Abstract:
In a method for enabling transmission of larger data quantities relatively rapidly in a data network, the sent data frames have a logical structure according to CAN Specification ISO 11898-1, the bit length in time within a data frame being able to assume at least two different values; for a first specifiable range within the data frame, the bit length in time being greater than, or equal to a specified minimum value of approximately one microsecond and in at least one second specifiable range within the data frame the bit length in time compared to the first range is at least halved, preferably less than halved; the change of the bit length in time being implemented by using at least two different scaling factors for setting the bus time unit relative to a shortest time unit or the oscillator clock pulse during running operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method and a device for the transmission of data between at least two users of a bus system, in which the time duration of the bits transmitted is able to be switched over between at least two different values. 
         [0003]    2. Description of the Related Art 
         [0004]    From published German patent application document DE 100 00 305 A1, for example, the Controller Area Network as well as a “Time-Triggered CAN” (TTCAN)-designated broadening of the CAN is known. The media access control method used in the CAN is based on a bit-wise arbitration. In a bit-wise arbitration, a plurality of user stations are simultaneously able to transmit data via the channel of the bus system, without thereby interfering with the data transmission. Furthermore, the user stations are able to ascertain the logical state (0 or 1) of the channel while transmitting a bit over the channel. If a value of the bit sent does not correspond to the ascertained logical state of the channel, the user station terminates the access to the channel. In CAN, the bit-wise arbitration is usually carried out in an arbitration field within a data frame that is to be transmitted via the channel. After a user station has sent the arbitration field completely to the channel, it knows that it has exclusive access to the channel. Consequently, the end of the transmission of the arbitration field corresponds to the beginning of an enable interval, within which the user station is able to use the channel exclusively. According to the protocol specification of the CAN, other user stations may not access the channel, that is, send data to the channel, until the sending user station has transmitted a checksum field (CRC field) of the data frame. Thus, an end point of the transmission of the CRC field corresponds to the end of the enable interval. 
         [0005]    Because of the bit-wise arbitration, a non-destructive transmission of the data frame over the channel is achieved. Because of this, good real time properties of the CAN come about, whereas media access control methods, in which the data frame sent by a user station is able to be destroyed during transmission via the channel, based on a collision with a further data frame sent by another station, have a clearly more unfavorable real time behavior since, based on the collision and the new transmission of the data frame required thereby, a delay in the data transmission comes about. 
         [0006]    The protocols of the CAN are particularly suitable for transmitting short messages under real time conditions. If larger data blocks are to be transmitted via the CAN domain, the relatively low bit rate of the channel becomes a limiting factor. In order to assure the correct functioning of the bit-wise arbitration, during the arbitration, for the transmission of a bit, a minimum duration, that is first of all a function of the extension of the bus system, the signal propagation speed on the channel and intrinsic processing times in the interface modules of the bus users, has to be maintained, because all the bus users have to have a uniform picture of the state of the bus (0 or 1) and equal priority access to the state of the bus. Therefore, the bit rate cannot be increased without any problem by reducing the duration of the individual bits. 
         [0007]    So as, nevertheless, to be able to transmit sufficiently rapidly a relatively large data block, required for the programming of a control unit, via a communications interface that is actually provided for connection to a CAN domain, German patent application document DE 101 53 085 A1 proposes switching over the communications interface for transmitting the data block temporarily to another communications mode, in which no bit-wise arbitration is carried out, and consequently a relatively high bit rate is possible. In this case, however, the communication with the protocols of the CAN has to be interrupted for a certain time. If, for example, based on an error, the operation of the bus system cannot be taken up again according to the CAN protocols, there is a failure of the bus system. In addition, the transmission of a relatively large data block will cause a considerable delay in the transmissions to be undertaken according to the protocols of the CAN, so that the real time properties of the CAN are impaired. 
         [0008]    Published German patent application document DE 103 11 395 A1 describes a system in which the asynchronous, serial communication is able to take place alternatively via an asymmetrical physical protocol or via the symmetrical physical CAN protocol, and because of this, a higher data transmission rate or data transmission security is achievable for the asynchronous communication. 
