I've been going over the spec of the CRSF protocol which is pretty simple: All packets are in the CRSF format [sync] [len] [type] [payload] [crc8] with a maximum total size of 64 bytes. CRC8 using poly 0xD5, includes all bytes from type (buffer[2]) to end of payload.
I am looking at raw packets from the receiver and no matter what I do, I can not reach the same checksum result. See raw packets below (italic is the data used to calculate CRC8):
C8 18 16 AE 70 85 2B 68 F1 8B 9F FC E2 17 7F F8 05 F8 28 08 00 00 4C 7C E2 63
C8 18 16 C0 03 1F FA 80 F7 8B 9F FC E2 17 7F F8 05 F8 28 08 00 00 4C 7C E2 9C
I've used multiple CRC8 calculators with different settings, but nothing works.
Any ideas?
Try to use the next code to debug the received CRSF frame:
#include <stdio.h>
#include <stdint.h>
#define CRSF_APP_CHANNELS_COUNT (12)
#define CRSF_PROTOCOL_SYNC_BYTE (0xC8)
#define CRSF_PROTOCOL_PACK_MAX_LEN (64)
#define CRSF_PROTOCOL_PACK_TYPE_PAYLOAD (0x16)
#pragma pack(push, 1)
typedef struct
{
uint16_t ch01: 11;
uint16_t ch02: 11;
uint16_t ch03: 11;
uint16_t ch04: 11;
uint16_t ch05: 11;
uint16_t ch06: 11;
uint16_t ch07: 11;
uint16_t ch08: 11;
uint16_t ch09: 11;
uint16_t ch10: 11;
uint16_t ch11: 11;
uint16_t ch12: 11;
} crsf_app_channels_t;
#pragma pack(pop)
uint16_t channels[CRSF_APP_CHANNELS_COUNT] = {0};
const uint8_t crc8_table[] = {
0x00, 0xD5, 0x7F, 0xAA, 0xFE, 0x2B, 0x81, 0x54, 0x29, 0xFC, 0x56, 0x83, 0xD7, 0x02, 0xA8, 0x7D,
0x52, 0x87, 0x2D, 0xF8, 0xAC, 0x79, 0xD3, 0x06, 0x7B, 0xAE, 0x04, 0xD1, 0x85, 0x50, 0xFA, 0x2F,
0xA4, 0x71, 0xDB, 0x0E, 0x5A, 0x8F, 0x25, 0xF0, 0x8D, 0x58, 0xF2, 0x27, 0x73, 0xA6, 0x0C, 0xD9,
0xF6, 0x23, 0x89, 0x5C, 0x08, 0xDD, 0x77, 0xA2, 0xDF, 0x0A, 0xA0, 0x75, 0x21, 0xF4, 0x5E, 0x8B,
0x9D, 0x48, 0xE2, 0x37, 0x63, 0xB6, 0x1C, 0xC9, 0xB4, 0x61, 0xCB, 0x1E, 0x4A, 0x9F, 0x35, 0xE0,
0xCF, 0x1A, 0xB0, 0x65, 0x31, 0xE4, 0x4E, 0x9B, 0xE6, 0x33, 0x99, 0x4C, 0x18, 0xCD, 0x67, 0xB2,
0x39, 0xEC, 0x46, 0x93, 0xC7, 0x12, 0xB8, 0x6D, 0x10, 0xC5, 0x6F, 0xBA, 0xEE, 0x3B, 0x91, 0x44,
0x6B, 0xBE, 0x14, 0xC1, 0x95, 0x40, 0xEA, 0x3F, 0x42, 0x97, 0x3D, 0xE8, 0xBC, 0x69, 0xC3, 0x16,
0xEF, 0x3A, 0x90, 0x45, 0x11, 0xC4, 0x6E, 0xBB, 0xC6, 0x13, 0xB9, 0x6C, 0x38, 0xED, 0x47, 0x92,
0xBD, 0x68, 0xC2, 0x17, 0x43, 0x96, 0x3C, 0xE9, 0x94, 0x41, 0xEB, 0x3E, 0x6A, 0xBF, 0x15, 0xC0,
0x4B, 0x9E, 0x34, 0xE1, 0xB5, 0x60, 0xCA, 0x1F, 0x62, 0xB7, 0x1D, 0xC8, 0x9C, 0x49, 0xE3, 0x36,
