PIC24F16KA102 datasheet(165/254 Pages) MICROCHIP

165 / 254 page

Preliminary

DS39927B-page 163

PIC24F16KA102 FAMILY

20.0

PROGRAMMABLE CYCLIC

REDUNDANCY CHECK (CRC)

GENERATOR

The programmable Cyclic Redundancy Check (CRC)

module in PIC24F devices is a software-configurable

CRC checksum generator. The CRC algorithm treats a

message as a binary bit stream and divides it by a fixed

binary number.

The remainder from this division is considered the

checksum. As in division, the CRC calculation is also

an iterative process. The only difference is that these

operations are done on modulo arithmetic based on

mod2. For example, division is replaced with the XOR

operation (i.e., subtraction without carry). The CRC

algorithm uses the term, polynomial, to perform all of

its calculations.

The divisor, dividend and remainder that are

represented by numbers are termed as polynomials

with binary coefficients.

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 CRCXOR (X<15:1>) bits

and the CRCCON (PLEN<3:0>) 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 provided in

Table 20-1.

TABLE 20-1:

EXAMPLE CRC SETUP

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 displayed

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. For more information on Program-

mable Cyclic Redundancy Check, refer to

the “PIC24F Family Reference Manual”,

Section

30.

“Programmable

Cyclic

Redundancy Check (CRC)”

(DS39714).

Bit Name

Bit Value

PLEN<3:0>

1111

X<15:1>

000100000010000

OUT

BIT 0

clk

OUT

BIT 1

clk

OUT

BIT 2

clk

OUT

BIT 15

clk

X15

XOR

DOUT

PLEN<3:0>

Hold

CRC Read Bus

CRC Write Bus

CRC Shift Register

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