National CP3BT26 manual Transmit Structure, Transmit Scheduling, Transmit Priority

CNSTAT status section will be 0101b, as the buffer was RX_FULL (0100b) before. After finally reading the last re- ceived message, the CPU can reset the buffer to RX_READY.

19.6TRANSMIT STRUCTURE

To transmit a CAN message, software must configure the message buffer by changing the buffer status to TX_NOT_ACTIVE. The buffer is configured for transmission if the ST[3] bit of the buffer status code (CNSTAT) is set. In TX_NOT_ACTIVE status, the buffer is ready to receive data from the CPU. After receiving all transmission data (ID, data bytes, DLC, and PRI), the CPU can start the transmission by writing TX_ONCE into the buffer status register. During the transmission, the status of the buffer is TX_BUSYx. Af- ter successful transmission, the CAN module will reset the buffer status to TX_NOT_ACTIVE. If the transmission pro- cess fails, the buffer condition will remain TX_BUSYx for re- transmission until the frame was successfully transmitted or the CPU has canceled the transmission request.

To Send a Remote Frame (Remote Transmission Request) to other CAN nodes, software sets the RTR bit of the mes- sage identifier (see Storage of Remote Messages on page

132)and changes the status of the message buffer to TX_ONCE. After this remote frame has been transmitted successfully, this message buffer will automatically enter the RX_READY state and is ready to receive the appropri- ate answer. Note that the mask bits RTR/XRTR need to be set to receive a data frame (RTR = 0) in a buffer which was configured to transmit a remote frame (RTR = 1).

To answer Remote Frames, the CPU writes TX_RTR in the buffer status register, which causes the buffer to wait for a remote frame. When a remote frame passes the accep-

tance filtering mask of one or more buffers, the buffer status will change to TX_ONCE_RTR, the contents of the buffer will be transmitted, and afterwards the CAN module will write TX_RTR in the status code register again.

If the CPU writes TX_ONCE_RTR into the buffer status, the contents of the buffer will be transmitted, and the successful transmission the buffer goes into the “wait for Remote Frame” condition TX_RTR.

19.6.1Transmit Scheduling

After writing TX_ONCE into the buffer status, the transmis- sion process begins and the BUSY bit is set. As soon as a buffer gets the TX_BUSY status, the buffer is no longer ac- cessible by the CPU except for the ST[3:1] bits of the CN- STAT register. Starting with the beginning of the CRC field of the current frame, the CAN module looks for another buff- er transmit request and selects the buffer with the highest priority for the next transmission by changing the buffer state from TX_ONCE to TX_BUSY. This transmit request can be canceled by the CPU or can be overwritten by anoth- er transmit request of a buffer with a higher priority as long as the transmission of the next frame has not yet started. This means that between the beginning of the CRC field of the current frame and the transmission start of the next frame, two buffers, the current buffer and the buffer sched- uled for the next transmission, are in the BUSY status. To cancel the transmit request of the next frame, the CPU must change the buffer state to TX_NOT_ACTIVE. When the transmit request has been overwritten by another request of a higher priority buffer, the CAN module changes the buffer state from TX_BUSY to TX_ONCE. Therefore, the transmit request remains pending. Figure 56 further illustrates the transmit timing.

CP3BT26

		ARBITRATION FIELD	DATA FIELD	CRC	ACK
BUS	SOF	+ CONTROL	(IF PRESENT)	FIELD	FIELD	EOF	IFS
IDLE	1 BIT	12/29 BIT + 6 BIT	n × 8 BIT	16 BIT	2 BIT	7 BIT	3 BIT

BUS

TX_BUSY current buffer

CPU write TX_ONCE in buffer status

TX_BUSY next buffer

Begin selection of next buffer
if new tx_request	DS040

Figure 56. Data Transmission

If the transmit process fails or the arbitration is lost, the transmission process will be stopped and will continue after the interrupting reception or the error signaling has finished (see Figure 56). In that case, a new buffer select follows and the TX process is executed again.

Note: The canceled message can be delayed by a TX re- quest of a buffer with a higher priority. While TX_BUSY is high, software cannot change the contents of the message buffer object. In all cases, writing to the BUSY bit will be ig- nored.

19.6.2Transmit Priority

The CAN module is able to generate a stream of scheduled messages without releasing the bus between two messag- es so that an optimized performance can be achieved. It will arbitrate for the bus immediately after sending the previous message and will only release the bus due to a lost arbitra- tion.

If more than one buffer is scheduled for transmission, the priority is built by the message buffer number and the prior- ity code in the CNSTAT register. The 8-bit value of the prior-

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