         [0009]    Published German patent application document DE 10 2007 051 657 A1 suggests using an asynchronous, rapid data transmission not conforming to the CAN in the exclusive time windows of the TTCAN protocols, so as to increase the transmitted data quantity. 
         [0010]    G. Cena and A. Valenzano, in “Overclocking of controller area networks” (Electronics Letters, vol. 35, No. 22 (1999), p. 1924) treat, from a theoretical point of view, the effects of overclocking the bus frequency in subsections of the data frame on the effectively achieved data rate, without, however, going into details of the methodology and the various states and state transitions of the bus users. 
         [0011]    It may be seen in the cited documents that the related art does not supply satisfactory results from every point of view. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    It is an object of the present invention to describe a method by which, in a CAN network, data frames are able to be transmitted in the shortest time and simultaneously essential properties of the CAN with regard to error detection as well as network-wide data consistency are maintained. For this purpose, a data transmission method (called Fast-CAN below) is proposed that is modified compared to the CAN protocol according to ISO 11898-1 to 4 (called standard CAN below). 
         [0013]    According to the present invention, the object is attained in that the bit length in time is able to assume at least two different values within a data frame, for a specifiable range within the data frame, the bit length in time remaining the same for all the users on the bus, changes in the bit length in time being signaled by an identification included in the same data frame, and the changes of the bit length in time being implemented by using at least two different scaling factors between a bus time unit and the shortest time unit or the oscillator clock pulse in the running operation. 
         [0014]    One advantage of this method is that the modification of the CAN protocol is thereby restricted to a minimum, and in particular, the structure of the CAN data frames is maintained at least for the range between the SOF and the CRC delimiter. The interface to the application program remains unchanged. 
         [0015]    Fast-CAN controllers may also be used in standard CAN networks. In one network, which includes exclusively users having Fast-CAN controllers, switch all users after the arbitration into the fastest mode, so that all synchronization mechanisms and error detection mechanisms are able to continue fulfilling their task. 
         [0016]    An additional advantage of this method is that a standard CAN controller only has to be changed minimally so that it is able to function as a Fast-CAN controller. A Fast-CAN controller which is also able to work as a standard CAN controller is only negligibly larger than a standard CAN controller. The application program does not have to be changed and considerable parts of the CAN Conformance Tests (ISO 16845) may be taken over. 
         [0017]    The shortening of the bit length takes place for event-controlled communication with arbitration only after arbitration has taken place, since bus-wide data consistency is required for the arbitration, as was explained above. Beyond that, it is also possible, however, to combine the Fast-CAN protocol with the TTCAN protocol, because in the TTCAN, too, all data are transmitted in CAN data frames, whose principal construction corresponds to the specifications of ISO 11898-1. In this case, the address field and the control field could be totally or partially transmitted using a shortened bit length, at least in the exclusive time windows of the TTCAN matrix, in which no arbitration takes place but the bus access is exclusively issued. 
         [0018]    Furthermore, it is advantageous that the transitions between the various bit lengths are able to be described by a simple status model having simply implementable transition conditions. 
         [0019]    It is also of advantage that the switching of the bit length is able to be made by a simple adjustment of the scaling factor between the oscillator period or the smallest time unit and the bus time unit, using the baud rate prescaler, for example. The assumption in this case is naturally that the oscillator period is sufficiently short. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  shows schematically a state diagram having the various states which a Fast-CAN controller is able to occupy with regard to the method according to the present invention, as well as the transition conditions. 
           [0021]      FIG. 2  shows an example for the different settings of the bit timing as a function of the transmission rate. 
           [0022]      FIG. 3  shows the structure of a CAN data frame in standard format and in extended format having the subdivision, according to the present invention, into ranges of different bit length and using the identification by a reserved bit. 
           [0023]      FIG. 4  shows an example for the broadening of the range of reduced bit length at a combination of the method with the time-controlled transmission method of the TTCAN protocol, represented by a system matrix. 
           [0024]      FIG. 5  shows the one possibility for subdividing a data frame in an exclusive TTCAN time window into ranges of different bit length. 
