Sports Tracker HRM2 Bluetooth HRM belt protocol

The protocol used by the ST HRM2 belt is unknown, there are no official documentation available.
This README describe a probable protocol specification of the ST-HRM2 Bluetooth HRM belt.

If you have anything to add to this, please let me know!

SPP Service, easy to connect with plain RFCOMM, data can be viewed with 

sudo cat /dev/rfcomm0 | hexdump

The data dumps are made with the simple helper script, bthrm-data-dumpper.sh

The device sends out data packets every second, the length varies, the contents
and possible meaning described below:

1. 0xFA Start of data packet
2. Sequence number going up from 0x44 to 0x47
3. Sequence number going down from 0xBB to 0xB8
** The sum of 2 and 3 seem to always be 0xFF
4. 0x81 Seen to be 0x01 on first connect, but after that stays at 0x81
5. 0x?? Varies, most likely battery charge information, as it is large after charge and then goes down.
   - Seen as 0x53 on first connect, if 4. is 0x01
   - 0x3C when fully charged
6. 0x?? Heart rate measurement, the value we are looking for
7. 0x00 7-10 sometimes have values, sometime they 0x00
8. 0x00 
9. 0x00
10 0x00 
10. Padding of 6,7,9 zeroes, amount varies
11. Packet ends with two bytes, most likely a checksum

Example packet
1  4518#    0xFA hex Start
2  4519#    0x47 hex 
3  4520#    0xB8 hex 0x47+0xB8==0xFF
4  4521#    0x81 hex 
5  4522#    0x24 hex Battery?
6  4523#    0x4C hex HR
7  4524#    0x30 hex ?
8  4525#    0x22 hex ?
9  4526#    0xF9 hex ?
10 4527#    0x00 hex 
11 4528#    0x00 hex 
12 4529#    0x00 hex 
13 4530#    0x00 hex 
14 4531#    0x00 hex 
15 4532#    0x00 hex 
16 4533#    0x14 hex 
17 4534#    0xA1 hex Checksum ?

Sometimes an unknown data packet is sent, currently the meaning is unknown:
4552#    0xB3 hex = -77 dec = 263 oct = _�_
4553#    0x22 hex = 034 dec = 042 oct = _"_
4554#    0x93 hex = -109 dec = 223 oct = _�_
4555#    0x72 hex = 114 dec = 162 oct = _r_
4556#    0x83 hex = -125 dec = 203 oct = _�_
4557#    0x67 hex = 103 dec = 147 oct = _g_
4558#    0x6E hex = 110 dec = 156 oct = _n_
4559#    0x6D hex = 109 dec = 155 oct = _m_
4560#    0xE1 hex = -31 dec = 341 oct = _�_
4561#    0xC7 hex = -57 dec = 307 oct = _�_
4562#    0xF7 hex = -09 dec = 367 oct = _�_
4563#    0xDB hex = -37 dec = 333 oct = _�_
4564#    0x19 hex = 025 dec = 031 oct = _^Y_
4565#    0x4F hex = 079 dec = 117 oct = _O_
4566#    0x51 hex = 081 dec = 121 oct = _Q_
4567#    0x08 hex = 008 dec = 010 oct = _^H_
4568#    0xED hex = -19 dec = 355 oct = _�_
4569#    0x1D hex = 029 dec = 035 oct = _^]_
4570#    0x1A hex = 026 dec = 032 oct = _^Z_
4571#    0x46 hex = 070 dec = 106 oct = _F_
4572#    0x32 hex = 050 dec = 062 oct = _2_
4573#    0x41 hex = 065 dec = 101 oct = _A_
4574#    0xC3 hex = -61 dec = 303 oct = _�_
4575#    0x68 hex = 104 dec = 150 oct = _h_