FP_B messages

Framing:

The structure of the FP_B message frame is:

Offs	Size	Type	Content
0	1	uint8_t	Sync 1: `f` = 102 = 0x66 = 0b0110‘0110
1	1	uint8_t	Sync 1: `!` = 33 = 0x21 = 0b0010‘0001
2	2	uint16_t	Message ID (`message_id`), 1…65534
4	2	uint16_t	Payload size (`payload_size`)
6	2	uint16_t	Monotonic time [ms] (`message_time`), overflows/wraps at ~65s
8…	…	…	Payload
8 + `payload_size`	4	uint32_t	Checksum

Notes:

For input messages, set the message_time to 0.

Example:

A short FP_B message frame with the bytes in order as they appear on the wire:

CODE

First byte                                   Last byte
|                                            |
66 21 34 12 04 00 21 43 01 02 03 04 61 c4 c5 9c
^^ ^^ ^^^^^ ^^^^^ ^^^^^ ^^^^^^^^^^^ ^^^^^^^^^^^
|  |  |     |     |     |           |
|  |  |     |     |     |           CRC (= 0x9cc5c461)
|  |  |     |     |     Payload (= [ 0x01, 0x02, 0x03, 0x04 ])
|  |  |     |     Message time (= 0x4321 = 17185 = 17.185s)
|  |  |     Payload size = (=x0004 = 4)
|  |  Message ID (= 0x1234)
|  Sync 2
Sync 1

Cyclic Redundancy Check (CRC):

The user must calculate a checksum at the end of the message to verify the integrity of the message. We use cyclic redundancy check (CRC) as our error-detecting code to achieve this. The CRC allows the detection of possible errors introduced during the transmission or storage of the message. The user can determine whether the data has been altered by comparing the computed checksum with the original checksum (previously computed and stored with the data).

For the FP_B message, the checksum is a 32 bit cyclic redundancy check (CRC) with the polynomial 0x32c00699. You can use the following script to generate the CRC:

