Why insert an inverse bit in the CAN signal transmission

Engineers familiar with CAN communication generally have seen the term "anti-code bit", but what exactly is it? What is the use? Perhaps many people do not have an in-depth understanding of it, this article will let everyone no longer be confused.

Data digital coding has many methods, such as non-return-to-zero (NRZ), Manchester or pulse width coding, which differ in the number of time slots used to represent one bit, as shown in FIG. The signal level of the non-return-to-zero level code remains constant throughout the bit time, so only one time slot is needed to represent one bit. The Manchester encoded signal changes over a bit time, so two time slots are needed to represent one bit. The advantage of Manchester encoding is that each bit has a signal edge for self-clocking code. However, compared to Manchester coding and non-return-to-zero coding, the bit rate can only reach half of the non-return to zero code at the same bit time (bit frequency). Since the non-return-to-zero encoded signal level can remain constant for a long time (depending on the data being transmitted), it is necessary to take appropriate measures to ensure that the maximum allowable time interval between the two signal edges is not exceeded. The maximum time interval between resynchronization points is determined by the error of the node oscillator. Resynchronization can be achieved by applying a "bit stuffing" approach. This method inserts an inverse code into the bit stream after a certain number of constant bit levels.

Figure 1 NRZ and Manchester coded bit representation

The CAN protocol uses NRZ signal encoding with bit stuffing (filling width of 5 bits), which guarantees the highest transmission performance with sufficient synchronization capability. This type of coding also provides very good signal radiation characteristics (the energy of the bus radiation is roughly proportional to the frequency of the transmitted signal and the number of signal edges).

The portion of the CAN protocol to which the NRZ code is applied includes the CRC sequence of the SOF, the arbitration field, the control field, the data field, the data frame, and the remote request frame. As soon as the transmitter detects five consecutive sequences of identical values, it inserts an inverse bit in the actual transmitted bit stream. The rest of the data frame or remote frame (CRC delimiter, ACK field, and EOF) has a fixed form (recessive level) and is transmitted without bit stuffing. The same is true for error and overload frames.
(The manuscript is provided by ZLG Zhiyuan Electronics)

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