30.1 Architectural Overview

The programmable CRC generator module can be divided into two parts: the control logic and the CRC engine. The control logic incorporates a register interface, data FIFO, an interrupt generator and a CRC engine interface. The CRC engine incorporates a CRC calculator, which is implemented using a serial shifter with XOR function. A simplified block diagram is shown in Figure 30-1.

Figure 30-1. Simplified Block Diagram of the Programmable CRC Generator

The checksum is a unique number associated with a message or a particular block of data containing several bytes. Whether it is a data packet for communication or a block of data stored in memory, a checksum, helps to validate it before processing. The simplest way to calculate a checksum is to add together all the data bytes present in the message. However, this method of checksum calculation fails badly when the message is modified by inverting or swapping groups of bytes. Also, it fails when null bytes are added anywhere in the message.

The Cyclic Redundancy Checksum (CRC) is a more complicated but robust error-checking algorithm. The main idea behind the CRC algorithm is to treat a message as a binary bit stream and divide it by a fixed binary number. The remainder from this division is considered to be the checksum. Like in division, the CRC calculation is also an iterative process. The only difference is that these operations are done on modulo arithmetic, based on mod 2. For example, division is replaced with the XOR operation (i.e., subtraction without carry). The CRC algorithm uses the term polynomial to perform all of its calculations. The divisor, dividend and remainder that are represented by numbers are termed as polynomials with binary coefficients. For example, the number 25h (11001) is represented as:

(1 * x⁴) + (1 * x³) + (0 * x²) + (0 * x¹) + (1 * x⁰) or x⁴ + x³ + x⁰

In order to perform the CRC calculation, a suitable divisor is first selected. This divisor is called the generator polynomial. Since CRC is used to detect errors, a generator polynomial of a suitable length needs to be chosen for a given application, as each polynomial has different error detection capabilities. Some polynomials are widely used for many applications, but the error detecting capabilities of any particular polynomial are beyond the scope of this reference section.

The CRC algorithm is straightforward to implement in software. However, it requires considerable CPU bandwidth to implement the basic requirements, such as shift, bit test and XOR. Moreover, CRC calculation is an iterative process, and additional software overhead for data transfer instructions puts an enormous burden on the MIPS requirement of a microcontroller. In contrast, the software-configurable CRC hardware module facilitates a fast CRC checksum calculation with minimal software overhead.