PIC24FJ128GA006 datasheet(175/258 Pages) MICROCHIP

175 / 258 page

 2005-2012 Microchip Technology Inc.

DS39747F-page 175

PIC24FJ128GA010 FAMILY

20.0

PROGRAMMABLE CYCLIC

REDUNDANCY CHECK (CRC)

GENERATOR

The programmable CRC generator offers the following

features:

• User-programmable polynomial CRC equation

• Interrupt output

• Data FIFO

20.1

Registers

There are four registers used to control programmable

CRC operation:

• CRCCON

• CRCXOR

• CRCDAT

• CRCWDAT

20.2

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 (PLEN<3:0>) bits, respectively.

Consider the following equation:

EQUATION 20-1:

CRC POLYNOMIAL

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

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

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.

FIGURE 20-1:

CRC SHIFTER DETAILS

Note:

This data sheet summarizes the features of

this group of PIC24F devices. It is not

intended to be a comprehensive reference

source. Refer to Section 30. “Program-

mable

Cyclic

Redundancy

Check

(CRC)”

(DS39714) in the “PIC24F Family

Reference Manual”

for more information.

Bit Name

Bit Value

PLEN<3:0>

1111

X<15:1>

000100000010000

x16 + x12 + x5 + 1

OUT

BIT 0

p_clk

OUT

BIT 1

p_clk

OUT

BIT 2

p_clk

OUT

BIT 15

p_clk

X15

XOR

DOUT

PLEN<3:0>

Hold

CRC Read Bus

CRC Write Bus

CRC Shift Register

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