Receiving float from Arduino "BLEFloatCharacteristic" via BLE

giorgiogandolfi_

Hello everyone,
I'm currently experiencing a problem while sending gyroscope and accelerometer data from an Arduino Sense BLE to a QT application.

The code I wrote on Arduino creates a single service containing two characteristics, one for sending the gyro data and one for the accelerometer data. Each one of them is a "BLEFloatCharacteristic", so a float value should be sent.

Using the notify property of BLE, I connected each one of the characteristic to the signal "characteristicChanged", as per own QT guide on BLE.

connect(ptr_dataService, &QLowEnergyService::characteristicChanged, this, &MainWindow::updateGyroscopeData);
connect(ptr_dataService, &QLowEnergyService::characteristicChanged, this, &MainWindow::updateAccelerometerData);

The SLOTs to get the data are very basic:

void MainWindow::updateGyroscopeData(const QLowEnergyCharacteristic &c, const QByteArray &value)
{
    if (c.uuid() != QBluetoothUuid(BLE_UUID_GYROSCOPE_CHARACTERISTIC))
        return;

    qDebug()<<"Gyro";
    qDebug()<<value;
}
//------------------------------------------------------------------------------
void MainWindow::updateAccelerometerData(const QLowEnergyCharacteristic &c, const QByteArray &value)
{
    if (c.uuid() != QBluetoothUuid(BLE_UUID_ACCELEROMETER_CHARACTERISTIC))
        return;

    qDebug()<<"Acc";
    qDebug()<<value;
}

What I get printed out is something like

Gyro
"\x00\x00z\xBF"
Gyro
"\x00\x00\xFA\xBD"
Acc
"\x00\x00\xDD<"
Acc
"\x00H^?"

How can I get a float number instead?
Thanks to everyone!

mpergand

Hi,

Warning: we enter the twillight zone :)

To convert QByteArray to float, I tried this:

    float f=12.5;
    QByteArray d((char*)&f,sizeof(f));
    
    float ff=*reinterpret_cast<float*>(d.data());
    qDebug()<<ff;

prints 12.5 so far so good
but what is the format of the float you send from the Ardiuno, a 4 bytes float like in C ?
Which endianess little/big endian ?

First try to print the QByteArray in a readable format with:
qDebug()<<value.toHex();

[EDIT] Seems to be valid floats:
-0.976563 giro 1
-0.12207 giro 2
0.0269775 acc 1
0.868774 acc2

giorgiogandolfi_

Yes, your suggestions worked to perfection, thank you very much!

Adding only a small note for potential future viewers:
because the functions I use to update gyro and accelerometer data get as argument

const QByteArray &value

I had to modify your suggestion in :

float fNum = *reinterpret_cast<const float*>(value.constData());

artwaw

@mpergand What is wrong with QByteArray::toFloat() that you offer the above?

mpergand

@artwaw
As far as I know, it only works with ASCII string:

char n[]="1.25";
QByteArray d(n);
bool ok;
float f=d.toFloat(&ok);
qDebug()<<f<<ok;

artwaw

@mpergand thanks.