PIC24FV32KA304 Datasheet, PDF (205/322 Pages) Microchip Technology – 20/28/44/48-Pin, General Purpose, 16-Bit Flash Microcontrollers with XLP Technology

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20.1.3 DATA SHIFT DIRECTION
The LENDIAN bit (CRCCON1<3>) is used to control
the shift direction. By default, the CRC will shift data
through the engine, MSb first. Setting LENDIAN (= 1)
causes the CRC to shift data, LSb first. This setting
allows better integration with various communication
schemes and removes the overhead of reversing the
bit order in software. Note that this only changes the
direction of the data that is shifted into the engine. The
result of the CRC calculation will still be a normal CRC
result, not a reverse CRC result.
20.1.4 INTERRUPT OPERATION
The module generates an interrupt that is configurable
by the user for either of two conditions. If CRCISEL is
‘0’, an interrupt is generated when the VWORD<4:0>
bits make a transition from a value of ‘1’ to ‘0’. If
CRCISEL is ‘1’, an interrupt will be generated after the
CRC operation finishes and the module sets the
CRCGO bit to ‘0’. Manually setting CRCGO to ‘0’ will
not generate an interrupt.
20.1.5 TYPICAL OPERATION
To use the module for a typical CRC calculation:
1. Set the CRCEN bit to enable the module.
2. Configure the module for the desired operation:
a) Program the desired polynomial using the
CRCXORL and CRCXORH registers, and
the PLEN<4:0> bits
b) Configure the data width and shift direction
using the DWIDTH and LENDIAN bits
c) Select the desired interrupt mode using the
CRCISEL bit
3. Preload the FIFO by writing to the CRCDATL
and CRCDATH registers until the CRCFUL bit is
set or no data is left.
4. Clear old results by writing 00h to CRCWDATL
and CRCWDATH. CRCWDAT can also be left
unchanged to resume a previously halted
calculation.
5. Set the CRCGO bit to start calculation.
6. Write remaining data into the FIFO as space
becomes available.
7. When the calculation completes, CRCGO is
automatically cleared. An interrupt will be
generated if CRCISEL = 1.
8. Read CRCWDATL and CRCWDATH for the
result of the calculation.
20.2 Registers
There are eight registers associated with the module:
• CRCCON1
• CRCCON2
• CRCXORL
• CRCXORH
• CRCDATL
• CRCDATH
• CRCWDATL
• CRCWDATH
The CRCCON1 and CRCCON2 registers (Register 20-1
and Register 20-2) control the operation of the module,
and configure the various settings. The CRCXOR
registers (Register 20-3 and Register 20-4) select the
polynomial terms to be used in the CRC equation. The
CRCDAT and CRCWDAT registers are each register
pairs that serve as buffers for the double-word, input
data and CRC processed output, respectively.
 2011 Microchip Technology Inc.
DS39995A-page 205