GPIO - TRIS, PORT and LAT

This page covers how to use TRIS, PORT, and LAT together in real code and UNEXPO labs. It builds on the ports guide and First LED — they come before this page in the sidebar and study path.

What GPIO is on the PIC18

GPIO (General Purpose Input/Output) means pins your program configures as digital inputs or outputs. On the PIC18F4550 you do not wire each pin separately to the CPU — you control ports: groups of 8 pins (A, B, C, D, E) through three SFRs per port.

Step 1 — Physical pin ↔ register bit

Each pin has a fixed hardware name:

Pin	Port	Bit in `TRISx` / `LATx` / `PORTx`
RB0	B	bit 0
RB3	B	bit 3
RD7	D	bit 7

Course rule: bit 0 = pin number 0. BCF TRISB, 0 affects only RB0; TRISB bits 1–7 do not change (Architecture — SFR).

Step 2 — Three registers, three roles

Register	Write?	Purpose
`TRISx`	Yes	Pin direction: 1 = input, 0 = output
`LATx`	Yes	Output value in the internal latch
`PORTx`	Read	Actual pin level (inputs, switches)

Do not swap roles: TRIS first, then LAT for outputs or PORT to read inputs.

Step 3 — Typical program flow

  Start / init_gpio
       ↓  TRISx — define inputs and outputs
       ↓  LATx  — initial output state (LED off, etc.)
  main loop
       ↓  PORTx — read switches / buttons
       ↓  LATx  — update LEDs / actuators
       ↓  GOTO loop

In sandwich style, init_gpio is usually a subroutine above; programa: calls CALL init_gpio and enters the loop.

Step 4 — How it fits with config bits

Config bits (PBADEN = OFF, LVP = OFF…) prepare hardware at power-on. GPIO registers (TRISB, LATB…) are configured while your code runs. If the LED does not respond, check both layers: crystal/FOSC and BCF TRISB, 0.

LED circuit on RB0 with current-limiting resistor — Digital output: TRISB,0 = 0 (output) and LATB,0 = 1 turn on the LED on RB0. — LED lab — UNEXPO

Registers per port — TRIS, PORT, and LAT

Port	Pins	TRIS	PORT	LAT	Course notes
A	RA0–RA5	TRISA	PORTA	LATA	RA4 not analog; several pins can be ANx
B	RB0–RB7	TRISB	PORTB	LATB	RB0 = INT0; RB4–RB7 = change interrupt
C	RC0–RC7	TRISC	PORTC	LATC	RC6/RC7 = UART (TX/RX)
D	RD0–RD7	TRISD	PORTD	LATD	LCD in labs
E	RE0–RE2	TRISE	PORTE	LATE	Analog inputs RE0–RE2

All three registers share bit numbering: bit 3 of TRISB, PORTB, and LATB is always pin RB3.

Step 1 — Set direction with TRIS

Before reading or writing, declare each pin as input or output.

One pin at a time (best while learning):

        BCF     TRISB, 0        ; RB0 → output (bit = 0)
        BSF     TRISB, 3        ; RB3 → input (bit = 1)

Whole port at once (Practice 1 — PORTB and PORTD as inputs):

        MOVLW   B'11111111'     ; all bits = 1 → all inputs
        MOVWF   TRISB
        MOVWF   TRISD

In TRISx, 1 = input and 0 = output. If TRISB = 0xFF, all of PORTB is input — classic partial exam question.

More exercises in Set direction with TRIS.

Step 2 — Write outputs with LAT

To drive an LED, relay, or digital line, write LATx, not PORTx:

        CLRF    LATB            ; clear all PORTB outputs
        BCF     TRISB, 0        ; RB0 output
        BSF     LATB, 0         ; RB0 high → LED ON
        BCF     LATB, 0         ; RB0 low → LED OFF

BSF/BCF on LATB touch one bit only — useful when inputs and outputs share a port (Bits and hex — masks).

Practical guide: Write outputs with LAT and First LED tutorial.

Step 3 — Read inputs with PORT

For switches, buttons, or digital sensors, read PORTx:

        BSF     TRISB, 3        ; RB3 input
        BTFSS   PORTB, 3        ; skip if RB3 = 1 (released, with pull-up)
        GOTO    button_pressed     ; jump → destination button_pressed:
        ; RB3 = 1 — button not pressed ...
        GOTO    done_read
button_pressed:
        ; RB3 = 0 — button pressed ...
done_read:

BTFSS PORTB, n / BTFSC PORTB, n are the most common course pattern. You can also MOVF PORTB, W and mask the bit (Bits and hex).

