Advanced PIC18 Projects—CAN Bus Projects:Overload Frame And Bit Stuffing

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Overload Frame

The overload frame is used by the receiving unit to indicate that it is not yet ready to receive frames. This frame consists of an overload flag and an overload delimiter. The overload flag consists of 6 dominant bits and has the same structure as the active error flag of the error frame. The overload delimiter consists of 8 recessive bits and has the same structure as the error delimiter of the error frame.

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.

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