CAN FD Protocol Bit-Time Segmentation for Arbitration Reliability
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Solution Overview
Problem
The existing CAN networks face limitations in data-rate due to the minimum bit time required for the CAN bus arbitration method and the relationship between data bits and frame bits, which restricts the effective data transmission rate.
Innovation Solution
The introduction of the CAN FD (CAN with Flexible Data-Rate) protocol, which increases the bit-rate by switching to a shorter bit time after arbitration and returns to the longer bit time at the CRC Delimiter, allowing longer data fields and using different CRC polynomials for frames beyond eight data bytes, thereby enhancing data transmission efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the minimum bit time is increased to ensure proper CAN bus arbitration function, then the arbitration reliability is improved, but the effective data-rate is reduced
Solution Approach 1:
The bit time is segmented into two distinct phases: arbitration phase with longer bit time for reliable arbitration, and data field phase with shorter bit time for high-speed data transmission. This segmentation allows each phase to be optimized independently for its specific function.
Solution Approach 2:
The system dynamically switches between two different bit times based on the communication phase. The controller automatically transitions from the longer arbitration bit time to the shorter data field bit time, enabling adaptive timing that optimizes both arbitration reliability and data transmission speed.
2Productivity
If the data field length is increased to improve data transmission capacity, then the data throughput is improved, but the frame structure complexity is increased
Solution Approach 1:
The system changes the bit time parameter dynamically based on the data field length. For extended data fields beyond 8 bytes, the controller switches to a shorter bit time, allowing the frame structure to accommodate longer data fields without proportionally increasing complexity.
Solution Approach 2:
The CAN FD protocol maintains compatibility with standard CAN frames while extending support to longer data fields. The same basic frame structure is used for both standard and extended frames, with only the data field length and corresponding bit time changing, thus avoiding increased structural complexity.
3Productivity
If the bit-rate is increased to improve data transmission speed, then the data-rate is improved, but the arbitration reliability may be compromised
Solution Approach 1:
The communication process is segmented into arbitration and data transmission phases with different bit time requirements. The arbitration phase uses longer bit time for reliable communication, while the data field phase uses shorter bit time for high-speed transmission, preventing compromise of arbitration reliability during high-speed data transfer.
Solution Approach 2:
The arbitration phase completes all necessary arbitration operations before the high-speed data field transmission begins. This preliminary completion of arbitration ensures that all control decisions are made at reliable speeds before switching to high-speed mode for data transmission.
Data Source
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AI summary
A method for the serial communication by exchange of data frames between a transmitting node and at least one receiving node who are connected by means of a bus system is described, wherein the roles of transmitter and receiver are assigned to nodes for each data frame by the arbitration procedure defined in the CAN-Standard ISO 11898-1, wherein the exchanged data frames have a logical structure according to the CAN-Standard ISO 11898-1, wherein the data frames are composed of a sequence of bits, wherein the logical structure of the data frames includes a Start-Of-Frame-Bit, an Arbitration Field, a Control Field, a Data Field, a CRC Field, an Acknowledge Field and an End-Of-Frame Field, wherein the Data Field may have a length of zero bits, wherein each other Field contains at least two bits, wherein each bit has a bit time, wherein each bit time is divided into Time Segments (SYNC_SEG, PROP_SEG, PHASE_SEG1, PHASE_SEG2), wherein the bit-rate is defined by the reciprocal value of the bit time, wherein for at least one first predeterminable part of the exchanged data frames the bit-rate in that part lies below a maximum value of 1Mbit/s, wherein for at least one second predeterminable part of the exchanged data frames the bit-rate in that part lies higher than in the at least one first predeterminable part, characterized in that at least two different sets of values of the Time Segments (SYNC_SEG, PROP_SEG, PHASE_SEG1, PHASE_SEG2) are predeterminable for the at least two different parts of the exchanged data frames.