MikroElektronika PIC Microcontrollers PIC18 Bedienungsanleitung PDF herunterladen (Seite 12)

9.5 Bit Stuffing

The CAN bus makes use of bit stuffing, a technique to periodically synchronize

transmit-receive operations to prevent timing errors between receive nodes. After 5

consecutive bits with the same level, one bit of inverted data is added to the sequence.

If, during sending of a data frame or remote frame, the same level occurs in 5

consecutive bits anywhere from the start of frame to the CRC sequence, an inverted

bit is inserted in the next (i.e., the sixth) bit. If, during receiving of a data frame or

remote frame, the same level occurs in 5 consecutive bits anywhere from the start of

frame to CRC sequence, the next (sixth) bit is deleted from the received frame. If the

deleted sixth bit is at the same level as the fifth bit, an error (stuffing error) is detected.

9.6 Types of Errors

The CAN bus identifies five types of errors:



Bit error



CRC error



Form error



ACK error



Stuffing error

Bit errors are detected when the output level and the data level on the bus do not

match. Both transmit and receive units can detect bit errors. CRC errors are detected

only by receiving units. CRC errors are detected if the calculated CRC from the

received message and the received CRC do not match. Form errors are detected

by the transmitting or receiving units when an illegal frame format is detected.

ACK errors are detected only by the transmitting units if the ACK field is found

recessive. Stuffing errors are detected when the same level of data is detected for 6

consecutive bits in any field that should have been bit-stuffed. This error can be

detected by both the transmitting and receiving units.

9.7 Nominal Bit Timing

The CAN bus nominal bit rate is defined as the number of bits transmitted every

second without resynchronization. The inverse of the nominal bit rate is the nominal

bit time. All devices on the CAN bus must use the same bit rate, even though each

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MikroElektronika PIC Microcontrollers PIC18 Bedienungsanleitung Seite 12