Practical guide: Read inputs with PORT.

Step 4 — PORT vs LAT — key difference

Action	Correct register	Typical mistake
Turn on LED on RB0	`BSF LATB, 0`	`BSF PORTB, 0`
Read button on RB3	`BTFSS PORTB, 3`	`BTFSS LATB, 3`
Turn off all of PORTC	`CLRF LATC`	`CLRF PORTC`

On pure outputs, writing PORTx sometimes “works” because it shares hardware with the latch, but in the course and datasheet the correct practice is LATx to write and PORTx to read.

Configure several pins at once

You want RB0–RB3 as outputs and RB4–RB7 as inputs:

        MOVLW   B'11110000'     ; bits 7..4 = 1 (input), 3..0 = 0 (output)
        MOVWF   TRISB

Equivalent pin by pin:

        BCF     TRISB, 0
        BCF     TRISB, 1
        BCF     TRISB, 2
        BCF     TRISB, 3
        BSF     TRISB, 4
        BSF     TRISB, 5
        BSF     TRISB, 6
        BSF     TRISB, 7

For TRIS, design the mask inverted vs what you want as output: output = 0, input = 1.

Analog pins and digital GPIO

Several PORTA/PORTB pins start as analog inputs if PBADEN = ON in config bits. For course digital GPIO use PBADEN = OFF in the CONFIG block (Config bits — PBADEN).

If you mix ADC and digital in one lab, set ADCON1 to declare which pins are analog and which digital — otherwise PORTx may read nonsense even when TRISx is correct.

Full walkthrough — LED on RB0

Minimal fragment linked to Assembly — LED example:

        LIST    P=18F4550
        #include <P18F4550.INC>
        CONFIG  FOSC = HS, WDT = OFF, LVP = OFF, PBADEN = OFF

        ORG     0x0000
        GOTO    programa         ; jump to main

init_gpio:
        CLRF    LATB
        BCF     TRISB, 0         ; RB0 output
        RETURN

programa:
        CALL    init_gpio
loop:
        BSF     LATB, 0          ; LED ON
        GOTO    loop             ; jump → destination loop: (above)

        END

Flow: config bits (hardware at power-on) → TRIS (direction) → LAT (output state) → simulate in Proteus with the .HEX from MPLAB.

Common mistakes

Mistake	Symptom	Fix
Write `PORTx` instead of `LATx`	Erratic output or odd reads	Use `BSF`/`BCF`/`CLRF` on `LATx`
Skip `TRISx` setup	Pin does not behave as expected	Always TRIS before LAT/PORT
`PBADEN = ON` without `ADCON1`	Digital GPIO “dead” on RA/RB	`PBADEN = OFF` or configure `ADCON1`
LED without resistor	Overloaded pin	220 Ω or 330 Ω in series
Mix bit and pin	“Set bit 3 but watch RB0”	Bit n = pin Rn on the same port

UNEXPO Practice 1 — how ports fit

Practica · Laboratory

UNEXPO Practice 1

PORTB and PORTD as inputs (operand switches). PORTC and PORTE as outputs (result LEDs). RA0 as a button to change operation. Implement add, subtract, multiply, OR, AND, XOR, complement, rotate, and software division. Before the lab, review the ALU guide if you use the PIC ALU.

Next step — buttons

When a pin is an input with a push button, you need pull-up and debounce. That is Buttons, pull-up, and debounce.

Exam-style questions

What does TRISB = 0xFF mean?

All of PORTB configured as input (every TRIS bit is 1).

Why write the LED on LATB and read the button on PORTB?

LATB controls the desired output of the latch; PORTB reflects the actual level on the pin — required for inputs with pull-up or external wiring.

What happens if you forget BCF TRISB, 0 before BSF LATB, 0?

RB0 may still be an input; the latch changes but the pin does not drive the LED correctly.

Ejercicio · Parcial

Trace the flow

In Proteus, change BSF LATB, 0 to BSF PORTB, 0 with RB0 as output. Does the LED behave the same? Note why the course prefers LATB.

Tutor IA