328
8331B–AVR–03/12
Atmel AVR XMEGA AU
26. CRC – Cyclic Redundancy Check Generator
26.1
Features
•
Cyclic redundancy check (CRC) generation and checking for
– Communication data
– Program or data in flash memory
– Data in SRAM and I/O memory space
•
Integrated with flash memory, DMA controller and CPU
– Continuous CRC on data going through a DMA channel
– Automatic CRC of the complete or a selectable range of the flash memory
– CPU can load data to the CRC generator through the I/O interface
•
CRC polynomial software selectable to
– CRC-16 (CRC-CCITT)
– CRC-32 (IEEE 802.3)
•
Zero remainder detection
26.2
Overview
A cyclic redundancy check (CRC) is an error detection technique test algorithm used to find
accidental errors in data, and it is commonly used to determine the correctness of a data trans-
mission, and data present in the data and program memories. A CRC takes a data stream or a
block of data as input and generates a 16- or 32-bit output that can be appended to the data and
used as a checksum. When the same data are later received or read, the device or application
repeats the calculation. If the new CRC result does not match the one calculated earlier, the
block contains a data error. The application will then detect this and may take a corrective
action, such as requesting the data to be sent again or simply not using the incorrect data.
Typically, an n-bit CRC applied to a data block of arbitrary length will detect any single error
burst not longer than n bits (any single alteration that spans no more than n bits of the data), and
will detect the fraction 1-2
-n
of all longer error bursts. The CRC module in XMEGA devices sup-
ports two commonly used CRC polynomials; CRC-16 (CRC-CCITT) and CRC-32 (IEEE 802.3).
•
CRC-16:
•
CRC-32:
Polynomial:
x
16
+
x
12
+
x
5
+1
Hex
value:
0x1021
Polynomial:
x
32
+
x
26
+
x
23
+
x
22
+
x
16
+
x
12
+
x
11
+
x
10
+
x
8
+
x
7
+
x
5
+
x
4
+
x
2
+
x
+1
Hex value:
0x04C11DB7
