Johnston Johnston - 6 months ago 31
iOS Question

How to get data out of bluetooth characteristic in Swift

I've got a Polar h7 device around me right now (it's BTLE) and I've got everything working but I am confused about how to get the BPM our of the

characteristic.value
now that it is updating. I have to turn some bytes into bpm...

My peripheral is updating:

func peripheral(peripheral: CBPeripheral!, didUpdateValueForCharacteristic characteristic: CBCharacteristic!, error: NSError!) {
if characteristic.UUID == CBUUID.UUIDWithString(heartRateChar) {
getInfoAboutHeartRate(characteristic)
}
}


I am getting info about the heart rate:

func getInfoAboutHeartRate(characteristic:CBCharacteristic) {
println(characteristic.value)
var bytes = characteristic.value.bytes
}


I know I need turn those bytes into the BPM.

According to the specs (this is where I get confused) at bluetooth.org, byte 0 is either going to be a
1
or a
0
.. If it is a
0
the heart rate value is an
uint8
and if it is a
1
then it is a
uint16
and I can get the BPM from that.

How do I find out if byte
0
is a
1
or a
0
? How to turn that into a
uint8
or
uint16
. And if I do that do I get the BPM straight up or do I have to do something else to it? Right now the BPM comes back as something like
<16447d03>
which makes sense.

Answer

This seems like it should be simple enough, but trying to confirm what you're saying. Does “bytes” end up with a value of “16447d03”? It's a pointer to the data, not the data itself, so you need to do something like this to get the actual value:

var data = characteristic.value
var values = [UInt8](count:data.length, repeatedValue:0)
data.getBytes(&values, length:data.length)

In this, “values” is an array containing the actual values.

From a private discussion we had, you listed the output as:

[22, 77, 22, 3] 
[22, 78, 27, 3, 18, 3] 
[22, 79, 2, 3] 
[22, 78, 15, 3] 

The first byte is the flags, which has been 22 in all the cases you've listed. This makes sense as it's all from the same heart rate hardware.

The bits are grouped like this: | 3 bits are reserved | 1 bit for RR-Interval | 1 bit for Energy Expended Status | 2 bits for Sensor Contact Status | 1 bit for Heart Rate Value Format | 22 is 00010110 in binary, which is | 000 | 1 | 0 | 11 | 0 |.

Heart Rate Value Format bit: 0 (Heart Rate Value Format is set to UINT8)
Sensor Contact Status bits: 3 (Sensor Contact feature is supported and contact is detected)
Energy Expended Status bit: 0 (Energy Expended field is not present)
RR-Interval bit: 1 (One or more RR-Interval values are present)

This means that the following byte is the heart rate (C1 field) and the remaining bytes are RR-Interval values, whatever they are (C4 field).

So for this data the heart rates were 77, 78, 79, 78.