Button Detection

1. Button Detection Overview

GPIO input is the most basic way for an MCU to interact with the outside world. This experiment reads the level of the BOOT button (GPIO0) to control the LED on/off with the button — press once to turn on, press again to turn off. The source code demonstrates the complete flow of GPIO input, software debouncing, edge detection, and a polling architecture.

Typical application scenarios:

2. Core Concepts

2.1 Button Bounce and Debouncing

When a mechanical button is pressed/released, the metal contacts bounce repeatedly for 5~20ms, generating a series of glitch pulses:

Ideal waveform:  ────┐             ┌────
              └────────────┘
Actual waveform:  ────┐ ┌┐ ┌┐ ┌┐ ┌┐ ┌┐ ┌┐ ┌────
              └┘ └┘ └┘ └┘ └┘ └┘ └┘
               ├─ bounce period ─┤
              (typically 5~20ms)

Software debouncing: after detecting a level change, wait 20ms before confirming, skipping the bounce region.

2.2 Button Connection and Pull-Up/Pull-Down

The BOOT button on this board has one end connected to GND and the other end directly to GPIO0:

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GND → Button → GPIO0

Pin.PULL_UP must be enabled, otherwise the pin floats when released and the level jumps randomly.

2.3 Edge Detection Logic

What is detected is the falling edge from "released" to "pressed":

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if last_key == 1 and current_key == 0:
    time.sleep_ms(20)       # avoid bounce
    if key.value() == 0:    # double-check
        led.toggle()        # toggle LED
last_key = current_key      # must update, otherwise detection fails next time

Only the falling edge (1→0) is detected to avoid repeated triggering while holding the button.

2.4 Core API — machine.Pin Input Mode

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key = Pin(0, Pin.IN, Pin.PULL_UP)  # input + internal pull-up
val = key.value()                   # read level: 0 or 1

3. Hardware Dependencies

The BOOT button is onboard; no external components are required.

4. Project Architecture and Flow

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Script execution (single-file .py)
  │
  ├─ led = Pin(43, Pin.OUT)          // GPIO43 output, on by default (0=on)
  ├─ key = Pin(0, Pin.IN, PULL_UP)   // GPIO0 input + pull-up
  ├─ led_state = 0, last_key = 1     // state variable initialization
  │
  └─ while True:                     // polling loop
       ├─ current_key = key.value()   // read button
       ├─ if last==1 and cur==0:      // falling edge detection
       │    ├─ sleep_ms(20)            // debounce
       │    ├─ if key.value() == 0:    // double-check
       │    │    ├─ led_state ^= 1     // toggle LED state
       │    │    └─ led.value(state)   // update LED
       └─ last_key = current_key      // update historical state

5. Source Code Details

Full source code: 5.Source Code → MicroPython → 1.Basic Peripherals → 2.KEY_LED → 2.KEY_LED.py

5.1 Import Modules and Initialization

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from machine import Pin  # Pin class: GPIO control
import time              # delay functions

led = Pin(43, Pin.OUT)                  # GPIO43 push-pull output, common-anode: 0=on,1=off
key = Pin(0, Pin.IN, Pin.PULL_UP)       # GPIO0 input + internal pull-up

led_state = 0  # LED state: 0=on, 1=off
last_key = 1   # last key reading

5.2 Main Loop — Polling with Debounce

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while True:
    current_key = key.value()  # read current key level

    # detect button press: falling edge 1→0, with debounce
    if last_key == 1 and current_key == 0:
        time.sleep_ms(20)       # 20ms debounce
        if key.value() == 0:    # confirm still pressed
            led_state = not led_state  # toggle LED
            led.value(led_state)       # apply to LED

    last_key = current_key  # save for next iteration

6. Operation Procedure

6.1 Flashing and Running

Thonny IDE steps: See the 1. Before Development → 3.Thonny IDE chapter.

1. Connect the development board with a USB cable

2. Select the ESP32S3 interpreter at the bottom right of Thonny

3. Open 2. KEY_LED.py → click the green Run button

4. Press the onboard BOOT button and observe the LED

6.2 Normal Phenomenon

• After power-up, the LED is on (led_state=0, 0=on in common-anode)

• Press BOOT once → LED off

• Press again → LED on

• A complete trigger is registered only after releasing and pressing again

6.3 Verification Methods

7. Common Problems