PIC24HJ32GP302_11 Datasheet, PDF (248/368 Pages) Microchip Technology – High-Performance, 16-bit Microcontrollers

PIC24HJ32GP302/304, PIC24HJ64GPX02/X04 AND PIC24HJ128GPX02/X04
FIGURE 23-2:
CRC GENERATOR RECONFIGURED FOR x¹⁶ + x¹² + x⁵ + 1
XOR
SDOx
DQ
BIT 0
DQ
BIT 4
DQ
BIT 5
DQ
BIT 12
DQ
BIT 15
p_clk
p_clk
p_clk
p_clk
p_clk
CRC Read Bus
CRC Write Bus
23.2 User Interface
23.2.1 DATA INTERFACE
To start serial shifting, a ‘1’ must be written to the
CRCGO bit.
The module incorporates a FIFO that is 8 deep when
PLEN (PLEN<3:0>) > 7, and 16 deep, otherwise. The
data for which the CRC is to be calculated must first be
written into the FIFO. The smallest data element that
can be written into the FIFO is one byte. For example,
if PLEN = 5, then the size of the data is PLEN + 1 = 6.
The data must be written as follows:
data[5:0] = crc_input[5:0]
data[7:6] = ‘bxx
Once data is written into the CRCWDAT MSb (as
defined by PLEN), the value of VWORD
(VWORD<4:0>) increments by one. The serial shifter
starts shifting data into the CRC engine when
CRCGO = 1 and VWORD > 0. When the MSb is
shifted out, VWORD decrements by one. The serial
shifter continues shifting until the VWORD reaches 0.
Therefore, for a given value of PLEN, it will take
(PLEN + 1) * VWORD number of clock cycles to
complete the CRC calculations.
When VWORD reaches 8 (or 16), the CRCFUL bit will
be set. When VWORD reaches 0, the CRCMPT bit will
be set.
To continually feed data into the CRC engine, the
recommended mode of operation is to initially “prime”
the FIFO with a sufficient number of words so no
interrupt is generated before the next word can be
written. Once that is done, start the CRC by setting the
CRCGO bit to ‘1’. From that point onward, the
VWORD<4:0> bits should be polled. If they read less
than 8 or 16, another word can be written into the FIFO.
To empty words already written into a FIFO, the
CRCGO bit must be set to ‘1’ and the CRC shifter
allowed to run until the CRCMPT bit is set.
Also, to get the correct CRC reading, it will be
necessary to wait for the CRCMPT bit to go high before
reading the CRCWDAT register.
If a word is written when the CRCFUL bit is set, the
VWORD Pointer will roll over to 0. The hardware will
then behave as if the FIFO is empty. However, the
condition to generate an interrupt will not be met;
therefore, no interrupt will be generated (See
Section 23.2.2 “Interrupt Operation”).
At least one instruction cycle must pass after a write to
CRCWDAT before a read of the VWORD bits is done.
23.2.2 INTERRUPT OPERATION
When the VWORD<4:0> bits make a transition from a
value of ‘1’ to ‘0’, an interrupt will be generated.
23.3 Operation in Power-Saving Modes
23.3.1 SLEEP MODE
If Sleep mode is entered while the module is operating,
the module will be suspended in its current state until
clock execution resumes.
23.3.2 IDLE MODE
To continue full module operation in Idle mode, the
CSIDL bit must be cleared prior to entry into the mode.
If CSIDL = 1, the module will behave the same way as
it does in Sleep mode; pending interrupt events will be
passed on, even though the module clocks are not
available.
DS70293E-page 248
© 2011 Microchip Technology Inc.