🛠 Temps réel : timers matériels et interruptions sur le GIGA R1

Objectives

Move beyond the "delay everywhere" model to generate an audio signal at an exact rate. The GIGA R1's 480 MHz Cortex-M7 features hardware timers and interrupts; we will use a hardware ticker (mbed::Ticker, exposed by the Arduino Mbed core) to sample a waveform on the 12-bit DAC while loop remains available. You will also learn about interrupt precautions related to documented bugs (#789 regarding attachInterrupt).

Hardware

• 1 Arduino GIGA R1 WiFi board
• 1 pair of headphones or an amplifier connected to the onboard jack (DAC0)
• 1 logic analyzer or a simple LED on a pin to observe the rate (optional)

Complete Code

// GIGA R1 WiFi — ton precis genere par ticker materiel, loop reste libre
#include <Arduino.h>
#include "mbed.h"

const int DAC_PIN = DAC0;          // jack audio embarque
mbed::Ticker ticker;
uint32_t phase = 0;
const uint32_t PAS = 32;           // pas de phase -> frequence du ton
volatile uint32_t compteur = 0;    // partage ISR <-> loop : volatile obligatoire

void echantillonner {            // s'execute en contexte d'interruption
  phase = (phase + PAS) & 0xFFF;   // onde 12 bits
  analogWrite(DAC_PIN, phase);
  compteur++;
}

void setup {
  Serial.begin(115200);
  analogWriteResolution(12);
  ticker.attach(echantillonner, 0.0001);  // 10 kHz constants
}

void loop {
  static uint32_t dernier = 0;
  if (compteur - dernier >= 10000) {      // ~1 s d'echantillons a 10 kHz
    dernier = compteur;
    Serial.print("echantillons 10 kHz : ");
    Serial.println(compteur);
    // loop peut faire autre chose : capteurs, reseau, I2C...
  }
}

Expected Verification

A continuous and stable sound comes out of the jack, without crackling even if loop performs long processing tasks. The Serial Monitor displays a counter that increases by exactly 10,000 per second, proving that the rate is maintained by the hardware timer and not by loop.

Common Errors

• Forgetting volatile on a variable shared between the ISR and loop: the compiler optimizes it away, and the loop never sees the updates.
• Blocking calls (Serial.print, delay) inside the interrupt routine: causes blocking and crashes; the ISR must remain as short as possible.
• attachInterrupt on poorly supported pins or modes: crashes have been documented (bug #789, closed); prefer mbed tickers when possible and update the core.
• Confusion between hardware ticker / software Ticker (millis): only the hardware ticker guarantees the rate under load.
• ISR frequency too high with the DAC: the M7 core is fast (480 MHz), but writing analogWrite in the ISR has a cost; measure before increasing it.