I came across a project where we needed to read and handle iButtons. The iButton, also known as a Dallas Key, looks like a flat battery. To use the iButton is pressed against a reader and the identitiy is transferred over the OneWire protocol.

There is a built in cyclic redundancy check, but it took som effort to actually verify it in code. Thus I think this can be helpful for anyone trying it out.

I have combined a table I found. I can’t rember where. The table is built using the specific polynominal used by Dallas Semiconductor Corporation for Dallas Keys, or iButtons as they are now called.

Once I combined the dallas table with the CRC8 impementation from Sanity-Free I finally got it working.

In my specific case we use a Raspberry Pi and a HAS7 for serial communication with the iButton. This reads the iButton backwards. Thus the first byte read should correspond to the CRC calculated using the remaing 7 bytes in reverse order.

[code language=”csharp”]
// BASED ON
// http://sanity-free.org/146/crc8_implementation_in_csharp.html
public static class Crc8
{
// This table comes from Dallas sample code where it is
// freely reusable, though Copyright (C) 2000 Dallas
// Semiconductor Corporation
static byte[] 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
};

public static byte ComputeChecksum(params byte[] bytes)
{
byte crc = 0;
if (bytes != null && bytes.Length > 0)
{
foreach (byte b in bytes)
{
crc = table[crc ^ b];
}
}
return crc;
}
}
[/code]