           [0025]      FIG. 6  shows acceptance criteria, broadened compared to the related art, for the CRC delimiter or the acknowledge bit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Furthermore, exemplary embodiments are described for the method and device according to the present invention. These specific examples are used to explain the embodiment, but they do not limit the scope of the inventive thought. 
         [0027]    First of all, in a first exemplary embodiment, with the aid of  FIGS. 1 through 3 , we shall describe the states of the Fast-CAN controller and the associated data transmission properties, as well as their transition and the transition conditions required for this. 
         [0028]      FIG. 1  represents the three operating states of the Fast-CAN controller: standard CAN  101 , Fast-CAN-Arbitration  102  and Fast-CAN-Data  103 . 
         [0029]    In the operating state standard CAN  101 , it works according to the standard CAN protocol. In the operating state, Fast-CAN arbitration  102  behaves like a standard CAN controller, but is also able to change into Fast-CAN data state  103 . In Fast-CAN data state  103 , it works like a standard CAN controller, but having a shorter bit time. After the switching-on, the controller is in Fast-CAN arbitration mode  102 , when this is requested by the application program. Otherwise, after switching-on, it is in standard CAN mode  101 . 
         [0030]    A change is provided in the bit length in time by a change in the scaling factor (“prescaler”) between the bus unit of time (“time quantum”) and the smallest time unit (“minimum time quantum”) or the oscillator clock pulse in running operation. The length of the bus time unit, and thus the length of the bits, is set thereby. The bit time segments, whose length is measured in bus time units, remain unchanged, and so do the rules for resynchronization and the position of the sample points. In the states Fast-CAN-Arbitration  102  and standard CAN  101 , the long bus time unit is used, and in the state Fast-CAN-Data  103  the short bus time unit. Alternatively, the settings of the bit time segments could also be changed, depending on the state and the bus time unit used, which will be mentioned in greater detail in connection with  FIG. 2 . 
         [0031]    In the Fast-CAN arbitration state  102 , for example, “reserved bit” RO, which in the CAN frame lies before data length code DLC, is sent recessively as identification. In the standard CAN protocol it is specified that this bit has to be sent in dominant fashion. When a Fast-CAN controller receives this bit dominantly, it changes durably to the standard CAN state (state change T 1  or T 2 ). This assures that Fast-CAN and standard CAN controllers are able to be used in the same network and that then both work in the standard CAN protocol. Another bit may also be selected as identification, for which a fixed value is specified in the standard CAN protocol. 
         [0032]    A Fast-CAN controller in the state Fast-CAN arbitration  102 , which as identification, for example, recessively receives “reserved bit” RO before the DLC or successfully sends it recessively, switches over, as of the sample point of this bit, to the shorter bus time unit by switching over the scaling factor, and changes to the state Fast-CAN data  103  (state change T 3 ). The state change may also take place using an at least approximately constant time distance or after expiration of a specified number of bus time units after the sample point. 
         [0033]    A Fast-CAN controller in the state Fast-CAN data  103  remains in this state until one of two conditions applies: 
         [0000]    (A) it sees a reason for starting a CAN-error frame, or
 
(B) the CRC delimiter is reached in the CAN frame.
 
         [0034]    If (A) or (B) is satisfied, the controller switches back into the state Fast-CAN arbitration  102  (state change T 4 ). 
         [0035]    In the range between DLC and CRC delimiter, according to the CAN protocol there are two reasons to start an error frame: (A1) the transmitter sees a bit error or (A2) a receiver sees a stuff error. At the end of the possibly superposed error flag, the beginning of the error delimiter, all the controllers in the network are in state Fast-CAN arbitration  102 . 
         [0036]    Both in (A1) and (A2), and also in (B), change T 4  takes place into state Fast-CAN arbitration  102 , and consequently the switching over of the scaling factor at the sample point at which the condition applies, or having an at least approximately constant time distance from this. The state change may also take place after the expiration of a specified number of bus time units after the sample point, for instance, at the end of phase buffer segment  2  (compare  FIG. 2 ). 
