How to interface LEDs with PIC18F4550 Microcontroller




It is necessary to understand basic I/O operations of PIC18F4550 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.

Interfacing-LED-with-PIC

PIC18F4550 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
7
RC0-RC2, RC4-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 PIC18F4550 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.




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