Datasheet

dsPIC33EPXXXGM3XX/6XX/7XX
DS70000689D-page 406  2013-2014 Microchip Technology Inc.
FIGURE 29-2: CRC SHIFT ENGINE DETAIL
29.1 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 29-1.
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 29-1: CRC SETUP EXAMPLES FOR
16 AND 32-BIT POLYNOMIAL
CRCWDATH CRCWDATL
Bit 1 Bit n
(2)
X(1)
(1)
Read/Write Bus
Shift Buffer
Data
Bit 2
X(2)
(1)
X(n)
(1)
Note 1: Each XOR stage of the shift engine is programmable. See text for details.
2: Polynomial Length n is determined by ([PLEN<4:0>] + 1).
Bit 0
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