PIC24FJ128GA Datasheet, PDF (164/232 Pages) Microchip Technology – General Purpose, 16-Bit Flash Microcontrollers

PIC24FJ128GA FAMILY
19.2 Overview
The module implements a software configurable CRC
generator. The terms of the polynomial and its length
can be programmed using the CRCXOR (X<15:1>) bits
and the CRCCON (PLEN3:PLEN0) bits, respectively.
Consider the CRC equation:
x¹⁶ + x¹² + x⁵ + 1
To program this polynomial into the CRC generator,
the CRC register bits should be set as shown in
Table 19-1.
FIGURE 19-1:
CRC SHIFTER DETAILS
TABLE 19-1: EXAMPLE CRC SETUP
Bit Name
Bit Value
PLEN3:PLEN0
X<15:1>
1111
000100000010000
Note that for the value of X<15:1>, the 12th bit and the
5th bit are set to ‘1’, as required by the equation. The
0th bit required by the equation is always XORed. For
a 16-bit polynomial, the 16th bit is also always
assumed to be XORed; therefore, the X<15:1> bits do
not have the 0th bit or the 16th bit.
The topology of a standard CRC generator is shown in
Figure 19-2.
PLEN<3:0>
0
Hold
XOR
DOUT
OUT
IN
BIT 0
p_clk
1
2
X1
Hold
CRC Shift Register
X2
Hold
0
OUT
IN
BIT 1
1
p_clk
0
OUT
IN
BIT 2
1
p_clk
15
X3 X15
Hold
0
0
OUT
IN
1
1
BIT 15
p_clk
CRC Read Bus
CRC Write Bus
DS39747C-page 162
Preliminary
© 2006 Microchip Technology Inc.