PIC24FJ256GB106-I Datasheet, PDF (247/328 Pages) Microchip Technology – 64/80/100-Pin, 16-Bit Flash Microcontrollers with USB On-The-Go (OTG)

PIC24FJ256GB110 FAMILY
20.0 PROGRAMMABLE CYCLIC
REDUNDANCY CHECK (CRC)
GENERATOR
Note:
This data sheet summarizes the features
of this group of PIC24F devices. It is not
intended to be a comprehensive reference
source. For more information, refer to the
“PIC24F Family Reference Manual”,
”Section 30. Programmable Cyclic
Redundancy Check (CRC)” (DS39714).
The programmable CRC generator offers the following
features:
• User-programmable polynomial CRC equation
• Interrupt output
• Data FIFO
The module implements a software configurable CRC
generator. The terms of the polynomial and its length
can be programmed using the X15:X1 bits
(CRCXOR<15:1>) and the PLEN3:PLEN0 bits
(CRCCON<3:0>), respectively.
FIGURE 20-1:
CRC SHIFTER DETAILS
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 20-1.
TABLE 20-1: EXAMPLE CRC SETUP
Bit Name
Bit Value
PLEN3:PLEN0
X15:X1
1111
000100000010000
Note that for the value of X15:X1, the 12th bit and the
5th bit are set to ‘1’, as required by the equation. The
0 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 0 bit or the 16th bit.
The topology of a standard CRC generator is shown in
Figure 20-2.
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
© 2008 Microchip Technology Inc.
Preliminary
DS39897B-page 245