MCP2515-E/PRB2 Microchip Technology, MCP2515-E/PRB2 Datasheet - Page 32

• Manufacturer Part Number: MCP2515-E/PRB2
• Description: CAN controller with SPI interface, 125 deg C, -40C to +125C, 18-PDIP, TUBE
• Manufacturer: Microchip Technology

MCP2515
4.5
The message acceptance filters and masks are used to
determine if a message in the message assembly
buffer should be loaded into either of the receive
buffers (see
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.
4.5.1
When receiving standard data frames (11-bit identifier),
the DSTEMP automatically applies 16 bits of masks
and
identifiers to the first 16 bits of the data field (data bytes
0 and 1).
apply to extended and standard data frames.
Data byte filtering reduces the load on the MCU when
implementing Higher Layer Protocols (HLPs) that filter
on the first data byte (e.g., DeviceNet™).
4.5.2
The
Register 4-17) are used to determine which bits in the
identifier are examined with the filters. A truth table is
shown in
FIGURE 4-4:
DS21801E-page 32
* The two MSb (EID17 and EID16) mask and filter bits are not used.
ID10
ID10
filters
filter
Message Acceptance Filters and
Masks
Table 4-2
Figure 4-4
DATA BYTE FILTERING
FILTER MATCHING
11-bit ID Standard frame
Figure
masks
normally
that indicates how each bit in the
4-5). Once a valid message has
illustrates how masks and filters
MASKS AND FILTERS APPLY TO CAN FRAMES
(see
associated
Register 4-14
Masks and Filters apply to the entire 29-bit ID field
with
ID0
ID0
extended
EID17
*
through
Standard Data Frame
Extended Frame
Data Byte 0
identifier is compared to the masks and filters to deter-
mine if the 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, that bit will automatically be accepted,
regardless of the filter bit.
TABLE 4-2:
As shown in the receive buffers block diagram
(Figure
filter mask RXM0) are associated with RXB0. Filters
RXF2, RXF3, RXF4, RXF5 and mask RXM1 are
associated with RXB1.
Mask Bit
Note:
n
0
1
1
1
1
16-bit data filtering *
4-2), acceptance filters RXF0 and RXF1 (and
X = don’t care
Filter Bit n
FILTER/MASK TRUTH TABLE
X
0
0
1
1
© 2007 Microchip Technology Inc.
Message
Identifier
Data Byte 1
bit
X
0
1
0
1
Accept or
Reject bit
Accept
Accept
Accept
Reject
Reject
EID0
n