MCP2515
If the BUKT bit is clear, there are six codes
corresponding to the six filters. If the BUKT bit is set,
there are six codes corresponding to the six filters, plus
two additional codes corresponding to RXF0 and RXF1
filters that roll over into RXB1.
4.5.3
FILHIT BITS
Filter matches on received messages can be
determined by the FILHIT bits in the associated
RXBnCTRL register. RXB0CTRL.FILHIT0 for buffer 0
and RXB1CTRL.FILHIT<2:0> for buffer 1.
The three FILHIT bits for receive buffer 1 (RXB1) are
coded as follows:
4.5.4
MULTIPLE FILTER MATCHES
If more than one acceptance filter matches, the FILHIT
bits will encode the binary value of the lowest
numbered filter that matched. For example, if filter
RXF2 and filter RXF4 match, FILHIT will be loaded with
the value for RXF2. This essentially prioritizes the
acceptance filters with a lower-numbered filter having
higher priority. Messages are compared to filters in
ascending order of filter number. This also insures that
the message will only be received into one buffer. This
implies that RXB0 has a higher priority than RXB1.
- 101= Acceptance Filter 5 (RXF5)
- 100= Acceptance Filter 4 (RXF4)
- 011= Acceptance Filter 3 (RXF3)
- 010= Acceptance Filter 2 (RXF2)
- 001= Acceptance Filter 1 (RXF1)
- 000= Acceptance Filter 0 (RXF0)
Note: 000and 001can only occur if the BUKT bit
in RXB0CTRL is set, allowing RXB0
messages to roll over into RXB1.
4.5.5
CONFIGURING THE MASKS AND
FILTERS
RXB0CTRL contains two copies of the BUKT bit and
the FILHIT<0> bit.
The mask and filter registers can only be modified
when the MCP2515 is in Configuration mode (see
Section 10.0 “Modes of Operation”).
The coding of the BUKT bit enables these three bits to
be used similarly to the RXB1CTRL.FILHIT bits and to
distinguish a hit on filter RXF0 and RXF1 in either
RXB0 or after a roll over into RXB1.
- 111= Acceptance Filter 1 (RXB1)
- 110= Acceptance Filter 0 (RXB1)
- 001= Acceptance Filter 1 (RXB0)
- 000= Acceptance Filter 0 (RXB0)
FIGURE 4-5:
MESSAGE ACCEPTANCE MASK AND FILTER OPERATION
Acceptance Filter Register
Acceptance Mask Register
RXFn0
RXMn0
RXMn1
RxRqst
RXFn1
RXFnn
RXMnn
Message Assembly Buffer
Identifier
© 2005 Microchip Technology Inc.
Preliminary
DS21801D-page 33
MICROCHIP [ MICROCHIP ]