CODE

// Generated using https://github.com/madler/crcany.git with the following parameters:
// width =32 poly =0 x32c00699 init =0 x00000000 refin = false refout = false xorout =0 x00000000 check =0 x00000000 residue =0 x000000 name =" FF - FPB -32"
static uint32_t const k_crc32_fpb[] = {
    0x00000000, 0x32c00699, 0x65800d32, 0x57400bab, 0xcb001a64, 0xf9c01cfd,
    0xae801756, 0x9c4011cf, 0xa4c03251, 0x960034c8, 0xc1403f63, 0xf38039fa,
    0x6fc02835, 0x5d002eac, 0x0a402507, 0x3880239e, 0x7b40623b, 0x498064a2,
    0x1ec06f09, 0x2c006990, 0xb040785f, 0x82807ec6, 0xd5c0756d, 0xe70073f4,
    0xdf80506a, 0xed4056f3, 0xba005d58, 0x88c05bc1, 0x14804a0e, 0x26404c97,
    0x7100473c, 0x43c041a5, 0xf680c476, 0xc440c2ef, 0x9300c944, 0xa1c0cfdd,
    0x3d80de12, 0x0f40d88b, 0x5800d320, 0x6ac0d5b9, 0x5240f627, 0x6080f0be,
    0x37c0fb15, 0x0500fd8c, 0x9940ec43, 0xab80eada, 0xfcc0e171, 0xce00e7e8,
    0x8dc0a64d, 0xbf00a0d4, 0xe840ab7f, 0xda80ade6, 0x46c0bc29, 0x7400bab0,
    0x2340b11b, 0x1180b782, 0x2900941c, 0x1bc09285, 0x4c80992e, 0x7e409fb7,
    0xe2008e78, 0xd0c088e1, 0x8780834a, 0xb54085d3, 0xdfc18e75, 0xed0188ec,
    0xba418347, 0x888185de, 0x14c19411, 0x26019288, 0x71419923, 0x43819fba,
    0x7b01bc24, 0x49c1babd, 0x1e81b116, 0x2c41b78f, 0xb001a640, 0x82c1a0d9,
    0xd581ab72, 0xe741adeb, 0xa481ec4e, 0x9641ead7, 0xc101e17c, 0xf3c1e7e5,
    0x6f81f62a, 0x5d41f0b3, 0x0a01fb18, 0x38c1fd81, 0x0041de1f, 0x3281d886,
    0x65c1d32d, 0x5701d5b4, 0xcb41c47b, 0xf981c2e2, 0xaec1c949, 0x9c01cfd0,
    0x29414a03, 0x1b814c9a, 0x4cc14731, 0x7e0141a8, 0xe2415067, 0xd08156fe,
    0x87c15d55, 0xb5015bcc, 0x8d817852, 0xbf417ecb, 0xe8017560, 0xdac173f9,
    0x46816236, 0x744164af, 0x23016f04, 0x11c1699d, 0x52012838, 0x60c12ea1,
    0x3781250a, 0x05412393, 0x9901325c, 0xabc134c5, 0xfc813f6e, 0xce4139f7,
    0xf6c11a69, 0xc4011cf0, 0x9341175b, 0xa18111c2, 0x3dc1000d, 0x0f010694,
    0x58410d3f, 0x6a810ba6, 0x8d431a73, 0xbf831cea, 0xe8c31741, 0xda0311d8,
    0x46430017, 0x7483068e, 0x23c30d25, 0x11030bbc, 0x29832822, 0x1b432ebb,
    0x4c032510, 0x7ec32389, 0xe2833246, 0xd04334df, 0x87033f74, 0xb5c339ed,
    0xf6037848, 0xc4c37ed1, 0x9383757a, 0xa14373e3, 0x3d03622c, 0x0fc364b5,
    0x58836f1e, 0x6a436987, 0x52c34a19, 0x60034c80, 0x3743472b, 0x058341b2,
    0x99c3507d, 0xab0356e4, 0xfc435d4f, 0xce835bd6, 0x7bc3de05, 0x4903d89c,
    0x1e43d337, 0x2c83d5ae, 0xb0c3c461, 0x8203c2f8, 0xd543c953, 0xe783cfca,
    0xdf03ec54, 0xedc3eacd, 0xba83e166, 0x8843e7ff, 0x1403f630, 0x26c3f0a9,
    0x7183fb02, 0x4343fd9b, 0x0083bc3e, 0x3243baa7, 0x6503b10c, 0x57c3b795,
    0xcb83a65a, 0xf943a0c3, 0xae03ab68, 0x9cc3adf1, 0xa4438e6f, 0x968388f6,
    0xc1c3835d, 0xf30385c4, 0x6f43940b, 0x5d839292, 0x0ac39939, 0x38039fa0,
    0x52829406, 0x6042929f, 0x37029934, 0x05c29fad, 0x99828e62, 0xab4288fb,
    0xfc028350, 0xcec285c9, 0xf642a657, 0xc482a0ce, 0x93c2ab65, 0xa102adfc,
    0x3d42bc33, 0x0f82baaa, 0x58c2b101, 0x6a02b798, 0x29c2f63d, 0x1b02f0a4,
    0x4c42fb0f, 0x7e82fd96, 0xe2c2ec59, 0xd002eac0, 0x8742e16b, 0xb582e7f2,
    0x8d02c46c, 0xbfc2c2f5, 0xe882c95e, 0xda42cfc7, 0x4602de08, 0x74c2d891,
    0x2382d33a, 0x1142d5a3, 0xa4025070, 0x96c256e9, 0xc1825d42, 0xf3425bdb,
    0x6f024a14, 0x5dc24c8d, 0x0a824726, 0x384241bf, 0x00c26221, 0x320264b8,
    0x65426f13, 0x5782698a, 0xcbc27845, 0xf9027edc, 0xae427577, 0x9c8273ee,
    0xdf42324b, 0xed8234d2, 0xbac23f79, 0x880239e0, 0x1442282f, 0x26822eb6,
    0x71c2251d, 0x43022384, 0x7b82001a, 0x49420683, 0x1e020d28, 0x2cc20bb1,
    0xb0821a7e, 0x82421ce7, 0xd502174c, 0xe7c211d5
};

uint32_t Crc32fpb(const uint8_t* data, const int size) {
    uint32_t crc = 0;
    if (data != NULL) {
        for (int ix = 0; ix < size; ix++) {
            crc = (crc << 8) ^ k_crc32_fpb[((crc >> 24) ^ data[ix]) & 0xff];
        }
    }
    return crc;
}

The following CRC tables were generated using the following repository and catalog: 'https://github.com/madler/crcany.git ', 'https://reveng.sourceforge.io/crc-catalogue/all.htm '. The polynomials were extracted from 'http://users.ece.cmu.edu/~koopman/crc/crc32.html '.