70353C Datasheet, PDF (39/76 Pages) Microchip Technology – Section 21. Enhanced Controller Area Network

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:
1. Configure the message buffer for transmission and assign a priority to the buffer.
2. 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.
3. 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:
1. ECAN Node 1 sends an RTR (using message buffer 1).
2. ECAN Node 2 receives the request and responds by sending the data frame (using
message buffer 7).
3. The data frame is received by ECAN Node 1.
4. The data frame is stored in message buffer 14 of ECAN Node 1.
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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.
© 2008-2011 Microchip Technology Inc.
DS70353C-page 21-39