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impaginato corretto fullone_Layout 1 13/10/15 12.08 Pagina 19 Department of Engineering, ICT and Technologies for Energy and Transport Patent Title Title Preventing bit-stuffing in the transmission of frames Preventing bit stuffing in the transmission of frames over communication lines. over communication lines. Ref. CNR 10288 Ref. CNR 10288 Assignee(s): CNR CNR Institute: IEIIT Main Inventor: Gianluca Cena Countries: IT, EP Priority date: 17/02/2014 Abstract Abstract The bit-stuffing mechanism adopted in Controller Area Network (CAN) to encode the bit stream at the The bit stuffing mechanism adopted in Controller Area Network (CAN) to encode the bit stream at the physical layer leads to an unwanted variability, or jitter, in frame transfer times, since the number of physical layer leads to an unwanted variability, or jitter, in frame transfer times, since the number of stuff bits is variable and depends on the frame content in a non-trivial way. Solutions like 8B9B and VHCC stuff bits is variable and depends on the frame content in a non trivial way. Solutions like 8B9B and VHCC were proved to be optimal in order to prevent stuff bits in the payload of the frame. This invention were proved to be optimal in order to prevent stuff bits in the payload of the frame. This invention makes it possible to prevent the occurrence of stuff bits in the Cyclic Redundancy Check (CRC) field as makes it possible to prevent the occurrence of stuff bits in the Cyclic Redundancy Check (CRC) field as well. Doing so ensures extremely accurate and completely deterministic transfer times, regardless of the well. Doing so ensures extremely accurate and completely deterministic transfer times, regardless of the message content. This is of great importance in distributed control systems to improve their basic message content. This is of great importance in distributed control systems to improve their basic characteristics, such as timing accuracy and, as a consequence, quality of control. Moreover, the characteristics, such as timing accuracy and, as a consequence, quality of control. Moreover, the invention also reduces noticeably the residual error probability, hence increasing system reliability. invention also reduces noticeably the residual error probability, hence increasing system reliability. Background Background CAN is used for real-time communications among devices in automotive, industrial, and embedded CAN is used for real time communications among devices in automotive, industrial, and embedded systems. Bit-stuffing in CAN leads to jitters in frame transfer times, which worsen the timing accuracy of systems. Bit stuffing in CAN leads to jitters in frame transfer times, which worsen the timing accuracy of distributed control applications. Several solutions to this issue were proposed in the past years, either distributed control applications. Several solutions to this issue were proposed in the past years, either based on statistical grounds or suitable encoders, which prevent the insertion of stuff bits by modifying the based on statistical grounds or suitable encoders, which prevent the insertion of stuff bits by modifying the content of the data field. However, they are completely unable to cope with the CRC field. content of the data field. However, they are completely unable to cope with the CRC field. Technology Technology By reserving 3 bits at the end of the data field (denoted as the tuning field), this invention permits By reserving 3 bits at the end of the data field (denoted as the tuning field), this invention permits steering CRC calculation to a value which does not cause the insertion of any stuff bit in the CRC field by steering CRC calculation to a value which does not cause the insertion of any stuff bit in the CRC field by the CAN controller. The invention aims at complementing techniques, like 8B9B, which prevent variability the CAN controller. The invention aims at complementing techniques, like 8B9B, which prevent variability in stuff bits added to the other parts of the frame. Complete compatibility is ensured with legacy CAN in stuff bits added to the other parts of the frame. Complete compatibility is ensured with legacy CAN controllers. controllers. Advantages and Applicatons Advantages and Applications This invention has two main advantages. Firstly, it reduces jitters, which may affect the duration of frame This invention has two main advantages. Firstly, it reduces jitters, which may affect the duration of frame transmissions in CAN by up to about 20 bit times, to well below one bit time. In competing solutions, transmissions in CAN by up to about 20 bit times, to well below one bit time. In competing solutions, residual jitters can be as high as 4 bit times. Secondly, it reduces the residual error probability of CAN by residual jitters can be as high as 4 bit times. Secondly, it reduces the residual error probability of CAN by about two orders of magnitude. It can be used in low-cost, high-precision, high-reliability real-time control about two orders of magnitude. It can be used in low cost, high precision, high reliability real time control systems. systems. Development stage Development stage A highly-optimized prototype (codec), written in C for the NXP LPC2468 μC, has been implemented and Ahighly optimized prototype (codec), written in C for the NXP LPC2468 C, has been implemented and tested for correctness and performance. The resulting jitter, measured at the application level, is in the tested for correctness and performance. The resulting jitter, measured at the application level, is in the order of 500ns. A testbed has been developed. 19 order of 500ns. A testbed has been developed.