🛠 Ajouter un deuxième port série avec SoftwareSerial (GPS sur broches 10/11)

Objectives

The Duemilanove has only one hardware UART (pins 0/1), and it is already occupied by the FTDI USB-to-serial bridge. The official page states this explicitly: "the SoftwareSerial library allows serial communication on any digital pin." You will connect a GPS to pins 10/11 and create a bidirectional serial bridge between the GPS and the USB monitor — the technique that has allowed generations of GPS/GSM/MP3 projects to live on AVR.

Hardware

• 1 Arduino Duemilanove (ATmega328 recommended: the 1 KB of SRAM on the 168 limits program size)
• 1 TTL GPS module (e.g., NEO-6M, 4800 or 9600 baud) — at 5 V it is compatible with the Duemilanove's 5 V logic, no level shifter needed
• 4 male-to-male wires, mini-USB cable

Complete code

// Pont moniteur USB <-> GPS sur SoftwareSerial (RX=10, TX=11)
#include <SoftwareSerial.h>

SoftwareSerial gps(10, 11);   // RX, TX

void setup {
  Serial.begin(9600);         // USB (FTDI)
  gps.begin(4800);            // NEO-6M par defaut : 4800
  Serial.println("Pont GPS actif — attendez le fix");
}

void loop {
  if (gps.available) Serial.write(gps.read);   // GPS -> moniteur
  if (Serial.available) gps.write(Serial.read); // moniteur -> GPS
}

Expected verification

The serial monitor (9600) displays NMEA sentences $GPGGA, $GPRMC… as soon as the GPS acquires a signal (a few minutes for a cold start, near a window). Sending a UBX command or a valid sentence from the monitor forwards it to the GPS. The GPS status LED blinks every second.

Common errors

• Believing SoftwareSerial is full-duplex at 9600+ on AVR: it works at 4800; at 9600 and above, bytes are lost if loop does anything else — this is the physical limit of software emulation on a 16 MHz AVR.
• Reversing the crossover: GPS TX → pin 10 (software RX), GPS RX ← pin 11 (software TX); without crossover, there is total silence.
• Using pin 11 for PWM at the same time: SoftwareSerial on 10/11 monopolizes the timer used by PWM on pins 9/10/11 — analogWrite calls on these pins degrade.
• Connecting the GPS to pins 0/1: direct conflict with the FTDI, the monitor and the GPS talk over each other, and uploading becomes impossible.
• Choosing pins 2/3 for the GPS: possible, but they are often reserved for external interrupts; 10/11 is the classic choice.
• Expecting a fix indoors: without a view of the sky, sentences arrive empty — go outside to test.
• Forgetting the delay(1000) after opening the port in a host script: the DTR auto-reset causes the loss of the first half-second (see the button tutorial).