🛠 Bouton poussoir et moniteur série sur la Duemilanove (avec ses 0,5 s de silence)

Objectives

Read a push button and display its state on the serial monitor — the famous DigitalReadSerial — while discovering two documented specifics of the Duemilanove: the 20 to 50 kΩ internal pull-ups activatable by software, and the auto-reset via the FTDI DTR line: each time the port is opened, the board resets and the ATmegaBOOT bootloader swallows the first bytes for ~0.5 seconds. Your automation scripts must keep this in mind.

Hardware

• 1 Arduino Duemilanove (ATmega328 recommended)
• 1 push button (4 pins: pairs are connected two by two)
• 1 male-to-male wire
• Mini-USB cable; FTDI driver installed

Complete code

// Bouton sur broche 2 -> GND, pull-up interne active
const byte BOUTON = 2;      // les interruptions externes sont aussi sur 2 et 3

void setup {
  pinMode(BOUTON, INPUT_PULLUP);   // pull-up interne 20-50 kOhm
  Serial.begin(9600);              // USB via FTDI FT232RL
  delay(1000);                     // laisser passer le bootloader (~0,5 s)
  Serial.println("Duemilanove prete");
}

void loop {
  // rappel : avec pull-up, bouton enfonce = LOW
  byte etat = digitalRead(BOUTON);
  Serial.println(etat == LOW ? "ETAT=0 (appuye)" : "ETAT=1 (libere)");
  delay(100);
}

Expected verification

The serial monitor (9600 baud) continuously displays "ETAT=1 (libere)" [STATE=1 (released)]; holding the button down changes the lines to "ETAT=0 (appuye)" [STATE=0 (pressed)]. Each time the monitor is opened/closed, the board blinks (DTR reset) and the line "Duemilanove prete" [Duemilanove ready] reappears: this is the normal behavior of the auto-reset, not a bug.

Common errors

• Waiting for data in the half-second following the port opening: the ATmegaBOOT bootloader keeps the port for itself and the first bytes sent by the host are lost — wait ~1 s before talking to the board (documented on the official page).
• Button wired to 5 V without an external resistor while the pull-up is not activated in the code: the input floats and the state changes on its own.
• Inverting the logic: with INPUT_PULLUP, button pressed = LOW; many beginners test == HIGH and think the button is dead.
• Button on pins 0 or 1: they are connected to the FTDI (USB RX/TX) and hijack the serial monitor — prefer 2 and 3 (which are also the two external interrupts).
• Wrong baud rate on the monitor: the code opens at 9600, set the monitor to 9600.
• On older PCs, the COM port changes with each different USB port — check in the Device Manager.
• Exceeding 40 mA per pin (official spec): here the button consumes nothing, but do not plug an LED without a resistor out of habit.