08. GPIO Input — Reading Buttons/Switches

What you'll learn

• 1Explain why you write 1 before reading on a quasi-bidirectional port
• 2Read an input pin via sbit and branch
• 3Understand pull-up/active-low buttons (1 idle, 0 pressed)
• 4Mirror button state onto an LED
• 5Detect edges (1→0) to catch only the press instant

Introduction

Reading digital signals from buttons·switches·sensors so the program reacts. Understanding the 8051 quasi-bidirectional-port trap is the heart of this lesson.

Key concepts

1) Quasi-bidirectional — write 1 before reading

8051 pins share an output latch and input buffer. With 0 latched, the pin is strongly held Low and external High won't read. Write 1 to "release" pins you'll use as input.

sbit BTN = P3 ^ 2;
BTN = 1;            /* prepare input: pin to 1 (weak pull-up) */
if (BTN == 0) { }   /* now external 0 reads */

2) Pull-up + active-low button

So "pressed" is `BTN == 0`.

3) Mirroring vs edge detection

if (prev_btn == 1 && btn == 0) {   /* falling edge = just pressed */
    led = !led;                    /* toggle (once) */
}
prev_btn = btn;

Core example

#include <reg52.h>
sbit BTN = P3 ^ 2;   /* input */
sbit LED = P1 ^ 0;   /* output (active-low) */
void main(void) {
    BTN = 1;                 /* prepare input */
    while (1) {
        if (BTN == 0) LED = 0;   /* pressed → LED ON */
        else          LED = 1;   /* released → LED OFF */
    }
}

Simulator: click the P3.2 bit in the Port 3 window to "press" the button and watch P1.0 follow in Port 1.

Common mistakes

Q. The input always reads the same value.

A. The quasi-bidirectional trap. If you ever wrote output 0 to that pin, input is blocked. Write BTN = 1; before reading.

Q. One press toggles the LED several times.

A. A too-fast loop mistakes "while held" for many toggles → use edge detection. If it's chatter, you need debouncing (lesson 21).

Q. if (P3 & 0x04 == 0) looks wrong.

A. Precedence trap: == binds before &, so 0x04==0 is first. Parenthesize: if ((P3 & 0x04) == 0), or use sbit: if (BTN == 0).

Summary

• A quasi-bidirectional port needs a 1 written before reading as input
• Pull-up + active-low button: 1 idle, 0 pressed — test BTN == 0
• Mirroring follows the current state; edge detection catches the press instant
• Edge detection compares previous and current state (prev==1 && cur==0)
• Contact bounce is solved by debouncing (lesson 21)

Exercises

1. Write edge-detection code that moves the lit LED one step per press
2. Predict the toggle result for {1,1,0,0,1,0} by hand, then verify in pc_test
3. Read two buttons (P3.2/P3.3) to control two LEDs independently