How to get data from an object returned from a Bluetooth device using BLE

I have managed to write a program that connects to my power meter via Bluetooth, I have set up a profile for the power data and retrieved it via BLE. When I try and decode the output I get the following error:

"Details: Could not find symbol 'decodeNumber'"

I imagine this means that the value I am getting in my power characteristic is not a ByteArray but an Object, I am unsure how to get the data I need from this if that is the case. Here is my profile manager code:

import Toybox.BluetoothLowEnergy;

class ProfileManager {

public const POWER_METER_SERVICE = BluetoothLowEnergy.longToUuid(0x6E400001B5A3F393L, 0xE0A9E50E24DCCA9EL);

public const CYCLING_POWER_SERVICE = BluetoothLowEnergy.longToUuid(0x0000181800001000L, 0x800000805F9B34FBL);
public const CYCLING_POWER_MEASURMENT_CHARACTERISTIC = BluetoothLowEnergy.longToUuid(0x00002A6300001000L, 0x800000805F9B34FBL);
public const CYCLING_POWER_FEATURE_CHARACTERISTIC = BluetoothLowEnergy.longToUuid(0x00002A6500001000L, 0x800000805F9B34FBL);
public const CYCLING_POWER_CP_CHARACTERISTIC = BluetoothLowEnergy.longToUuid(0x00002A6600001000L, 0x800000805F9B34FBL);

private const _cyclingPowerDef = {
:uuid => CYCLING_POWER_SERVICE,
:characteristics => [{
:uuid => CYCLING_POWER_MEASURMENT_CHARACTERISTIC
}, {
:uuid => CYCLING_POWER_FEATURE_CHARACTERISTIC
}, {
:uuid => CYCLING_POWER_CP_CHARACTERISTIC
}]
};

//! Register the bluetooth profile
public function registerProfiles() as Void {
BluetoothLowEnergy.registerProfile(_cyclingPowerDef);
}

And my startRead code in device manager:

private function startRead() as Void {
System.println("Start Read");
var device = _device;
if (device != null) {
_cycleService = device.getService(_profileManager.CYCLING_POWER_SERVICE);
var cycleService = _cycleService;
if (cycleService != null) {
_feature = cycleService.getCharacteristic(_profileManager.CYCLING_POWER_FEATURE_CHARACTERISTIC);
_powerMeterData = cycleService.getCharacteristic(_profileManager.CYCLING_POWER_MEASURMENT_CHARACTERISTIC);
}
if (_powerMeterData instanceof Toybox.Lang.ByteArray){
System.println("yup");
}

value.decodeNumber(Lang.NUMBER_FORMAT_UINT8, {});
var pd = _powerMeterData.decodeNumber(Lang.NUMBER_FORMAT_UINT16, {});
System.println(pd);

System.println(_powerMeterData.toString());
}

Any help would be greatly appreciated

Top Replies

jim_m_58 over 1 year ago in reply to li2niu +1

Yo won't see any data coming back in onCharacteristicWrite -it's just it won't be called until the sensor has processed it if you ask for a response

In onCharacteristicWrite(), you can do a…

All Replies

jim_m_58 over 3 years ago

What you get back is a byte array. Assume here that "value" is that byte array you get when a read or notify occurs:

var battery=value.decodeNumber(Lang.NUMBER_FORMAT_UINT8, null);

I'm reading an unsigned 8 bit value from the beginning of the byte array.

EuanW over 3 years ago in reply to jim_m_58

Yeah that what I thought initially but when I try var pd = _powerMeterData.decodeNumber(Lang.NUMBER_FORMAT_UINT16, {}); I get an error saying that "decodeNumber" isnt recognised

I also have this code that displays nothong:

if (_powerMeterData instanceof Toybox.Lang.ByteArray){
System.println("ba");
}

jim_m_58 over 3 years ago in reply to EuanW

_powerMeterData isn't a byte array. How is it created based on what was passed to onCharacteristicRead() or onCharacteristicChange()?

EuanW over 3 years ago in reply to jim_m_58

I don't think I have those functions which I imagine is what is causing the error

jim_m_58 over 3 years ago in reply to EuanW

That's how you see the data in the BLE Delegate.

EuanW over 3 years ago in reply to jim_m_58

Is there an example of a onCharacteristicRead in the sample files? I cant seem to find one

EuanW over 3 years ago in reply to EuanW

Ive looked at the Nordic Thingy examples and neither seems to have an onCharacteristicRead function or a requestRead call as far as I can see

jim_m_58 over 3 years ago in reply to EuanW

From the API doc. onCharactericRead occurs when you do a specific read, while onCharacteristicChanged when you have notify turned on for the characteristic. Looks like the Nordic same only uses notify. I tend to do an inital read when setting notify on.

onCharacteristicChanged(characteristic as BluetoothLowEnergy.Characteristic, value as Lang.ByteArray) as Void

After enabling notifications or indications on a characteristic by enabling the appropriate bit of the CCCD of the characteristic this function will be called after every change to the characteristic.
onCharacteristicRead(characteristic as BluetoothLowEnergy.Characteristic, status as BluetoothLowEnergy.Status, value as Lang.ByteArray) as Void

After requesting a read operation on a characteristic using Characteristic.requestRead() this function will be called when the operation is completed.

EuanW over 3 years ago in reply to jim_m_58

I have the following function in my delegate:

public function onCharacteristicRead(characteristic as BluetoothLowEnergy.Characteristic, value as Lang.ByteArray) as Void {
var onCharRead = _onCharRead;
if ((null != _onCharRead) && _onCharRead.stillAlive()) {
(_onCharRead.get() as DeviceManager).procCharWrite(value);

}
}

And when I call _powerMeterData.requestRead() in device manager I get the following error:

Error: Too Many Arguments Error
Details: Failed invoking <symbol>
Stack:
- onCharacteristicRead() at C:\Users\PC-User\eclipse-workspace\PowerMeterBLE\source\PowerMeterBLEDelegate.mc:64 0x100002ca

This points to the declarative line of onCharacteristicRead

jim_m_58 over 3 years ago in reply to EuanW

onCharacteristicRead gets 3 parameters - the characteristic, the status, and the byte array. That's why you get the "Too Many Arguments" error. It's trying to be called with 3 arguments, where you're only specifying two.