0x19, 0xCC, 0x66, 0xB3, 0xE7, 0x32, 0x98, 0x4D, 0x30, 0xE5, 0x4F, 0x9A, 0xCE, 0x1B, 0xB1, 0x64,
0x72, 0xA7, 0x0D, 0xD8, 0x8C, 0x59, 0xF3, 0x26, 0x5B, 0x8E, 0x24, 0xF1, 0xA5, 0x70, 0xDA, 0x0F,
0x20, 0xF5, 0x5F, 0x8A, 0xDE, 0x0B, 0xA1, 0x74, 0x09, 0xDC, 0x76, 0xA3, 0xF7, 0x22, 0x88, 0x5D,
0xD6, 0x03, 0xA9, 0x7C, 0x28, 0xFD, 0x57, 0x82, 0xFF, 0x2A, 0x80, 0x55, 0x01, 0xD4, 0x7E, 0xAB,
0x84, 0x51, 0xFB, 0x2E, 0x7A, 0xAF, 0x05, 0xD0, 0xAD, 0x78, 0xD2, 0x07, 0x53, 0x86, 0x2C, 0xF9
};
uint8_t crc8_calc(const uint8_t* data_p, uint32_t length) {
uint8_t crc = 0;
for (uint32_t i = 0; i < length; i++) {
crc = crc8_table[crc ^ *data_p++];
}
return crc;
}
static void crsf_app_handle_packet(const uint8_t* packet_p, uint32_t length) {
// Params check
if ((packet_p == NULL) || (length > CRSF_PROTOCOL_PACK_MAX_LEN)) {
printf("Params error!\r\n");
return;
}
// Sync byte check
uint8_t sync_byte = packet_p[0];
if (sync_byte != CRSF_PROTOCOL_SYNC_BYTE) {
printf("Wrong sync byte!\r\n");
return;
}
// Length check
uint8_t len_byte = packet_p[1];
if ((len_byte + 2) != length) {
printf("Wrong length: %d, %d!\r\n", len_byte, length);
return;
}
uint8_t crc_byte = packet_p[length - 1];
uint8_t crc_calc = crc8_calc(packet_p + 2, length - 3);
if (crc_byte != crc_calc) {
printf("Wrong CRC!\r\n");
printf("Expected: 0x%02X\r\n", crc_byte);
printf("Actual: 0x%02X\r\n", crc_calc);
return;
}
// Handle packet type
uint8_t pack_type = packet_p[2];
switch (pack_type) {
case CRSF_PROTOCOL_PACK_TYPE_PAYLOAD: {
const crsf_app_channels_t* d = (crsf_app_channels_t*)(packet_p + 3);
// Save channels
channels[0] = d->ch01;
channels[1] = d->ch02;
channels[2] = d->ch03;
channels[3] = d->ch04;
printf("CH0: %d\r\n", channels[0]);
printf("CH1: %d\r\n", channels[1]);
printf("CH2: %d\r\n", channels[2]);
printf("CH3: %d\r\n", channels[3]);
break;
}
default: {
return;
}
}
return;
}
int main() {
// const uint8_t crsf_packet[] = {
// 0xC8, 0x18, 0x16, 0xAE, 0x70, 0x85, 0x2B, 0x68,
// 0xF1, 0x8B, 0x9F, 0xFC, 0xE2, 0x17, 0x7F, 0xF8,
// 0x05, 0xF8, 0x28, 0x08, 0x00, 0x00, 0x4C, 0x7C,
// 0xE2, 0x63
// };
const uint8_t crsf_packet[] = {
0xC8, 0x18, 0x16, 0xE0, 0x03, 0x9F, 0x2B, 0xC0,
0xF7, 0x8B, 0x5F, 0xFC, 0xE2, 0x17, 0xBF, 0xF8,
0x85, 0xE6, 0x5C, 0x03, 0x00, 0x00, 0x4C, 0x3C,
0xD7, 0xBD
};
crsf_app_handle_packet(crsf_packet, sizeof(crsf_packet));
return 0;
}
This code works well with STM32F0 and BETAFPV ELRS Lite.