CODEWITHSUNDEEP
bluetoothbluetooth-lowenergy
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Reputation: 2227

Bluetooth heartrate monitor byte decoding

Problem :

I am having trouble with understanding the returned data of the BLE Heart Rate Characteristic(service 180d, characteristic 2a37).

According to the specification there will be either 6 or 7 bytes of data (when base64-decoded), i fully understand how to deal with it when this is the case.

But sometimes it won't return 6 or 7 bytes but 8 and more rarely 4 bytes, i have no idea why there are more/less bytes and what the meaning of the added bytes is or which bytes are left out.

I could skip all the cases where there are not 6 or 7 bytes but i want to fully understand this.

I am certain that the converting the base64-encoded to byte-array is done correctly, i made a function for it and checked it using manual base64-decode combined with charCodeAt(index) and truly manually checked it using good ol' pencil, paper and brain (not necessarily in that order).

TL;DR :

BLE Heart Rate (180d,2a37) sometimes does not return the expected amount of bytes (4 and 8 when it should be either 6 or 7 bytes). What exactly happened and why?

Example :

// Example results in byte-array's
["00010110", "01110111", "00000100", "00000010"] // unexpected 4 byte
["00010110", "01111000", "11111111", "00000001", "11111111", "00000001", "00001100", "00000001"] // unexpected 8 byte
["00010110", "01110111", "00001000", "00000010", "00001000", "00000010"] // normal 6 byte

// Example results in hex-array's (easier to read on small screens)
["0x16","0x77","0x04","0x02"] // unexpected 4 byte
["0x16","0x78","0xFF","0x01","0xFF","0x01","0x0C","0x01"] // unexpected 8 byte
["0x16","0x77","0x08","0x02","0x08","0x02"] // normal 6 byte

Byte Explanation :

  1. Flags. The first bit (most right) is on if the heart rate is in 16 bit format (i only got 8 bit).
  2. heartrate, if the heart rate is in 16 bit format there will be 2 bytes here
  3. energy expended
  4. energy expended
  5. rr interval
  6. rr interval

Upvotes: 1

Views: 2331

Answers (1)

Ifor
Ifor

Reputation: 2835

Energy expenditure is optional check the bit 3 of the flags in your sample data case it is not present. There are a variable number of rr intervals. with 4 bytes you have just 1 with 6 bytes you have 2 and with 8 bytes you have 3 you could in theory get 10 and 4.

You should decode the bytes using the flags then if rr's are present the number of bytes left / 2 is the number of rr's you have.

See the XML-Definition file for more details.

Upvotes: 3

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