Datasheet

© 2009 Microchip Technology Inc. DS39637D-page 335
PIC18F2480/2580/4480/4580
24.7 Message Reception
24.7.1 RECEIVING A MESSAGE
Of all receive buffers, the MAB is always committed to
receiving the next message from the bus. The MCU
can access one buffer while the other buffer is available
for message reception or holding a previously received
message.
When a message is moved into either of the receive
buffers, the associated RXFUL bit is set. This bit must
be cleared by the MCU when it has completed process-
ing the message in the buffer in order to allow a new
message to be received into the buffer. This bit
provides a positive lockout to ensure that the firmware
has finished with the message before the module
attempts to load a new message into the receive buffer.
If the receive interrupt is enabled, an interrupt will be
generated to indicate that a valid message has been
received.
Once a message is loaded into any matching buffer,
user firmware may determine exactly what filter caused
this reception by checking the filter hit bits in the
RXBnCON or BnCON registers. In Mode 0,
FILHIT<3:0> of RXBnCON serve as filter hit bits. In
Mode 1 and 2, FILHIT<4:0> bits of BnCON serve as fil-
ter hit bits. The same registers also indicate whether
the current message is an RTR frame or not. A
received message is considered a standard identifier
message if the EXID bit in the RXBnSIDL or the
BnSIDL register is cleared. Conversely, a set EXID bit
indicates an extended identifier message. If the
received message is a standard identifier message,
user firmware needs to read the SIDL and SIDH regis-
ters. In the case of an extended identifier message,
firmware should read the SIDL, SIDH, EIDL and EIDH
registers. If the RXBnDLC or BnDLC register contain
non-zero data count, user firmware should also read
the corresponding number of data bytes by accessing
the RXBnDm or the BnDm registers. When a received
message is an RTR and if the current buffer is not con-
figured for automatic RTR handling, user firmware
must take appropriate action and respond manually.
Each receive buffer contains RXM bits to set special
Receive modes. In Mode 0, RXM<1:0> bits in
RXBnCON define a total of four Receive modes. In
Mode 1 and 2, RXM1 bit, in combination with the EXID
mask and filter bit, define the same four receive modes.
Normally, these bits are set to ‘00’ to enable reception
of all valid messages as determined by the appropriate
acceptance filters. In this case, the determination of
whether or not to receive standard or extended mes-
sages is determined by the EXIDE bit in the accep-
tance filter register. In Mode 0, if the RXM bits are set
to ‘01’ or ‘10’, the receiver will accept only messages
with standard or extended identifiers, respectively. If an
acceptance filter has the EXIDE bit set such that it does
not correspond with the RXM mode, that acceptance
filter is rendered useless. In Mode 1 and 2, setting
EXID in the SIDL Mask register will ensure that only
standard or extended identifiers are received. These
two modes of RXM bits can be used in systems where
it is known that only standard or extended messages
will be on the bus. If the RXM bits are set to ‘11’ (RXM1
= 1 in Mode 1 and 2), the buffer will receive all mes-
sages regardless of the values of the acceptance fil-
ters. Also, if a message has an error before the end of
frame, that portion of the message assembled in the
MAB before the error frame will be loaded into the buf-
fer. This mode may serve as a valuable debugging tool
for a given CAN network. It should not be used in an
actual system environment as the actual system will
always have some bus errors and all nodes on the bus
are expected to ignore them.
In Mode 1 and 2, when a programmable buffer is
configured as a transmit buffer and one or more accep-
tance filters are associated with it, all incoming messages
matching this acceptance filter criteria will be discarded.
To avoid this scenario, user firmware must make sure
that there are no acceptance filters associated with a
buffer configured as a transmit buffer.
24.7.2 RECEIVE PRIORITY
When in Mode 0, RXB0 is the higher priority buffer and
has two message acceptance filters associated with it.
RXB1 is the lower priority buffer and has four acceptance
filters associated with it. The lower number of acceptance
filters makes the match on RXB0 more restrictive and
implies a higher priority for that buffer. Additionally, the
RXB0CON register can be configured such that if RXB0
contains a valid message and another valid message is
received, an overflow error will not occur and the new
message will be moved into RXB1 regardless of the
acceptance criteria of RXB1. There are also two
programmable acceptance filter masks available, one for
each receive buffer (see Section 24.5 “CAN Message
Buffers”).
In Mode 1 and 2, there are a total of 16 acceptance
filters available and each can be dynamically assigned
to any of the receive buffers. A buffer with a lower
number has higher priority. Given this, if an incoming
message matches with two or more receive buffer
acceptance criteria, the buffer with the lower number
will be loaded with that message.
Note: The entire contents of the MAB are moved
into the receive buffer once a message is
accepted. This means that regardless of
the type of identifier (standard or
extended) and the number of data bytes
received, the entire receive buffer is over-
written with the MAB contents. Therefore,
the contents of all registers in the buffer
must be assumed to have been modified
when any message is received.