Datasheet

The corresponding Channel Enable bit in the Channel Control A register is cleared
(CHCTRLA.ENABLE=0) when the channel is disabled.
The corresponding DMAC Enable bit in the Control register is cleared (CTRL.DMAENABLE=0) when the
entire DMAC module is disabled.
22.6.3.8 CRC Operation
A Cyclic Redundancy Check (CRC) is an error detection technique used to find errors in data. It is
commonly used to determine whether the data during a transmission, or data present in data and
program memories has been corrupted or not. 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 data is received, 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.
The CRC engine in DMAC supports two commonly used CRC polynomials: CRC-16 (CRC-CCITT) and
CRC-32 (IEEE 802.3). Typically, applying CRC-n (CRC-16 or CRC-32) to a data block of arbitrary length
will detect any single alteration that is ≤n bits in length, and will detect the fraction 1-2-n of all longer error
bursts.
• CRC-16:
– Polynomial: x
16
+ x
12
+ x
5
+ 1
– Hex value: 0x1021
• CRC-32:
– 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
The data source for the CRC engine can either be one of the DMA channels or the APB bus interface,
and must be selected by writing to the CRC Input Source bits in the CRC Control register
(CRCCTRL.CRCSRC). The CRC engine then takes data input from the selected source and generates a
checksum based on these data. The checksum is available in the CRC Checksum register
(CRCCHKSUM). When CRC-32 polynomial is used, the final checksum read is bit reversed and
complemented, as shown in Figure 22-19.
The CRC polynomial is selected by writing to the CRC Polynomial Type bit in the CRC Control register
(CRCCTRL.CRCPOLY), the default is CRC-16. The CRC engine operates on byte only. When the DMA is
used as data source for the CRC engine, the DMA channel beat size setting will be used. When used
with APB bus interface, the application must select the CRC Beat Size bit field of CRC Control register
(CRCCTRL.CRCBEATSIZE). 8-, 16-, or 32-bit bus transfer access type is supported. The corresponding
number of bytes will be written in the CRCDATAIN register and the CRC engine will operate on the input
data in a byte by byte manner.
SAM D5x/E5x Family Data Sheet
DMAC – Direct Memory Access Controller
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 395