MikroElektronika PIC Microcontrollers PIC18 Manual do Utilizador descarregar pdf (Página 10)

9.1.3 Control Field

The control field is 6 bits wide, consisting of 2 reserved bits and 4 data length code

(DLC) bits, and indicates the number of data bytes in the message being transmitted.

This field is coded as shown in Table 9.1, where up to 8 transmit bytes can be coded

with 6 bits.

9.1.4 Data Field

The data field carries the actual content of the message. The data size can vary from

0 to 8 bytes. The data is transmitted with the MSB first.

9.1.5 CRC Field

The CRC field, consisting of a 15-bit CRC sequence and a 1-bit CRC delimiter, is

used to check the frame for a transmission error. The CRC calculation includes the

start of frame, arbitration field, control field, and data field. The calculated CRC

and the received CRC sequence are compared, and if they do not match, an error

is assumed.

Table 9.1: Coding the control field

No. of data bytes DLC3 DLC2 DLC1 DLC0

0DDDD

1DDDR

2DDRD

3DDRR

4DRDD

5DRDR

6DRRD

7DRRR

8 R DorR DorR DorR

D: Dominant level, R: Recessive level.

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MikroElektronika PIC Microcontrollers PIC18 Manual do Utilizador Página 10