Tutorial — First LED

Learn by programming: follow each step, write your own code, then validate with the lab questions and exam. Adapt pins and routines to your board when needed.

What you will build

An assembly program that configures RB0 as output and turns on an LED. This is the foundation for most of the course.

1
Create the MPLAB project

Open MPLAB IDE 8.88 → File → New Project. Select PIC18F4550, MPASM compiler, and your programmer (PICkit 2/3) or Simulator if you have no board yet. Create a `.asm` file in the project.

Tip: Setup guide: /en/fundamentos/mplab-proteus/
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2
Write CONFIG directives

At the top of the file declare the processor and disable the watchdog. With a 20 MHz crystal use `FOSC = HS`. These lines do not generate executable code—they tell the assembler how to configure the PIC when programming.
```
								        LIST    P=18F4550
        #include <P18F4550.INC>

        CONFIG  FOSC = HS
        CONFIG  WDT = OFF
        CONFIG  LVP = OFF
							
```
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3
Set RB0 as output

`TRISB` sets direction: bit `1` = input, bit `0` = output. To drive an LED on RB0 you need that pin as output. Use `BCF TRISB, 0` (clears bit 0). Clear `LATB` first for a known state.
```
								inicio:
        CLRF    LATB        ; Apaga todas las salidas de PORTB
        BCF     TRISB, 0    ; RB0 como salida
							
```
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4
Turn the LED on via LATB

Write to `LATB`, not `PORTB`, to set the output level. `BSF LATB, 0` drives RB0 high. If your LED goes RB0 → resistor → cathode to ground, it should light up.
```
								        BSF     LATB, 0     ; RB0 en alto — LED encendido

bucle:
        GOTO    bucle       ; El PIC no debe “terminar”
							
```
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5
Simulate in Proteus and adapt to your build

Import PIC18F4550, 20 MHz crystal, LED on RB0 with 220–330 Ω resistor. Build in MPLAB → load `.hex` in Proteus. Adapt the pin if your board uses another port: change `TRISB`/`LATB` and the matching bit.

Tip: Exercise: move the LED to RB1. Which two lines change?
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Before you code — pre-lab

Pre-laboratory — reference research

Introduction to your first assembly program: digital GPIO outputs. Foundation before formal UNEXPO labs.

Q 1 What does the TRISB register do?

TRISB sets each PORTB pin direction:

- Bit = 1 → input (high impedance).
- Bit = 0 → output (driven by LATB/PORTB).

To drive an LED on RB0 use `BCF TRISB, 0` (bit 0 = output). Without TRIS setup the pin may stay input and the LED won't respond.

Q 2 PORTB vs LATB when writing

For digital outputs write LATB (*latch*): the value is stored without read-modify-write issues on PORT when the pin reads external level.

`BSF LATB, 0` drives RB0 high (LED on if cathode to ground with resistor). `BCF LATB, 0` turns it off.

Q 3 LED wiring with resistor

Typical wiring: RB0 → 220–330 Ω resistor → LED anode → LED cathode → GND.

The resistor limits current (~10–15 mA) to protect the LED and stay within the PIC pin limit (~25 mA max per pin; less is better).

This lab is shorter but uses the same design flow (pseudocode, flowchart, exam).

Design your solution

Write your pseudocode

Include: CLRF LATB, BCF TRISB,0, BSF LATB,0 and infinite loop.

0 characters (minimum 80)

My pseudocode draft

My flowchart draft

Reference pseudocode

START
    LATB ← 0
    TRISB.0 ← 0          ; RB0 output
    LATB.0 ← 1           ; LED on
REPEAT FOREVER
    ; (future: blink with delay)
END REPEAT

Reference flowchart

First LED practice flowchart — Minimal flow: set TRIS → turn LED on → loop.

Build and adapt the code

Practica · Beginner

Turn on an LED on RB0

Goal: configure RB0 as output and write a high level to turn on an LED connected correctly with its resistor.

LED on RB0 schematic with 330 ohm resistor — Basic wiring: RB0 as output, 220-330 ohm resistor and LED to ground. — First LED practice — UNEXPO

Full reference code

        LIST    P=18F4550
        #include <P18F4550.INC>

        CONFIG  FOSC = HS
        CONFIG  WDT = OFF
        CONFIG  LVP = OFF

        ORG     0x0000
        GOTO    start

start:
        CLRF    LATB        ; Clear PORTB outputs
        BCF     TRISB, 0    ; RB0 as output
        BSF     LATB, 0     ; RB0 high

loop:
        GOTO    loop

        END

What you should be able to explain

Why TRISB is used.
Why BCF TRISB, 0 makes RB0 an output.
Why the code writes to LATB and not necessarily to PORTB.
Why the program ends in an infinite loop.

Exercise

Modify the program so the LED is on RB1. Then explain which lines changed and why.

After building — post-lab

Post-laboratory — reference research

Reflection after assembling and simulating your first program.

Q 1 Why does the program end in an infinite loop (`GOTO bucle`)?

A microcontroller has no OS holding control: after `inicio` the CPU keeps fetching instructions. Without a loop it would run uninitialized memory. `GOTO bucle` keeps the LED state until reset or reprogramming.

Q 2 What would you change to blink the LED every 500 ms?

You must toggle LATB.0 periodically with a delay (~500 ms via nested loops or Timer0). Pseudocode:

`loop: toggle RB0 → wait 500ms → goto loop`

This introduces timing, covered with Timer0 in the course.

Try moving the LED to RB1 and document the line changes.

Check what you learned

Tutorial hub · Exercises

Tutor IA

Tutorial — First LED

Create the MPLAB project

Write CONFIG directives

Set RB0 as output

Turn the LED on via LATB

Simulate in Proteus and adapt to your build