basic ECC spec

specs/data.md

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+# Data card
+
+The data card deals with binary data, thus there must be a simple format.
+
+## Hamming ECC
+
+Each nibble (4-bit sequence) uses a Hamming (7, 4) code encoded in 1 byte, with the ability to detect 2 bit errors (but not correct them)
+
+Given N bytes to encode, the final encoded output will take 2N bytes.
+This also means when decoding, the length must be even.
+
+A byte is split into 2 nibbles, with the one comprised of the lowest bits first.
+Each nibble is ECC encoded separately.
+In a nibble there are 4 bits d0, d1, d2 and d3, where dN refers to the bit with value 2^N (2 to the N).
+In the output, there are p0, p1, p2, p3 bits, where p1 and above represent hamming code parity data, and p0 is a parity check on the whole block.
+
+### Nibble arrangement
+
+```
+| d0 | d1 | d2 | d3 |
+```
+
+### Hamming code arrangement
+
+```
+| p0 | p1 | p2 | d0 |
+| p3 | d1 | d2 | d3 |
+```
+
+This creates a fairly generic hamming code structure. Do note that p0 is the bit for checking the whole block's parity.