Datasheet

PIC18F2480/2580/4480/4580
DS39637D-page 336 © 2009 Microchip Technology Inc.
24.7.3 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 buffers deep. The entire FIFO must consist of con-
tiguous receive buffers. The FIFO head begins at
RXB0 buffer and its tail spans toward B5. The maxi-
mum length of the FIFO is limited by the presence or
absence of the first transmit buffer starting from B0. If a
buffer is configured as a transmit buffer, the FIFO
length is reduced accordingly. For instance, if B3 is
configured as a transmit buffer, the actual FIFO will
consist of RXB0, RXB1, B0, B1 and B2, a total of 5 buf-
fers. If B0 is configured as a transmit buffer, the FIFO
length will be 2. If none of the programmable buffers
are configured as a transmit 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 the FP<3:0> bits to access the RXFUL bit in
the current buffer. If RXFUL is cleared, the FIFO is con-
sidered to be empty. If it is set, the FIFO may contain
one or more messages. In Mode 2, the module also
provides 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.
24.7.4 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
(CIOCON<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.
24.8 Message Acceptance Filters
and Masks
The message acceptance filters and masks are used to
determine if a message in the Message Assembly Buf-
fer 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 24-2 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 buf-
fer. 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 24-2: FILTER/MASK TRUTH TABLE
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.
Mask
bit n
Filter
bit n
Message
Identifier
bit n001
Accept or
Reject
bit n
0x xAccept
10 0Accept
10 1Reject
11 0Reject
11 1Accept
Legend: x = don’t care