         [0037]      FIG. 2  describes the subdivision of each transmitted bit into bit time segments, whose length is measured in bus time units. These settings are usually configured in each bus user, and they are used to balance out signal propagation times on the bus and tolerances among the clock generators or oscillators used. In Fast-CAN controllers according to the present invention, it may now be provided that the settings of the bit time segments be taken care of individually, depending on the state and/or the currently used bus time unit. For this, the corresponding registers in which the configuration settings are stored, are doubly provided. In the example stated, the individual segments for a bit  210  are shown at a bus time unit of 200 ns, and the segments for four successive bits  220  at a bus time unit of 50 ns. For bit  210 , the propagation time segment has a length of only one bus time unit, while phase buffer segment  1  and  2  occupy  4  bus time units each. However, in the case of each bit from 220, the length of the propagation time segment and phase buffer segment  1  and  2  amounts to 3 bus time units each. 
         [0038]    In the states Fast-CAN arbitration  102  and standard CAN  101 , the long bus time unit is used and the bit time segments correspond to those of bit  210  shown, and in state Fast-CAN data  103 , the short bus time unit is used, and the bit time segments correspond to those of bit  220 . 
         [0039]    In particular, it may be advantageous, in the case according to the present invention, in the state Fast-CAN data to select the propagation time segment to be as small as possible, that is, for example, only one bus time unit in length, and to select the two phase buffer segments correspondingly as large as possible, in order to be able to balance out the oscillator tolerances, which are able to become relevant particularly at the high transmission rates in the state Fast-CAN data, as well as possible by the CAN resynchronization mechanism. 
         [0040]    The design of the data frames used, the ranges having different bit lengths, their dependence upon the respective state of the controller and the identification according to the present invention will be explained below, with reference to  FIG. 3 . 
         [0041]      FIG. 3  shows the structure of a CAN data frame according to ISO11898-1 in the two possible variants, the standard format and the extended format. For both variants the regions are drawn in which, according to the present invention, switching over takes place between states Fast-CAN arbitration  102  and Fast-CAN data  103 . Shown also is the switching over of the bit length that goes along with this, as well as the corresponding change in the scaling factor. Finally shown, is the position selected still in this exemplary embodiment of the identification according to the present invention, in “reserved bit” RO, which is transmitted before the DLC. 
         [0042]    The benefit of the method shown in the first exemplary embodiment for the data transmission rate is illustrated by the following calculation: We assume a length of the data field of 8 bytes, a data frame in the standard format having 11-bit addressing, as well as a baud rate of 500 kbit/s. Moreover, it is assumed that the scaling factor according to “reserved bit” RO is increased by a factor of four. In this case, the bit length, that is, according to “reserved bit” RO of 2 microseconds would be reduced to 0.5 microseconds. Ignoring possible stuff bits, in this example, per data frame, 27 bits (SOF, identifier, RTR, IDE, r0, ACK field, EOF, intermission) having normal bit length and 84 bits (DLC, data, CRC, CRC delimiter) having shortened bit length are transmitted, which yields an effective transmission performance of 111 bits in 96 microseconds. At the same assumed bus loading, this corresponds to a data transmission rate that is increased compared to the unmodified standard CAN transmission by a factor of 2.3. 
         [0043]    In the case of otherwise equal conditions, if we assume an extended format having 29 bit addressing, then, per data frame, 47 bits having normal bit lengths and 84 bits having the shortened bit length are transmitted, which yields an effective transmission performance of 131 bits in 136 microseconds. At the same assumed bus loading, this corresponds to a data transmission rate that is increased compared to the normal transmission performance by a factor of 1.9. 
         [0044]    One additional exemplary embodiment is shown in the following, with reference to  FIGS. 4 and 5 . 
         [0045]      FIG. 4  shows a system matrix of a TTCAN network, according to ISO11898-4, having the base cycles and time windows described. There are time windows that are designated by “message A”, “message C”, etc., which are available exclusively for the transmission of certain data frames, while in other time windows that are designated by “arbitration”, the bus access is issued by ordinary CAN arbitration. 
