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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.

16.6

TRANSMIT STRUCTURE

To transmit a CAN message, software must configure the

message

buffer

changing

the

buffer

status

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

98) 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.

16.6.1

Transmit 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 44 further illustrates the

transmit timing.

Figure 44.

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 44). 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.

16.6.2

Transmit 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-

SOF

1 BIT

IFS

3 BIT

EOF

7 BIT

TX_BUSY

current buffer

CPU write TX_ONCE

in buffer status

Begin selection of next buffer

if new tx_request

TX_BUSY

next buffer

BUS

IDLE

ACK

FIELD

2 BIT

CRC

FIELD

16 BIT

DATA FIELD

(IF PRESENT)

n × 8 BIT

ARBITRATION FIELD

+ CONTROL

12/29 BIT

+ 6 BIT

BUS

DS040

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