WIP: ESP32 with MMA7455 test communication

Added two projects for sending accelerometer data wirelessly.

esp32_espnow/espnow_receiver/espnow_receiver.ino

@@ -0,0 +1,71 @@
+/*
+  Rui Santos & Sara Santos - Random Nerd Tutorials
+  Complete project details at https://RandomNerdTutorials.com/esp-now-esp32-arduino-ide/  
+  Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files.
+  The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+*/
+
+#include <esp_now.h>
+#include <WiFi.h>
+
+const bool DEBUG = true;
+
+char debug_string_buffer[20];
+// sprintf + serial of 20 bytes takes ~200us
+// sprintf + serial of 10 bytes takes ~144us
+// sprintf + serial of  5 bytes takes ~108us
+#define debug(formato, valor) \
+  sprintf(debug_string_buffer, formato, valor); \
+  Serial.print(debug_string_buffer)
+
+// Structure example to receive data
+// Must match the sender structure
+typedef struct accel_packet_t {
+    float x;
+    float y;
+    float z;
+} accel_packet_t;
+
+// Create a accel_packet_t called accelPacket
+accel_packet_t accelPacket;
+
+unsigned long timestamp = 0;
+
+// callback function that will be executed when data is received
+void OnDataRecv(const uint8_t * mac, const uint8_t *incomingData, int len) {
+  memcpy(&accelPacket, incomingData, sizeof(accelPacket));
+  //debug("X:% 01.3f ", accelPacket.x);
+  //debug("Y:% 01.3f ", accelPacket.y);
+  //debug("Z:% 01.3f ", accelPacket.z);
+  Serial.print(accelPacket.x); Serial.print(" ");
+  Serial.print(accelPacket.y); Serial.print(" ");
+  Serial.print(accelPacket.z); Serial.print("\n");
+  if (DEBUG) {
+    Serial.print(micros() - timestamp); Serial.print("\n");
+    timestamp = micros();
+  }
+}
+
+void setup() {
+  // Initialize Serial Monitor
+  Serial.begin(115200);
+
+  // Set device as a Wi-Fi Station
+  WiFi.mode(WIFI_STA);
+
+  // Init ESP-NOW
+  if (esp_now_init() != ESP_OK) {
+    while (true) {
+      Serial.println("Error initializing ESP-NOW");
+      delay(1000);
+    }
+  }
+
+  // Once ESPNow is successfully Init, we will register for recv CB to
+  // get recv packer info
+  esp_now_register_recv_cb(esp_now_recv_cb_t(OnDataRecv));
+}
+
+void loop() {
+
+}