         [0046]    In the second exemplary embodiment, all data frames for which nothing else is described are treated according to the method of the first exemplary embodiment. In addition, for certain time windows fixed ahead of time that are exclusively allocated, a shortening of the bit length is undertaken, by adjusting the scaling factor even earlier, for instance, as of the SOF bit and maintained, for instance, to the end of the CRC field. One example of a data frame transmitted in such a modified manner is shown in  FIG. 5 . As identification for the imminent rapid transmission, for example, a reserved bit of the preceding reference message may be drawn upon. Setting this bit would signal, in the case described, that the data frames, which are transmitted in the following base cycle in exclusive time windows, are transmitted in an accelerated manner already as of the SOF bit and to the end of the CRC field, which means at reduced bit length. 
         [0047]    In one preferred specific embodiment it is conceivable that only those exclusive data frames which are transmitted in each base cycle, that is, having repetition factor one, are additionally accelerated by the method. This case is shown in  FIG. 4 . This correlation is represented in  FIG. 4 . In the system matrix shown in exemplary fashion, the data frames designated by “message A” and “message C” would then be transmitted in an accelerated manner, according to the method explained, having a corresponding identification in the respectively preceding reference message. 
         [0048]    For the method described in the second exemplary embodiment, it is also possible to do without the identification and to establish that, in all exclusive time windows, the data frames are basically transmitted in an established range, such as between the SOF bit and the end of the CRC field having shortened bit length. It is for this reason that, in  FIG. 4 , the identification is marked “optional”. 
         [0049]    The usefulness of the method in the second exemplary embodiment shown is greater than in the first example, since the bits of arbitration and control field are rapidly transmitted within the exclusive time windows. The data transmission rate actually achieved depends at least on the proportion of the exclusive time windows and the type of addressing. 
         [0050]    A method modified compared to the method specified in ISO 11898-1 may be required in the Fast-CAN controller for handling of the transmission confirmation (CRC delimiter and acknowledge slot), as is explained in greater detail in  FIG. 6 . 
         [0051]    Shown in  FIG. 6  under “A” is the ideal sequence of the transition from the state Fast-CAN data to Fast-CAN arbitration at very short internal processing times and signal propagation times. The sender sends the CRC delimiter as a single recessive bit, and, according to the previously described exemplary embodiments of the present invention, changes, for example, at the sample point of this CRC delimiter bit or after the passing of phase buffer segment  2 , into the Fast-CAN arbitration state. At this bit position, for instance, the receivers, too, change to the state Fast-CAN arbitration. These state transitions T 4  having the resetting of the scaling factor are able to take place in the various bus users at times that do not match exactly, possibly because of signal propagation times or internal processing times. That is, the participating bus users, at times that do not exactly agree, set their scaling factor for the bus time unit again to the initial state. From this, different starting, time points of the next bit result for the bus users. 
         [0052]    After receipt of the CRC delimiter, if its CRC check was positive, each receiver sends a single dominant acknowledge bit. If this happens relatively late, because, for example, the receivers are connected at distant ends of the bus, the recessive CRC delimiter bit is able to appear longer than one bit. This case is shown in  FIG. 6  under “B”. By the superposition of acknowledge bits, the acknowledge slot may additionally appear longer than a bit, as shown in  FIG. 6  under “C”. In order to balance the phase-shifted sending times of these acknowledge bits, if necessary, the handling of these bits in Fast-CAN controllers is able to be changed to the extent that, in the state Fast-CAN arbitration, a dominant acknowledge slot of one or two bit length, which begins after the CRC delimiter, or even one bit later, is acknowledged as a valid acknowledge. 
         [0053]    Because of the falling edge of the acknowledge bit, the bus users are subsequently synchronized again within the scope of the usual resynchronization mechanism. After the first bit of the CRC delimiter, if the sender receives not only one but two additional recessive bits, this, to him, is an acknowledge error. After the second dominant acknowledge bit, if a third dominant bit is received, this is a format error to all. 
         [0054]    Upon the acknowledge slot, as in the standard CAN, there follows a recessive acknowledge delimiter, which is one bit long. As in the standard CAN, a Fast-CAN receiver, which has detected a CRC error, will start the error frame only in the bit after the acknowledge delimiter. 
         [0055]    In summary, because of the present invention, there is a design approach for describing a method by which, in a CAN network, data frames are able to be transmitted in a shorter time and simultaneously essential properties of the CAN with regard to error detection as well as network-wide data consistency are maintained.