70353C Datasheet, PDF (43/76 Pages) Microchip Technology – Section 21. Enhanced Controller Area Network

Section 21. Enhanced Controller Area Network (ECAN™)
21.7
RECEIVING ECAN MESSAGES
The ECAN module can receive both standard and extended frames on the CAN bus node. This
module has the additional capability of automatically transferring the received messages to the
user-defined buffers in device RAM, thereby eliminating the need for the user application to
explicitly copy messages from hardware registers to the user-defined buffers. The storage format
of each message within the DMA buffer is identical to that of transmit buffers, with each
message (including the associated status register) occupying eight words in device RAM.
The two main stages that constitute the ECAN reception process are described below, with a
simplified reception process example illustrated in Figure 21-10 and Figure 21-13.
21.7.1 Message Reception and Acceptance Filtering
As illustrated in Figure 21-10, every incoming message on the bus is received into a Message
Assembly Buffer, and its identifier field is compared with a set of 16 user-defined acceptance
filters. Each received standard data frame contains an 11-bit SID, and each extended data frame
contains an 11-bit SID and an 18-bit EID. If all bits in the incoming identifier completely match the
corresponding bits in any of the acceptance filters, the ECAN module generates a DMA transfer
request to the DMA Controller so that the message can be received into the appropriate buffer
in device RAM.
Figure 21-10: Message Reception and Acceptance Filtering 7
SID EID
ECAN™ Data Frames
21
Word 0
Word 1
Word 2
Word 3
Word 4
Word 5
Word 6
Word 7
Message
Assembly
Buffer
Identifier
Comparison
Filter 0
Filter 1
Filter 2
Filter 3
Filter 4
Filter 5
Filter 6
Filter 7
Filter 8
Filter 9
Filter 10
Filter 11
Filter 12
Filter 13
Filter 14
Filter 15
Filter Match (DMA Transfer Request)
User-Defined
Acceptance
Filters (0-15)
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DS70353C-page 21-43