70353C datasheet(39/76 Pages) MICROCHIP

39 / 76 page

DS70353C-page 21-39

Section 21. Enhanced Controller Area Network (ECAN™)

21.6.2

Aborting a Transmit Message

Setting the Abort All Pending Transmissions bit (ABAT) in the ECAN Control Register 1

(CiCTRL1<12>) requests an abort of all pending messages. To abort a specific message, the

Message Send Request (TXREQm) bit (CiTRmnCON<3>) associated with that message buffer

must be cleared. In either case, the message is only aborted if the ECAN module has not started

transmitting the message on the bus.

21.6.3

Transmitting and Responding Remote Frames

21.6.3.1 TRANSMIT A REMOTE FRAME

A node expecting to receive a data frame with a specific identifier value can initiate the

transmission of the respective data by another node by sending the remote frame. The remote

frame can be either in the Standard format or Extended format.

A remote frame is similar to a data frame, with the following exceptions:

• The RTR bit is recessive (RTR = 1)

• There is no data field

• The value of the DLC bits is 0 ≤ DLC ≤ 8

To transmit a remote frame, the user application must perform these tasks:

Configure the message buffer for transmission and assign a priority to the buffer.

Write the remote frame in the appropriate message buffer. The transmitted identifier must

be identical to the identifier of the data frame to be received.

Set the transmit request bit for the buffer to initiate transmission of the remote frame.

21.6.3.2 RESPOND TO A REMOTE FRAME

The node acting as the source to respond to the remote frame request needs to configure an

acceptance filter to match the identifier of the Remote Frame. Message buffers 0-7 can respond

to remote frames, so the Acceptance Filter Buffer Pointer (FnBP) should point to one of the eight

message buffers. The TX/RX Buffer Selection (TXENn) and Auto-Remote Transmit Enable

(RTRENm) bits in the ECAN Transmit/Receive Control register (CiTRMNCON<7> and

CiTRmnCON<2>) must be set to respond to the Remote Frame.

This is the only case where the Acceptance Filter Buffer Pointer (FnBP) points to a message

buffer that is configured for transmission (TXENn = 1).

Figure 21-9 illustrates the remote frame handling process:

ECAN Node 1 sends an RTR (using message buffer 1).

ECAN Node 2 receives the request and responds by sending the data frame (using

message buffer 7).

The data frame is received by ECAN Node 1.

The data frame is stored in message buffer 14 of ECAN Node 1.

Note:

When configured for automatic response to remote frames (RTREN = 1), the ECAN

module ignores the value of the DLC bits in the incoming RTR message. If the

application needs to transmit a data payload size specified by DLC bits in the

received RTR message, it should not enable automatic RTR response. The

application should process the RTR message like any other received message.

Check if the RTR bit is set and then transmit a message whose payload size is

equal to the DLC bits in the receive RTR message.

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