Controller Area Network (CAN) - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 660

7 6 5 4 3 2 1 0

IDR0 ID28 ID27 ID25 ID24 ID22

ID26 ID23 ID21

IDR1 ID20 ID19 ID18 SRR(=1) IDE(=1) ID17 ID16 ID15

ID14 ID11 ID9

IDR2 ID13 ID12 ID10 ID8 ID7

ID4 ID3 ID2 ID1 ID0 RTR

IDR3 ID6 ID5

Figure 13.34 ■ Receive/transmit message buffer extended identifier

7 6 5 4 3 2 1 0

IDR0 ID10 ID9 ID8 ID7 ID6 ID5 ID4 ID3

IDR1 ID2 ID1 ID0 RTR IDE(=0)

IDR2

IDR3

Figure 13.35 ■ Receive/transmit message buffer standard identifier

The identifier consists of 29 bits (ID28,ID0) for the extended format. The ID28 bit is the
most significant bit and is transmitted first. The CAN protocol uses the identifier to arbitrate
simultaneous transmissions. The priority of an identifier is defined to be highest for the small-
est binary number. The identifier consists of 11 bits (ID10,ID0) for the standard format. The
ID10 bit is the most significant bit and is transmitted first. Similar to the extended identifier,
the priority of a standard identifier is defined to be highest for the smallest binary number.
The SRR bit is used only in the extended format and must be set to 1. The IDE bit is used
to indicate whether the identifier is an extended identifier. When set to 1, the identifier is used
as an extended identifier.
The RTR bit is used to solicit transmission from other CAN nodes. In the case of a receive
buffer, it indicates the status of the received frame and supports the transmission of an answer-
ing frame in software.

D ATA S EGMENT R EGISTERS (DSR0 = DSR7)
The eight data segment registers, each with its DB7,DB0, contain the data to be transmit-
ted or received. The number of bytes to be transmitted or received is determined by the data
length code in the corresponding DLR.

D ATA L ENGTH R EGISTER (DLR)
The data length register has only the lowest 4 bits implemented and is used to indicate the
number of data bytes contained in the message.

T RANSMIT B UFFER P RIORITY R EGISTER (TBPR)
This register defines the local priority of the associated message buffer. The local priority is
used for the internal prioritization process of the MSCAN and is defined to be highest for the small-
est binary number. The MSCAN implements the following internal prioritization mechanism:
• All transmission buffers with a cleared TXEx flag (bit 2, 1, or 0 of the CAN x TFL register)
participate in the prioritization immediately before the SOF (start of frame) is sent.
• The transmission buffer with the lowest local priority field wins the prioritization.