATD0

PORTE:

PortE has 4 I/O pins. Pin3 can be used as input pin only. RDPU bit is used to enable/disable internal pull-ups of PortD.

	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
TRISE	–	–	–	–	–	TRISE2	TRISE1	TRISE0
PORTE	RPDU	–	–	–	RE3	RE2	RE1	RE0
LATE	–	–	–	–	–	LATE2	LATE1	LATE0

I/O configuration:

The TRISx register is configured to set a pin as input or output. The High value (1) sets a pin as input pin and Low value (0) sets a pin as output. An easy way to remember this is to consider the resemblance of 1 with the letter I (for input) and 0 with the letter O (for output).

For example suppose a switch is connected at RB0 and an LED is connected to RB7 of PortB. Now the pins 0 & 7 have to be configured as input and output respectively. So the bit TRISB0 is set to 1 to configure RB0 as input pin & bit TRISB7 is set to 0 to configure RB7 as output pin.

TRISB	TRISB7	TRISB6	TRISB5	TRISB4	TRISB3	TRISB2	TRISB1	TRISB0
Value	0	…	…	…	…	…	…	1

The unused bits are set to 0.

TRISB	TRISB7	TRISB6	TRISB5	TRISB4	TRISB3	TRISB2	TRISB1	TRISB0
Value	0	0	0	0	0	0	0	1

So the overall value of TRISB register becomes:

Using this knowledge, the objective of this article can be achieved which is to glow a set of LEDs in alternate blinking fashion. The LEDs here are connected to PORTB pins as shown in the circuit diagram.

Programming steps:

1.      Configure the TRISB register to make PortB as output port.

2.      Set all the bits of LATB register High (1) to glow all LEDs.

3.      Provide some delay.

4.      Set all the bits of LATB register Low (0) to turn off the LEDs.

5.      Provide some delay.

6.      Repeat the process from step 2.

Pin Assignment with atmega8

	Point LEDs	PIC18F4520  direction	PIC18F4520 Lines	LED Selection	Connections
DIGITAL OUTPUTS	LED.0	TRISx=0xfe	PORTx=0x01		Connect Any one PORT from PORTA,PORTB, PORTC, PORTD
	LED.1	TRISx=0xfd	PORTx=0x02
	LED.2	TRISx=0xfb	PORTx=0x04
	LED.3	TRISx=0xf7	PORTx=0x08
	LED.4	TRISx=0xef	PORTx=0x10
	LED.5	TRISx=0xdf	PORTx=0x20
	LED.6	TRISx=0xbf	PORTx=0x40
	LED.7	TRISx=0x7f	PORTx=0x80

Block Diagram

Schematic:

 

Code:

// **********************************************
// Project: Interfacing Single LED to PIC18F4520
// Author: Hack Projects
// Module description: Operate single LED
// **********************************************
#include<p18f4520.h>
#pragma config OSC=HS, FCMEN=ON, WDT=OFF, IESO=OFF, XINST=OFF, LVP=OFF
void main(void)
{
ADCON1=0x0F;
TRISB=0;
for(;;)
{
PORTB=0x01;
}
}

Downloads:




The code was compiled in Atmel Studio 6 and simulation was made in Proteus v7.7.
To download code and proteus simulation click here.



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