MCP2515
4.2
Receive Priority
4.0
4.1
MESSAGE RECEPTION
Receive Message Buffering
RXB0, the higher priority buffer, has one mask and two
message acceptance filters associated with it. The
received message is applied to the mask and filters for
RXB0 first.
The MCP2515 includes two full receive buffers with
multiple acceptance filters for each. There is also a
separate Message Assembly Buffer (MAB) that acts as
a third receive buffer (see Figure 4-2).
RXB1 is the lower priority buffer, with one mask and
four acceptance filters associated with it.
In addition to the message being applied to the RB0
mask and filters first, the lower number of acceptance
filters makes the match on RXB0 more restrictive and
implies a higher priority for that buffer.
4.1.1
MESSAGE ASSEMBLY BUFFER
Of the three receive buffers, the MAB is always
committed to receiving the next message from the bus.
The MAB assembles all messages received. These
messages will be transferred to the RXBn buffers (See
Register 4-4 to Register 4-9) only if the acceptance
filter criteria is met.
When a message is received, bits <3:0> of the
RXBnCTRL register will indicate the acceptance filter
number that enabled reception and whether the
received message is a remote transfer request.
4.1.2
RXB0 AND RXB1
4.2.1
ROLLOVER
The remaining two receive buffers, called RXB0 and
RXB1, can receive a complete message from the
protocol engine via the MAB. The MCU can access one
buffer, while the other buffer is available for message
Additionally, the RXB0CTRL 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.
reception, or for holding
message.
a previously received
4.2.2
RXM BITS
Note:
The entire contents of the MAB is 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
overwritten 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.
The RXBnCTRL.RXM bits set special receive modes.
Normally, these bits are cleared 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 messages is determined by the
RFXnSIDL.EXIDE bit in the acceptance filter register.
If the RXBnCTRL.RXM bits are set to 01 or 10, the
receiver will only accept messages with standard or
extended identifiers, respectively. If an acceptance
filter has the RFXnSIDL.EXIDE bit set such that it does
not correspond with the RXBnCTRL.RXM mode, that
acceptance filter is rendered useless. These two
modes of RXBnCTRL.RXM bits can be used in
systems where it is known that only standard or
extended messages will be on the bus.
4.1.3
RECEIVE FLAGS/INTERRUPTS
When a message is moved into either of the receive
buffers, the appropriate CANINTF.RXnIF bit is set. This
bit must be cleared by the MCU in order to allow a new
message to be received into the buffer. This bit
provides a positive lockout to ensure that the MCU has
finished with the message before the MCP2515
attempts to load a new message into the receive buffer.
If the RXBnCTRL.RXM bits are set to 11, the buffer will
receive all messages, regardless of the values of the
acceptance filters. Also, if a message has an error
before the EOF, that portion of the message assem-
bled in the MAB before the error frame will be loaded
into the buffer. This mode has some value in debugging
a CAN system and would not be used in an actual
system environment.
If the CANINTE.RXnIE bit is set, an interrupt will be
generated on the INT pin to indicate that a valid
message has been received. In addition, the
associated RXnBF pin will drive low if configured as a
receive buffer full pin. See Section 4.4 “RX0BF and
RX1BF Pins” for details.
© 2005 Microchip Technology Inc.
Preliminary
DS21801D-page 23
MICROCHIP [ MICROCHIP ]
