PIC24FJ256DA210 Datasheet, PDF (298/408 Pages) Microchip Technology – 64/100-Pin, 16-Bit Flash Microcontrollers with Graphics Controller and USB On-The-Go (OTG)

PIC24FJ256DA210 FAMILY
21.1 User Interface
21.1.1 POLYNOMIAL INTERFACE
The CRC module can be programmed for CRC
polynomials of up of up the 32^nd order, using up to 32 bits.
Polynomial length, which reflects the highest exponent
in the equation, is selected by the PLEN<4:0> bits
(CRCCON2<4:0>).
The CRCXORL and CRCXORH registers control which
exponent terms are included in the equation. Setting a
particular bit includes that exponent term in the equa-
tion; functionally, this includes an XOR operation on the
corresponding bit in the CRC engine. Clearing the bit
disables the XOR.
For example, consider two CRC polynomials, one a
16-bit and the other a 32-bit equation.
EQUATION 21-1: 16-BIT, 32-BIT CRC
POLYNOMIALS
X16 + X12 + X5 + 1
and
X32+X26 + X23 + X22 + X16 + X12 + X11 + X10 +
X8 + X7 + X5 + X4 + X2 + X + 1
To program these polynomial into the CRC generator,
set the register bits as shown in Table 21-1.
Note that the appropriate positions are set to ‘1’ to indi-
cate they are used in the equation (for example, X26
and X23). The ‘0’ bit required by the equation is always
XORed; thus, X0 is a don’t care. For a polynomial of
length 32, it is assumed that the 32^nd bit will be used.
Therefore, the X<31:1> bits do not have the 32^nd bit.
21.1.2 DATA INTERFACE
The module incorporates a FIFO that works with a vari-
able data width. Input data width can be configured to
any value between one and 32 bits using the
DWIDTH<4:0> bits (CRCCON2<12:8>). When the
data width is greater than 15, the FIFO is four words
deep. When the DWITDH bits are between 15 and 8,
the FIFO is 8 words deep. When the DWIDTH bits are
less than 8, the FIFO is 16 words deep.
The data for which the CRC is to be calculated must
first be written into the FIFO. Even if the data width is
less than 8, the smallest data element that can be writ-
ten into the FIFO is one byte. For example, if DWIDTH
is five, then the size of the data is DWIDTH + 1 or six.
The data is written as a whole byte; the two unused
upper bits are ignored by the module.
Once data is written into the MSb of the CRCDAT reg-
isters (that is, MSb as defined by the data width), the
value of the VWORD<4:0> bits (CRCCON1<12:8>)
increments by one. For example, if DWIDTH is 24, the
VWORD bits will increment when bit 7 of CRCDATH is
written. Therefore, CRCDATL must always be written
to before CRCDATH.
The CRC engine starts shifting data when the CRCGO
bit is set and the value of VWORD is greater than zero.
Each word is copied out of the FIFO into a buffer regis-
ter, which decrements VWORD. The data is then
shifted out of the buffer. The CRC engine continues
shifting at a rate of two bits per instruction cycle, until
VWORD reaches zero. This means that for a given
data width, it takes half that number of instructions for
each word to complete the calculation. For example, it
takes 16 cycles to calculate the CRC for a single word
of 32-bit data.
When VWORD reaches the maximum value for the
configured value of DWIDTH (4, 8 or 16), the CRCFUL
bit becomes set. When VWORD reaches zero, the
CRCMPT bit becomes set. The FIFO is emptied and
the VWORD<4:0> bits are set to ‘00000’ whenever
CRCEN is ‘0’.
At least one instruction cycle must pass after a write to
CRCWDAT before a read of the VWORD bits is done.
TABLE 21-1: CRC SETUP EXAMPLES FOR 16 AND 32-BIT POLYNOMIALS
CRC Control Bits
16-Bit Polynomial
Bit Values
32-Bit Polynomial
PLEN<4:0>
X<31:16>
X<15:0>
01111
0000 0000 0000 0001
0001 0000 0010 000X
11111
0000 0100 1100 0001
0001 1101 1011 011x
DS39969B-page 298
 2010 Microchip Technology Inc.