MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur télécharger pdf (Page 7)

There are basically four message frames in CAN: data, remote, error, and overload. The

data and remote frames need to be set by the user. The other two are set by the CAN

hardware.

9.1 Data Frame

The data frame is in two formats: standard (having an 11-bit ID) and extended (having a

29-bit ID). The data frame is used by the transmitting device to send data to the

receiving device, and the data frame is the most important frame handled by the user.

Figure 9.7 shows the data frame’s structure. A standard data frame starts with the

start of frame (SOF) bit, which is followed by an 11-bit identifier and the remote

transmission request (RTR) bit. The identifier and the RTR form the 12-bit arbitration

field. The control field is 6 bits wide and indicates how many bytes of data are in

the data field. The data field can be 0 to 8 bytes. The data field is followed by the

Application

Presentation

Session

Transport

Netwok

Data Link

Physical

Medium Access Control

Medium Dependent Interface

Physical Medium Attachment

Physical Signaling

Logical Link Control

Figure 9.6: CAN and the OSI model

RTR

Control

11-bit

identifier

Start of

frame

Data

CRC

ACK

End of

frame

Figure 9.7: Standard data frame

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MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur Page 7