Controller Area Network (CAN) - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 637

Interframe
space

Arbitration field Control field

Start-of-frame 11-bit identifier RTR IDE r0 DLC

(a) Standard format

Arbitration field

Control field

Start-of-
frame

11-bit identifier SRR IDE 18-bit identifier RTR r1 r0 DLC

(b) Extended format

Figure 13.3 ■ Arbitration field

The identifier of the extended format comprises two sections: an 11-bit base ID and an
18-bit extended ID. Both the base ID and the extended ID are transmitted most significant bit
first. The base ID defines the base priority of the extended frame.
The remote transmission request (RTR) bit in data frames must be dominant (0). Within a
remote frame, the RTR bit has to be recessive (1).
The substitute remote request (SRR) bit is a recessive bit (1). The SRR bit of an extended
frame is transmitted at the position of the RTR bit in the standard frame and therefore substi-
tutes for the RTR bit in the standard frame. As a consequence, collisions between a standard
frame and an extended frame, where the base IDs of both frames are identical, are resolved in
such a way that the standard frame prevails over the extended frame.
The identifier extension (IDE) bit belongs to the arbitration field for the extended format
and the control field for the standard format. The IDE bit in the standard format is transmitted
dominant, whereas in the extended format the IDE bit is recessive.

C ONTROL F IELD
The contents of this field are shown in Figure 13.4. The format of the control field is dif-
ferent for the standard format and the extended format. Frames in standard format include the
data length code; the IDE bit, which is transmitted dominant; and the reserved bit r0 . Frames
in extended format include the data length code and two reserved bits, r0 and r1 . The reserved
bits must be sent dominant, but the receivers accept dominant and recessive bits in all com-
binations. The data length code specifies the number of bytes contained in the data field. Data
length can be 0 to 8, as encoded in Table 13.1.

Arbitration
field

Data field
or CRC field

Control field

IDE/r1 r0 DLC3 DLC2 DLC1 DLC0

Reserved bits

Data length code

Figure 13.4 ■ Control field