CRC
In Embedded Systems, CRC usually stands for cyclic redundancy check. A CRC is in many respect similar to a checksum, and in fact, this is what they are typically used for. CRCs come in all kinds of sizes, from 4 bits to 64 bits, but there is no upper limit.
Generating CRCs in Hardware
CRCs can be generated very easily in hardware, by shifting the data through a shift registers with feedback at certain bit positions. A 32-bit CRC requires not much more than a 32-bit shift registers. This is ideal for applications in which the data is actually transferred bit-by-bit. A lot of MCUs have built-in CRC circuits, that allow computation of typically a specific CRC, such as 0x1021 (16-bit). The downside of these hardware units are 2 things:
- Fixed to a specific CRC
- Not thread safe: The CRC unit can only be used from one thread(task) at a time, so the software needs to be designed in a way that makes sure this is so, typically using the unit only in a single thread, or by securing it using a Mutex/Semaphore/Critical region or simply disabling interrupts
Generating CRCs in software
Generating CRCs is something that needs to done a lot. Again, there are a lot of ways to do it. The easiest way is to do the same thing that is easy in hardware, and compute the CRC bit-by-bit. Unfortunately, this is also a slow way of doing it. More advance ways of doing it are table based. The tables can be any size, so typically 16 entries for 4 bits at a time or 256 entries for 8 bits (one byte) at a time.
More to come.
