🛠 Tracer une courbe d'accélération en temps réel

Objectives

Draw the acceleration magnitude curve in real time on the 240×135 screen (rotation 1) using a circular buffer of 240 samples — one per pixel column — and M5GFX graphics primitives (drawLine, fillRect). This is a fundamental technique for any oscilloscope or chart recorder on this stick.

Hardware

• 1× M5StickC Plus2 (v2)
• USB-C cable for uploading
• Arduino IDE / PlatformIO, M5Unified library (includes M5GFX)

Complete Code

// Oscilloscope d'accélération — tampon circulaire 240 colonnes (écran 240x135)
#include <M5Unified.h>

const int W = 240, H = 135, N = 240;
float buf[N];
int tete = 0;                 // prochaine position d'écriture
const float GMIN = -0.5f, GMAX = 3.0f;   // échelle verticale en g

int yDe(float g) {            // g -> coordonnée écran
  if (g < GMIN) g = GMIN; if (g > GMAX) g = GMAX;
  return H - 1 - (int)((g - GMIN) / (GMAX - GMIN) * (H - 1));
}

void setup {
  auto cfg = M5.config;
  M5.begin(cfg);
  M5.Display.setRotation(1);
  M5.Display.fillScreen(TFT_BLACK);
  M5.Display.setTextColor(TFT_DARKGREY, TFT_BLACK);
  M5.Display.setTextSize(1);
  for (int g = 0; g <= 3; g++)          // grille : 0, 1, 2, 3 g
    M5.Display.drawFastHLine(0, yDe(g), W, TFT_DARKGREY);
}

void loop {
  M5.update;
  float ax, ay, az;
  M5.Imu.getAccel(&ax, &ay, &az);
  float norme = sqrtf(ax * ax + ay * ay + az * az);
  int yAvant = yDe(buf[tete]);          // ancienne valeur de la colonne
  buf[tete] = norme;
  int yApres = yDe(norme);
  M5.Display.drawLine(tete, yAvant, tete, yApres, TFT_BLACK);  // efface colonne
  M5.Display.drawLine(tete, yApres, (tete + 1) % N, yDe(buf[(tete + 1) % N]), TFT_GREEN);
  M5.Display.setCursor(150, 0);
  M5.Display.setTextColor(TFT_GREEN, TFT_BLACK);
  M5.Display.printf("|a| %4.2f g", norme);
  tete = (tete + 1) % N;
  vTaskDelay(20 / portTICK_PERIOD_MS);
  if (M5.BtnB.wasPressed) {           // B = remise à zéro de l'écran
    M5.Display.fillScreen(TFT_BLACK);
    for (int g = 0; g <= 3; g++) M5.Display.drawFastHLine(0, yDe(g), W, TFT_DARKGREY);
  }
}

Expected Verification

At rest, the green curve is flat at 1 g; tapping the table or shaking the stick produces visible peaks. The gray grid marks 0/1/2/3 g. Button B clears and redraws the grid.

Common Errors

• Redrawing the entire screen every loop: causes flickering; draw column by column as done here.
• Forgetting to clear the old column value before writing the new one → vertical trail.
• Poorly bounded vertical scale: without clamping in yDe, peaks go off-screen and the trace jumps to the top/bottom.
• Using portrait coordinates while setRotation(1) is active: the buffer must be 240 columns, not 135.
• Too much vTaskDelay: the curve becomes jagged; too little: the IMU's I2C (bus G21/G22 shared with the RTC) saturates the rendering.