Hey there,
I've got a sparkfun ADXL345 Breakout connecter over I2C, measuring with 200Hz. It works fine for quite a while, but all of a sudden, it reads wrong values:
in the beginning:
2892;-4.04;10.04;-0.75
2896;-4.08;10.08;-0.94
2901;-4.16;9.96;-0.98
2906;-4.12;10.12;-0.90
First is time in ms, the three values is x,y,z.
then, all of a sudden:
766754;-4.12;10.08;-0.86
766759;-4.08;10.00;-0.94
766764;-4.16;10.08;-0.82
766769;-4.12;10.08;-1.10
766774;-4.08;10.00;-0.82
766779;-4.20;10.12;-0.71
766834;0.00;80.02;-6.59
766836;-32.01;-0.04;-0.04
766840;-32.01;80.34;-8.79
766845;-32.01;80.34;-8.79
766850;-32.01;80.34;-8.79
766855;-32.01;80.34;-8.79
766860;-32.01;80.34;-8.79
766865;-32.01;80.34;-8.79
766870;-32.01;80.34;-8.79
766875;-32.01;80.34;-8.79
but still changing sometimes:
767760;-32.01;80.34;-8.79
767765;-32.01;80.34;-8.79
767770;-32.01;80.34;-8.79
767775;-32.01;80.34;-8.79
767780;-32.01;80.34;-8.79
767785;62.76;-33.89;178.87
767790;62.76;-33.89;178.87
767795;62.76;-33.89;178.87
767800;62.76;-33.89;178.87
767805;62.76;-33.89;178.87
767810;62.76;-33.89;178.87
did anyone had this behaviour before and could point me in a direction? A search didn't help me yet 
relevant parts of my code are:
ADXL setup:
void setup_adxl(void)
{
DEBUG_PRINTLN("Initializing Accelerometer:");
// Initialise the sensor
//DEBUG_PRINTBIN(accel.readRegister(ADXL345_REG_DEVID));
int counter = 0;
while (!accel.begin() && counter < 100)
{
if (counter > 98) {
// There was a problem detecting the ADXL345 ... check your connections
DEBUG_PRINTLN("Ooops, no ADXL345 detected ... Check your wiring!");
}
counter++;
delayMicroseconds(10);
}
DEBUG_PRINTLN(counter);
accel.setDataRate(ADXL345_DATARATE_800_HZ);
accel.setRange(ADXL345_RANGE_8_G);
// Setup: Int1 => Activity (AC gekoppelt, niedriger Threshold); Int2 => Freefall
accel.writeRegister(ADXL345_REG_ACT_INACT_CTL, 0b11111111); //AC coupled, ACT: X Y Z
accel.writeRegister(ADXL345_REG_THRESH_ACT, 2); // scale factor 62.5mg/LSB
//accel.writeRegister(ADXL345_REG_THRESH_INACT, 2); // scale factor 62.5mg/LSB
//accel.writeRegister(ADXL345_REG_TIME_INACT, 10); // scale factor 1s/LSB
accel.writeRegister(ADXL345_REG_THRESH_FF, 5); // scale factor 62.5mg/LSB
accel.writeRegister(ADXL345_REG_TIME_FF, 10); // scale factor 5ms/LSB
accel.writeRegister(ADXL345_REG_INT_MAP, 0b00000100); //set to INT1 Pin (INT1 = 0, INT2 = 1)
accel.writeRegister(ADXL345_REG_INT_ENABLE, 0b00010100); // Enable activity interrupt
// Display some basic information on this sensor
displaySensorDetails();
// Display additional settings (outside the scope of sensor_t)
displayDataRate();
displayRange();
DEBUG_PRINTLN("");
accel.readRegister(ADXL345_REG_INT_SOURCE);
accel.readRegister(ADXL345_REG_INT_SOURCE);
String accMessage = "Measurement (Millis,X,Y,Z in m/s^2) startet at: " + getBackLocalTime();
DEBUG_PRINTLN(accMessage);
}
Where I basically read it:
//Task1code: Core 0
void Task1code( void * pvParameters ) {
Serial.print("Task1 running on core ");
Serial.println(xPortGetCoreID());
delay(1000);
for (;;) {
if (millis() - end_time >= 5) { // maximum request rate 200Hz -> 5
end_time = millis();
buffer_S += msg_adxl(false);
buffer_S += "\n";
//Serial.println(msg_adxl(false));
}
if (millis() - last_send_to_queue > 900 && recorded == false) {
buffer_BME = msg_temp_hum();
recorded = true;
strcpy( TxBmeBuffer , buffer_BME.c_str() );
if (pdTRUE == xQueueSend(queue, TxBmeBuffer, 100))
{
Serial.println("Task2: Successfully sent BME data");
buffer_BME = "";
}
else
{
Serial.println("Sending BME Failed");
}
}
if (millis() - last_send_to_queue > 1000) {
strcpy( TxBuffer , buffer_S.c_str() );
if (pdTRUE == xQueueSend(queue, TxBuffer, 100))
{
Serial.println("Task2: Successfully sent ACC data");
buffer_S = "";
}
else
{
Serial.println("Sending Failed");
}
last_send_to_queue = millis();
recorded = false;
} else {
//buffer_S += "\n";
}
}
}
where I generate the string:
String msg_adxl(boolean freefall)
{
// Get a new sensor event
sensors_event_t event;
accel.getEvent(&event);
String accMessage = "";
accMessage += millis();
accMessage += ";";
accMessage += event.acceleration.x;
accMessage += ";";
accMessage += event.acceleration.y;
accMessage += ";";
accMessage += event.acceleration.z;
if (freefall) {
accMessage += ";";
accMessage += "Freefall";
}
return accMessage;
//writeAccString(accMessage);
}
I know, that string call is not that good at all, but this is not the problem in this case, as it happens with direct printing to UART as well. And it is not a problem due to the task thin, as it also happens without task-management.