One of the key differences between the CAN protocol and other fieldbus protocols is its use of synchronous data transfer instead of asynchronous character-based transmission. This approach allows for more efficient data transmission, but it also requires a more sophisticated bit synchronization mechanism. Unlike asynchronous protocols, where each character is synchronized using a start bit, CAN relies on a single start bit at the beginning of a frame, making continuous resynchronization essential.
In character-oriented protocols, synchronization is straightforward—each character’s start bit serves as a reference point. However, in CAN, the single start bit at the beginning of a frame is often insufficient to keep the receiver and transmitter aligned throughout the entire message. To maintain accurate timing, the receiver must continuously resynchronize by detecting signal edges and adjusting the sampling point accordingly. This process involves compensating for any time discrepancies between the transmitting and receiving nodes using a "phase buffer segment" before and after the nominal sample point.
Additionally, since CAN uses non-destructive bus arbitration and explicit acknowledgment bits, the signal must travel from the transmitter to the receiver and back within a single bit time. This necessitates the inclusion of a "Transmission Delay Segment" to account for signal propagation delays and internal node processing times.
The nominal bit time in CAN is divided into four non-overlapping segments: the Sync_Seg, Prop_Seg, Phase_Seg1, and Phase_Seg2. These segments help manage timing and synchronization across the network. The Sync_Seg ensures that the receiver aligns with the start of the frame, while the Prop_Seg accounts for signal propagation delays. Phase_Seg1 and Phase_Seg2 provide flexibility for resynchronization, allowing the system to adjust for minor timing errors.
Synchronization can occur in two ways: "hard sync" at the start of a frame and "resync" during the bit time. Hard sync resets the bit timer based on the initial signal edge, while resynchronization adjusts the bit time based on detected signal transitions. Resynchronization is crucial for maintaining accuracy, especially when there are variations in oscillator frequencies between nodes.
During resynchronization, the phase error (the difference between the actual signal edge and the expected position) determines how much the phase buffer segments are adjusted. The maximum adjustment allowed is defined by the "Synchronous Jump Width" (SJW), which limits how much the bit time can be extended or shortened. This ensures stability and prevents excessive jitter in the communication process.
The bit time can be configured with specific values for each segment, depending on the network requirements. For example, the Sync_Seg is fixed at one time unit, while the Prop_Seg, Phase_Seg1, and Phase_Seg2 can vary within certain ranges. The baud rate prescaler and SJW settings further influence the timing accuracy and overall performance of the CAN network.
Proper configuration of these parameters is essential for ensuring reliable communication, especially in real-time applications where timing precision is critical. The CAN protocol's design allows for flexible and robust synchronization, making it well-suited for industrial and automotive environments where reliability and efficiency are paramount.
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