COMMELL MPX-2515 User Manual
Page 18
Commell MPX-2515 User’s Guide Rev 0100
Taiwan Commate Computer Inc.
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62
RXBnD7
2's Complement Checksum
63
Checksum
Table 7 Firmware Message Buffer (FMB) format
Notes:
•
RXB0 / RXB1: 0X60 if the message is from RXB0. 0X70 if the message is
from RXB1.
•
n value: 0 if the message is from RXB0; 1 if the message if from RXB1.
•
RXBnCTRL: Contains the RXBnCTRL register value of MCP2515.
•
EFLG: The value of EFLG register of MCP2515.
•
Timestamp: Four bytes unsigned integer value. in 100 us unit.
•
RXBnSIDH, RXBnSIDL, RXBnEID8, RXBnEID0, RXBnDLC, RXBnD0,
RXBnD1, RXBnD2, RXBnD3, RXBnD4, RXBnD5, RXBnD6, and
RXBnD7 are values from MCP2515 corresponding registers.
1.7 CCP_CAN_RX_MASKS_FILTERS Service
1.7.1 Introduction to Message Acceptance Filters and Masks
The CCP_CAN_RX_MASKS_FILTERS uses 0X22 as its command category code.
This service provides ways to configure the masks and filters registers of the
MCP2515 CAN bus controller.
The MCP2515 includes two full receive buffers (RxB0 and RxB1) with multiple
acceptance filters for each. There is also a separate Message Assembly Buffer
(MAB) that acts as a third receive buffer.
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 only if the acceptance filter criteria is met.
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. 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.
FILTER MATCHING
The filter masks are used to determine which bits in the identifier are examined
with the filters. A truth table is shown below that indicates how each bit in the
identifier is compared to the masks and filters to determine if the message should be
loaded into a receive buffer. The mask essentially determines which bits to apply