The structure of a FIT file.

The FIT Protocol docs

The official FIT protocol docs are here.

Tools

• first download the FIT SDK
• it contains C# and Java example implementations. The C# one is about 60 000 LOC, so good luck.
• the most useful thing there is the java CSV tool which you can find at /java/FitCSVTool.jar. You can use it to explore existing .FIT files and to validate your implementation for errors using:

$ java -jar /fitsdk/java/FitCSVTool.jar /path/to/minimal-example.fit

A Minimal Example

This is a minimal example that is able to pass the validation. It has one Record with Power and Timestamp fields

The FIT file contains:

• FIT file header

[14,32, 8,40, 0, 0, 0,46,70,73,84,59,133]
  0  1  2  3  4  5  6  7  8  9 10 11  12

    0:     size of header in bytes (12 or 14)
    1:     protocol version (16 for v1, 32 for v2)
    2-3:   profile version  (21.40 * 100 = 2140)
    4-7:   size of data (only messages, no header or crc)
    8-11:  encodes the string '.FIT'
    11-13: CRC of header (byte 0 to 11)

• File Id definition message

[64, 0, 0, 0, 0, 3, 0, 1, 0, 1, 2,152, 2, 2,132]
  0  1  2  3  4  5  6  7  8  9 10  11 12 13 14

    0: definition message header (0b01000000)
    1: reserved
    2: architecture (0 = Little Endian)
    3-4: global message number
    5: number of following field definitions
    6-8:   definition of File type field
           - 0: field definition number under the global message numbers
           - 1: size (how many bytes -> 1)
           - 2: base type (the hex value from the table -> 0x00)
    9-11:  definition of Manufacturer field
    12-14: definition of Product field

• File Id data message

[0, 4,255, 0, 0, 0]
 0  1   2  3  4  5 

    0:   data message header (0b00000000)
    1:   4 is Activity file type
    2-3: 255 is development 'manufacturer'
    4-5: 0 for product

• Record definition message

[64, 0, 0,20, 0, 2, 7, 2,132,254, 4,134]
  0  1  2  3  4  5  6  7   8   9 10  11

   0:    definition message header
   ...
   6-8:  definition of Power field
   9-11: definition of Timestamp field

• Record data message

[0,44, 1,56,227,157,117]
 0  1  2  3   4   5   6

   0:   data message header (0b00000000)
   1-2: power value in watts resolution of 1
   3-6: timestamp

• CRC

[80,65]

Garmin Connect valid .FIT file

Now to be able to upload to Garmin Connect, the file must have a couple more message fields along with their definition messages. Those are:

• Event message field
• Lap message field
• Session message field
• Activity message field

If those are not present the file won't validate and the upload will be rejected.

... to be continued ...