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HoTT, XN297 emu: compute the hop and CRC xorout tables (-1.2 KB) (#1199)

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* HoTT: compute the hop table from the hop value (-1 KB)

The 16 hop tables are generated from the 16 bit value sent in packet[0..1]:
the C library rand() LCG (low 16 bits: x = x * 0x4E6D + 0x3039) seeded with
it drives forward Fisher-Yates shuffles, first of the order of the 5 bands of
15 channels, then of the channels of each band, hop i being in the band at
position i%5. This gives back the 16 tables bit for bit, and for any seed a
75 channel permutation with the same band structure.

* XN297 emu: compute the CRC xorout instead of the 4 tables

The XN297 CRC is CRC-16/CCITT with init 0xFFFF and xorout 0xFFFF over the
unscrambled address, pcf and payload, sent scrambled. The emulation computes
it from 0xB5D2 over the bits as sent, so the xorout for a CRC over n bits is
CRC(init 0xFFFF^0xB5D2) of n scramble (or zero) bits, xor 0xFFFF, xor the 16
scramble bits at the CRC. The scramble bits come from the LFSR behind
xn297_scramble[] (1110001, then s[i] = s[i-7] ^ s[i-3]), which also covers
the last enhanced entries, past the end of that table. It is the generator
Goebish gave for the scramble sequence in a comment on
https://mcumix.blogspot.com/2016/07/reverse-engineering-24ghz-rf-chip-xn297.html
(seed 0x71, feedback bit 6 ^ bit 2, MSB first).

