70353C datasheet(51/76 Pages) MICROCHIP

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DS70353C-page 21-51

Section 21. Enhanced Controller Area Network (ECAN™)

21.7.2.1 BUFFER SELECTION

There are four Acceptance Filter Buffer Pointer registers that select which message buffer the

received message is stored into for acceptance filters 0-15.

• CiBUFPNT1: ECAN Filter 0-3 Buffer Pointer Register

• CiBUFPNT2: ECAN Filter 4-7 Buffer Pointer Register

• CiBUFPNT3: ECAN Filter 8-11 Buffer Pointer Register

• CiBUFPNT4: ECAN Filter 12-15 Buffer Pointer Register

When the incoming message identifier is matched by one of the acceptance filters, the internal

logic looks up the buffer pointer (FnBP<3:0>) and uses that as an address for the corresponding

message buffer. The address is provided to the DMA channel by the peripheral. Therefore, the

DMA channel must be configured in the Peripheral Indirect mode.

The values for FnBP<3:0> are interpreted as follows:

• 1111 = Message is received in receive FIFO buffer

• 1110 = Message is received in message buffer 14

•

• 0001 = Message is received in message buffer 1

• 0000 = Message is received in message buffer 0

21.7.2.2 RECEIVING MESSAGES INTO MESSAGE BUFFERS 0-7

The message buffers 0-7 can be configured to transmit or receive the CAN messages using the

TX/RX Buffer Selection bit (TXENm) in the ECAN TX/RX Buffer m Control register

(CiTRmnCON<7>). The Acceptance Filter Buffer Pointer (FnBP) selects one of the message

buffers to store the received message, provided it is configured as a receive buffer (TXENm = 0).

If a message buffer is set up as a transmitter with the RTRENm bit (CiTRmnCON<2>) set, and

an acceptance filter pointing to that message buffer detects a message, the message buffer will

handle the RTR instead of storing the message. This is the only case where the Acceptance Filter

Buffer Pointer (FnBP) points to a message buffer that is configured for transmission (TXENn = 1).

21.7.2.3 RECEIVING MESSAGES INTO MESSAGE BUFFERS 8-14

The message buffers 8-14 are receive buffers. The Acceptance Filter Buffer Pointer (FnBP)

determines which message buffer to use.

21.7.2.4 RECEIVING MESSAGES INTO MESSAGE BUFFERS 15-31

The message buffers 15-31 are receive buffers and are only usable as FIFO buffers because the

Acceptance Filter Buffer Pointer bits (FnBP<3:0>) can only directly address 16 entities. When

FnBP<3:0> = 1111, the results of a hit on that filter will write to the next available buffer location

within the FIFO.

Note:

Multi message buffering can be implemented by a user application by configuring

multiple acceptance filters with the same value. In this case, a received message

may match multiple filters, and the ECAN module will assign the message to the

lowest-numbered matching filter pointing to an empty buffer.

Note:

The user application should not set the TXREQ bits (CiTRmnCON) when the buffer

is configured for receive operation (TXEN = 0). This could result in unpredictable

module behavior.

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