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Replaced 256 entry CRC8 table with tiny 2x16 entry table

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Arjen Lentz
2018-03-11 15:36:33 +10:00
parent ac0ea9a7fc
commit 83d99762b3
1 changed files with 15 additions and 27 deletions
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OneWire.cpp
+15 -27
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@@ -496,46 +496,34 @@ uint8_t OneWire::search(uint8_t *newAddr, bool search_mode /* = true */)
//
#if ONEWIRE_CRC8_TABLE
// This table comes from Dallas sample code where it is freely reusable,
// though Copyright (C) 2000 Dallas Semiconductor Corporation
static const uint8_t PROGMEM dscrc_table[] = {
0, 94,188,226, 97, 63,221,131,194,156,126, 32,163,253, 31, 65,
157,195, 33,127,252,162, 64, 30, 95, 1,227,189, 62, 96,130,220,
35,125,159,193, 66, 28,254,160,225,191, 93, 3,128,222, 60, 98,
190,224, 2, 92,223,129, 99, 61,124, 34,192,158, 29, 67,161,255,
70, 24,250,164, 39,121,155,197,132,218, 56,102,229,187, 89, 7,
219,133,103, 57,186,228, 6, 88, 25, 71,165,251,120, 38,196,154,
101, 59,217,135, 4, 90,184,230,167,249, 27, 69,198,152,122, 36,
248,166, 68, 26,153,199, 37,123, 58,100,134,216, 91, 5,231,185,
140,210, 48,110,237,179, 81, 15, 78, 16,242,172, 47,113,147,205,
17, 79,173,243,112, 46,204,146,211,141,111, 49,178,236, 14, 80,
175,241, 19, 77,206,144,114, 44,109, 51,209,143, 12, 82,176,238,
50,108,142,208, 83, 13,239,177,240,174, 76, 18,145,207, 45,115,
202,148,118, 40,171,245, 23, 73, 8, 86,180,234,105, 55,213,139,
87, 9,235,181, 54,104,138,212,149,203, 41,119,244,170, 72, 22,
233,183, 85, 11,136,214, 52,106, 43,117,151,201, 74, 20,246,168,
116, 42,200,150, 21, 75,169,247,182,232, 10, 84,215,137,107, 53};
// Dow-CRC using polynomial X^8 + X^5 + X^4 + X^0
// Tiny 2x16 entry CRC table created by Arjen Lentz
// See http://lentz.com.au/blog/calculating-crc-with-a-tiny-32-entry-lookup-table
static const uint8_t PROGMEM dscrc2x16_table[] = {
0x00, 0x5E, 0xBC, 0xE2, 0x61, 0x3F, 0xDD, 0x83,
0xC2, 0x9C, 0x7E, 0x20, 0xA3, 0xFD, 0x1F, 0x41,
0x00, 0x9D, 0x23, 0xBE, 0x46, 0xDB, 0x65, 0xF8,
0x8C, 0x11, 0xAF, 0x32, 0xCA, 0x57, 0xE9, 0x74
};
//
// Compute a Dallas Semiconductor 8 bit CRC. These show up in the ROM
// and the registers. (note: this might better be done without to
// table, it would probably be smaller and certainly fast enough
// compared to all those delayMicrosecond() calls. But I got
// confused, so I use this table from the examples.)
//
// and the registers. (Use tiny 2x16 entry CRC table)
uint8_t OneWire::crc8(const uint8_t *addr, uint8_t len)
{
uint8_t crc = 0;
while (len--) {
crc = pgm_read_byte(dscrc_table + (crc ^ *addr++));
crc = *addr++ ^ crc; // just re-using crc as intermediate
crc = pgm_read_byte(dscrc2x16_table + (crc & 0x0f)) ^
pgm_read_byte(dscrc2x16_table + 16 + ((crc >> 4) & 0x0f));
}
return crc;
}
#else
//
// Compute a Dallas Semiconductor 8 bit CRC directly.
// this is much slower, but much smaller, than the lookup table.
// this is much slower, but a little smaller, than the lookup table.
//
uint8_t OneWire::crc8(const uint8_t *addr, uint8_t len)
{