All the entries of the 4 tables are given back but two: entry 0 (3 byte
address, no payload) was a 0 placeholder, and scrambled entry 1 (3 byte
address, 1 byte payload) was 0x3448 where the formula gives 0xE348.
This commit is contained in:
Raphael Coeffic
2026-10-23 12:02:20 +02:00
committed by GitHub
parent ee984c403d
commit d3293afdab
4 changed files with 66 additions and 78 deletions
Download Patch File Download Diff File Expand all files Collapse all files
Multiprotocol/XN297_EMU.ino
+30 -50
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@@ -14,44 +14,30 @@ const uint8_t xn297_scramble[] = {
0x1B, 0x5D, 0x19, 0x10, 0x24, 0xD3, 0xDC, 0x3F,
0x8E, 0xC5, 0x2F, 0xAA, 0x16, 0xF3, 0x95 };
// scrambled, standard mode crc xorout table
const uint16_t PROGMEM xn297_crc_xorout_scrambled[] = {
0x0000, 0x3448, 0x9BA7, 0x8BBB, 0x85E1, 0x3E8C,
0x451E, 0x18E6, 0x6B24, 0xE7AB, 0x3828, 0x814B,
0xD461, 0xF494, 0x2503, 0x691D, 0xFE8B, 0x9BA7,
0x8B17, 0x2920, 0x8B5F, 0x61B1, 0xD391, 0x7401,
0x2138, 0x129F, 0xB3A0, 0x2988, 0x23CA, 0xC0CB,
0x0C6C, 0xB329, 0xA0A1, 0x0A16, 0xA9D0 };
// unscrambled, standard mode crc xorout table
const uint16_t PROGMEM xn297_crc_xorout[] = {
0x0000, 0x3D5F, 0xA6F1, 0x3A23, 0xAA16, 0x1CAF,
0x62B2, 0xE0EB, 0x0821, 0xBE07, 0x5F1A, 0xAF15,
0x4F0A, 0xAD24, 0x5E48, 0xED34, 0x068C, 0xF2C9,
0x1852, 0xDF36, 0x129D, 0xB17C, 0xD5F5, 0x70D7,
0xB798, 0x5133, 0x67DB, 0xD94E, 0x0A5B, 0xE445,
0xE6A5, 0x26E7, 0xBDAB, 0xC379, 0x8E20 };
// scrambled enhanced mode crc xorout table
const uint16_t PROGMEM xn297_crc_xorout_scrambled_enhanced[] = {
0x0000, 0x7EBF, 0x3ECE, 0x07A4, 0xCA52, 0x343B,
0x53F8, 0x8CD0, 0x9EAC, 0xD0C0, 0x150D, 0x5186,
0xD251, 0xA46F, 0x8435, 0xFA2E, 0x7EBD, 0x3C7D,
0x94E0, 0x3D5F, 0xA685, 0x4E47, 0xF045, 0xB483,
0x7A1F, 0xDEA2, 0x9642, 0xBF4B, 0x032F, 0x01D2,
0xDC86, 0x92A5, 0x183A, 0xB760, 0xA953 };
// unscrambled enhanced mode crc xorout table
// unused so far
#ifdef XN297DUMP_NRF24L01_INO
const uint16_t xn297_crc_xorout_enhanced[] = {
0x0000, 0x8BE6, 0xD8EC, 0xB87A, 0x42DC, 0xAA89,
0x83AF, 0x10E4, 0xE83E, 0x5C29, 0xAC76, 0x1C69,
0xA4B2, 0x5961, 0xB4D3, 0x2A50, 0xCB27, 0x5128,
0x7CDB, 0x7A14, 0xD5D2, 0x57D7, 0xE31D, 0xCE42,
0x648D, 0xBF2D, 0x653B, 0x190C, 0x9117, 0x9A97,
0xABFC, 0xE68E, 0x0DE7, 0x28A2, 0x1965 };
#endif
// XN297 CRC: CRC-16/CCITT (init 0xFFFF, xorout 0xFFFF) over the unscrambled
// address, pcf and payload, sent scrambled. The emulation computes it from 0xB5D2
// over the bits as sent: the xorout is the difference for a CRC over 'bits' bits,
// the CRC of the scramble bits plus the scramble of the CRC bits.
static uint16_t __attribute__((unused)) XN297_crc_xorout(bool scramble, uint16_t bits)
{
uint16_t xorout = 0xFFFF ^ 0xB5D2;
uint8_t lfsr = 0x71, s; // scramble bits (xn297_scramble[]): 1110001, then s[i] = s[i-7] ^ s[i-3]
for(uint16_t i = 0; i < bits + 16; i++)
{
s = scramble ? lfsr >> 6 : 0;
lfsr = ((lfsr << 1) | (((lfsr >> 6) ^ (lfsr >> 2)) & 1)) & 0x7F;
if(i < bits)
{
if((xorout >> 15) ^ s)
xorout = (xorout << 1) ^ 0x1021;
else
xorout <<= 1;
}
else
xorout ^= (uint16_t)s << (bits + 15 - i);
}
return ~xorout;
}
static bool __attribute__((unused)) XN297_Configure(bool crc_en, bool scramble_en, bool bitrate, bool force_nrf)
{
@@ -296,10 +282,7 @@ static void __attribute__((unused)) XN297_WritePayload(uint8_t* msg, uint8_t len
crc = 0xb5d2;
for (uint8_t i = offset; i < last; ++i)
crc16_update( buf[i], 8);
if(xn297_scramble_enabled)
crc ^= pgm_read_word(&xn297_crc_xorout_scrambled[xn297_addr_len - 3 + len]);
else
crc ^= pgm_read_word(&xn297_crc_xorout[xn297_addr_len - 3 + len]);
crc ^= XN297_crc_xorout(xn297_scramble_enabled, 8 * (xn297_addr_len + len));
buf[last++] = crc >> 8;
buf[last++] = crc & 0xff;
}
@@ -366,9 +349,9 @@ static void __attribute__((unused)) XN297_WriteEnhancedPayload(uint8_t* msg, uin
crc16_update( buf[i], 8);
crc16_update( buf[last] & 0xc0, 2);
if (xn297_scramble_enabled)
crc ^= pgm_read_word(&xn297_crc_xorout_scrambled_enhanced[xn297_addr_len-3+len]);
crc ^= XN297_crc_xorout(true, 8 * (xn297_addr_len + len) + 10);
//else
// crc ^= pgm_read_word(&xn297_crc_xorout_enhanced[xn297_addr_len - 3 + len]);
// crc ^= XN297_crc_xorout(false, 8 * (xn297_addr_len + len) + 10);
buf[last++] |= (crc >> 8) >> 2;
buf[last++] = ((crc >> 8) << 6) | ((crc & 0xff) >> 2);
@@ -446,10 +429,7 @@ static bool __attribute__((unused)) XN297_ReadPayload(uint8_t* msg, uint8_t len)
for (uint8_t i = 0; i < len; ++i)
crc16_update( buf[i], 8);
//xorout
if(xn297_scramble_enabled)
crc ^= pgm_read_word(&xn297_crc_xorout_scrambled[xn297_addr_len - 3 + len]);
else
crc ^= pgm_read_word(&xn297_crc_xorout[xn297_addr_len - 3 + len]);
crc ^= XN297_crc_xorout(xn297_scramble_enabled, 8 * (xn297_addr_len + len));
//test
if( (crc >> 8) == buf[len] && (crc & 0xff) == buf[len+1])
return true; // CRC OK
@@ -492,10 +472,10 @@ static uint8_t __attribute__((unused)) XN297_ReadEnhancedPayload(uint8_t* msg, u
crc16_update( buffer[pcf_size+1] & 0xc0, 2);
//xorout
if (xn297_scramble_enabled)
crc ^= pgm_read_word(&xn297_crc_xorout_scrambled_enhanced[xn297_addr_len-3+pcf_size]);
crc ^= XN297_crc_xorout(true, 8 * (xn297_addr_len + pcf_size) + 10);
#ifdef XN297DUMP_NRF24L01_INO
else
crc ^= pgm_read_word(&xn297_crc_xorout_enhanced[xn297_addr_len - 3 + pcf_size]);
crc ^= XN297_crc_xorout(false, 8 * (xn297_addr_len + pcf_size) + 10);
#endif
uint16_t crcxored=(buffer[pcf_size+1]<<10)|(buffer[pcf_size+2]<<2)|(buffer[pcf_size+3]>>6) ;
if( crc == crcxored)