MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur télécharger pdf (Page 8)

CRC field, which checks whether or not the received bit sequence is corrupted.

The ACK field is 2 bits and is used by the transmitter to receive acknowledgment of

a valid frame from any receiver. The end of the message is indicated by a 7-bit end

of frame (EOF) field. In an extended data frame, the arbitration field is 32 bits wide

(29-bit identifier þ1-bit IDE to define the message as an extended data frame þ1-bit

SRR which is unused þ1-bit RTR) (see Figure 9.8).

The data frame consists of the following fields:

9.1.1 Start of Frame (SOF)

The start of frame field indicates the beginning of a data frame and is common to

both standard and extended formats.

9.1.2 Arbitration Field

Arbitration is used to resolve bus conflicts that occur when several devices at

once start sending messages on the bus. The arbitration field indicates the priority

of a frame, and it is different in the standard and extended formats. In the standard

format there are 11 bits, and up to 2032 IDs can be set. The extended format

ID consists of 11 base IDs plus 18 extended IDs. Up to 2032  2

discrete IDs

can be set.

During the arbitration phase, each transmitting device transmits its identifier and

compares it with the level on the bus. If the levels are equal, the device continues

to transmit. If the device detects a dominant level on the bus while it is trying to

transmit a recessive level, it quits transmitting and becomes a receiving device.

After arbitration only one transmitter is left on the bus, and this transmitter continues

to send its control field, data field, and other data.

11-bit

identifier

SRR

18-bit

identifier

IDE RTR

CRC

Control

ACK

End of

frame

Start of

frame

Figure 9.8: Extended data frame

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MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur Page 8