PIC18F4580 Datasheet, PDF (332/484 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with ECAN Technology, 10-Bit A/D and nanoWatt Technology

PIC18F2480/2580/4480/4580
23.7.3 ENHANCED FIFO MODE
When configured for Mode 2, two of the dedicated
receive buffers in combination with one or more pro-
grammable transmit/receive buffers, are used to create
a maximum of an 8 buffer deep FIFO buffer. In this
mode, there is no direct correlation between filters and
receive buffer registers. Any filter that has been
enabled can generate an acceptance. When a mes-
sage has been accepted, it is stored in the next avail-
able receive buffer register and an internal write pointer
is incremented. The FIFO can be a maximum of 8 buff-
ers deep. The entire FIFO must consist of contiguous
receive buffers. The FIFO head begins at RXB0 buffer
and its tail spans toward B5. The maximum length of
the FIFO is limited by the presence or absence of the
first transmit buffer starting from B0. If a buffer is con-
figured as a transmit buffer, the FIFO length is reduced
accordingly. For instance, if B3 is configured as a trans-
mit buffer, the actual FIFO will consist of RXB0, RXB1,
B0, B1 and B2, a total of 5 buffers. If B0 is configured
as a transmit buffer, the FIFO length will be 2. If none
of the programmable buffers are configured as a trans-
mit buffer, the FIFO will be 8 buffers deep. A system
that requires more transmit buffers should try to locate
transmit buffers at the very end of B0-B5 buffers to
maximize available FIFO length.
When a message is received in FIFO mode, the inter-
rupt flag code bits (EICODE<4:0>) in the CANSTAT
register will have a value of ‘10000’, indicating the
FIFO has received a message. FIFO Pointer bits,
FP<3:0> in the CANCON register, point to the buffer
that contains data not yet read. The FIFO pointer bits,
in this sense, serve as the FIFO read pointer. The user
should use FP bits and read corresponding buffer data.
When receive data is no longer needed, the RXFUL bit
in the current buffer must be cleared, causing FP<3:0>
to be updated by the module.
To determine whether FIFO is empty or not, the user
may use FP<3:0> bits to access the RXFUL bit in the
current buffer. If RXFUL is cleared, the FIFO is consid-
ered to be empty. If it is set, the FIFO may contain one
or more messages. In Mode 2, the module also pro-
vides a bit called FIFO High Water Mark (FIFOWM) in
the ECANCON register. This bit can be used to cause
an interrupt whenever the FIFO contains only one or
four empty buffers. The FIFO high water mark interrupt
can serve as an early warning to a full FIFO condition.
23.7.4 TIME-STAMPING
The CAN module can be programmed to generate a
time-stamp for every message that is received. When
enabled, the module generates a capture signal for
CCP1, which in turn captures the value of either Timer1
or Timer3. This value can be used as the message
time-stamp.
To use the time-stamp capability, the CANCAP bit
(CIOCAN<4>) must be set. This replaces the capture
input for CCP1 with the signal generated from the CAN
module. In addition, CCP1CON<3:0> must be set to
‘0011’ to enable the CCP special event trigger for CAN
events.
23.8 Message Acceptance Filters
and Masks
The message acceptance filters and masks are used to
determine if a message in the Message Assembly
Buffer should be loaded into any of the receive buffers.
Once a valid message has been received into the MAB,
the identifier fields of the message are compared to the
filter values. If there is a match, that message will be
loaded into the appropriate receive buffer. The filter
masks are used to determine which bits in the identifier
are examined with the filters. A truth table is shown
below in Table 23-2 that indicates how each bit in the
identifier is compared to the masks and filters to deter-
mine if a message should be loaded into a receive
buffer. The mask essentially determines which bits to
apply the acceptance filters to. If any mask bit is set to
a zero, then that bit will automatically be accepted
regardless of the filter bit.
TABLE 23-2: FILTER/MASK TRUTH TABLE
Mask
bit n
Filter
bit n
Message
Identifier
bit n001
Accept or
Reject
bit n
0
x
x
1
0
0
1
0
1
1
1
0
1
1
1
Legend: x = don’t care
Accept
Accept
Reject
Reject
Accept
In Mode 0, acceptance filters RXF0 and RXF1 and filter
mask RXM0 are associated with RXB0. Filters RXF2,
RXF3, RXF4 and RXF5 and mask RXM1 are
associated with RXB1.
DS39637A-page 330
Preliminary
 2004 Microchip Technology Inc.