70291E Datasheet, PDF (309/436 Pages) Microchip Technology – High-Performance, 16-bit Digital Signal Controllers

dsPIC33FJ32MC302/304, dsPIC33FJ64MCX02/X04 AND dsPIC33FJ128MCX02/X04
26.0 PROGRAMMABLE CYCLIC
REDUNDANCY CHECK (CRC)
GENERATOR
Note 1: This data sheet summarizes the features
of the dsPIC33FJ32MC302/304,
dsPIC33FJ64MCX02/X04
and
dsPIC33FJ128MCX02/X04 family 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
web
site
(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.
The programmable CRC generator offers the following
features:
• User-programmable polynomial CRC equation
• Interrupt output
• Data FIFO
FIGURE 26-1:
CRC SHIFTER DETAILS
26.1 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 26-1: 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 26-1.
TABLE 26-1: EXAMPLE CRC SETUP
Bit Name
Bit Value
PLEN<3:0>
X<15:1>
1111
000100000010000
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 26-2.
PLEN<3:0>
0
Hold
XOR
DOUT
OUT
IN
BIT 0
p_clk
1
2
X1
Hold
0
OUT
IN
BIT 1
1
p_clk
CRC Shift Register
X2
Hold
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
© 2011 Microchip Technology Inc.
DS70291E-page 309