Datasheet

© 2007-2012 Microchip Technology Inc. DS70292G-page 301
dsPIC33FJ32GP302/304, dsPIC33FJ64GPX02/X04, AND dsPIC33FJ128GPX02/X04
25.0 PROGRAMMABLE CYCLIC
REDUNDANCY CHECK (CRC)
GENERATOR
The programmable CRC generator offers the following
features:
• User-programmable polynomial CRC equation
• Interrupt output
• Data FIFO
25.1 Overview
The module implements a software configurable CRC
generator. The terms of the polynomial and its length
can be programmed using the CRCXOR bits (X<15:1>)
and the CRCCON bits (PLEN<3:0>), respectively.
EQUATION 25-1: CRC EQUATION
To program this polynomial into the CRC generator,
the CRC register bits should be set as shown in
Table 25-1.
TABLE 25-1: EXAMPLE CRC SETUP
For the value of X<15:1>, the 12th bit and the 5th bit are
set to ‘1’, as required by the CRC equation. The 0th bit
required by the CRC 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 25-2.
FIGURE 25-1: CRC SHIFTER DETAILS
Note 1: This data sheet summarizes the features
of the dsPIC33FJ32GP302/304,
dsPIC33FJ64GPX02/X04, and
dsPIC33FJ128GPX02/X04 families of
devices. It is not intended to be a
comprehensive reference source. To
complement the information in this data
sheet, refer to Section 36.
“Programmable Cyclic Redundancy
Check (CRC)” (DS70298) of the
“dsPIC33F/PIC24H Family Reference
Manual”, which is available from the
Microchip website (www.microchip.com).
2: Some registers and associated bits
described in this section may not be
available on all devices. Refer to
Section 4.0 “Memory Organization” in
this data sheet for device-specific register
and bit information.
Bit Name Bit Value
PLEN<3:0> 1111
X<15:1> 000100000010000
x
16
x
12
x
5
1+++
IN
OUT
BIT 0
0
1
p_clk
X1
IN
OUT
BIT 1
0
1
p_clk
X2
IN
OUT
BIT 2
0
1
p_clk
X3
IN
OUT
BIT 15
0
1
p_clk
X15
XOR
D
OUT
01 2 15
PLEN<3:0>
Hold
Hold
Hold
Hold
CRC Read Bus
CRC Write Bus
CRC Shift Register