Datasheet

dsPIC33EPXXX(GP/MC/MU)806/810/814 and PIC24EPXXX(GP/GU)810/814
DS70616F-page 458 Preliminary © 2009-2012 Microchip Technology Inc.
27.1 Overview
The CRC module can be programmed for CRC
polynomials of up to the 32nd order, using up to 32 bits.
Polynomial length, which reflects the highest exponent
in the equation, is selected by the PLEN<4:0> bits
(CRCCON2<4:0>).
The CRCXORL and CRCXORH registers control which
exponent terms are included in the equation. Setting a
particular bit includes that exponent term in the
equation; functionally, this includes an XOR operation
on the corresponding bit in the CRC engine. Clearing
the bit disables the XOR.
For example, consider two CRC polynomials, one a
16-bit equation and the other a 32-bit equation:
To program these polynomials into the CRC generator,
set the register bits as shown in Table 2 7 - 1 .
Note that the appropriate positions are set to ‘1’ to
indicate that they are used in the equation (for example,
X26 and X23). The 0 bit required by the equation is
always XORed; thus, X0 is a don’t care. For a poly-
nomial of length N, it is assumed that the Nth bit will
always be used, regardless of the bit setting. Therefore,
for a polynomial length of 32, there is no 32nd bit in the
CRCxOR register.
TABLE 27-1: CRC SETUP EXAMPLES FOR
16 AND 32-BIT POLYNOMIAL
27.2 Programmable CRC Resources
Many useful resources related to Programmable CRC
are provided on the main product page of the Microchip
web site for the devices listed in this data sheet. This
product page, which can be accessed using this link,
contains the latest updates and additional information.
27.2.1 KEY RESOURCES
• Section 27. “Programmable Cyclic Redundancy
Check (CRC)” (DS70346)
• Code Samples
• Application Notes
• Software Libraries
• Webinars
• All related dsPIC33E/PIC24E Family Reference
Manuals Sections
• Development Tools
x16 + x12 + x5 + 1
and
x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7
+ x5 + x4 + x2 + x + 1
CRC
Control Bits
Bit Values
16-bit
Polynomial
32-bit
Polynomial
PLEN<4:0> 01111 11111
X<31:16>
0000 0000
0000 000x
0000 0100
1100 0001
X<15:0>
0001 0000
0010 000x
0001 1101
1011 011x
Note: In the event you are not able to access the
product page using the link above, enter
this URL in your browser:
http://www.microchip.com/wwwproducts/
Devices.aspx?dDocName=en554310