Abstract:
The present invention relates to a digital bus system ( 1; 1   a ) of low power consumption. The bus system ( 1; 1   a ) is adapted to establish at least one parameter (M; M 1 ) that indicates the number of transmitter units ( 9.1 - 9. N) in the bus system ( 1; 1   a ) which need to send data on a data bus ( 3 ). A clock signal (CLK 2 ) that indicates a rate at which data is sent is generated with regard to the established parameter (M; M 1 ) in accordance with a predetermined pattern. In this respect, the clock signal (CLK 2 ) is generated in a manner such that the fewer the transmitter units ( 9.1 -9.N) needing to send data, the lower the data rate on the data bus ( 3 ). This reduces the average power consumption of the transmitter units ( 9.1 - 9. N) and receiver ( 15 ). This lower power consumption is achieved without appreciably affecting waiting times in respect of transmitter units ( 9.1 - 9. N) being allowed to send data, since the clock signal (CLK 2 ) is generated so that the data rate will be adapted in relation to the number of transmitter units ( 9.1 - 9. N) that need to send data.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates to the field of digital bus systems, and more particularly to that part of this field in which the bus system includes means for generating a clock signal that indicates the rate at which data is sent over a data bus.  
         DESCRIPTION OF THE BACKGROUND ART  
         [0002]    In this technical context, there is often found the need to send data between units in a technical system. A well-known method of achieving an exchange of data between the units is to use a digital bus system. The digital bus system will normally include a data bus that has one or more data lines. A plurality of transmitter units and one or more receiver units are normally connected to the data bus. In order to prevent “collisions” on the data bus between different transmitter units, the digital bus system will often include an arbitrator, which is adapted to determine and control which of the transmitter units may have access to the data bus. Alternatively, the transmitter units may include means for detecting collisions on the data bus. When one of the transmitter units has detected a collision, the transmitter unit will normally wait for a randomly chosen time period before making a fresh attempt to send data. The digital bus system normally includes a clock, which generates a clock signal that indicates the rate at which data shall be sent on the data bus. The frequency of the clock signal is constant and normally calculated with regard to a predetermined maximum traffic load in the digital bus system.  
           [0003]    Although the digital bus system has many advantages, it also has some drawbacks. One drawback is that the digital bus system is highly power consuming. This is particularly true when the bus system includes a large number of transmitters, which is not unusual in many technical fields, such as data communications and telecommunications, for instance.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention mainly addresses the problem of reducing present-day power consumption of a digital bus system.  
           [0005]    Briefly speaking, the problem is solved with a bus system that includes a data bus to which a plurality of transmitter units and at least one receiver is connected. The bus system also includes means for generating a clock signal that controls the rate at which data is sent on the data bus. It is proposed in accordance with the invention that the clock signal is generated so that the rate at which data is transmitted on the data bus will vary. Since the data rate varies, the transmitter units and the receiver will not always operate at a high frequency that is adapted for a maximum traffic load in the bus system, which results in lower average power consumption of the transmitter units and the receiver than would otherwise be the case.  
           [0006]    The present invention is thus intended mainly to reduce the power consumption in a digital bus system in relation to known systems.  
           [0007]    More specifically, the problem formulated above is solved as follows. The bus system includes means for determining at least one parameter that provides an indication of how many of the transmitter units need to send data on the data bus at that particular moment in time. The means that generate the clock signal are adapted to generate said signal with regard to the established parameter in accordance with a predetermined pattern. In this respect, the clock signal is generated so that the fewer transmitter units that need to send data, the lower the data rate on the data bus. Because the clock signal is generated such as to vary the data rate, the mean power in the transmitter units and the receiver will decrease. Moreover, this lower power consumption is achieved without waiting times for the transmitter units to transmit data being appreciably affected, since the clock signal is generated so that the data rate will be adapted with respect to the number of transmitter units that need to transmit data.  
           [0008]    Thus, one significant advantage afforded by the invention is that power consumption decreases. This is, of course, beneficial in many technical fields, and in particular in applications where a large number of transmitter units are connected to the data bus, or where the need to save power is of particular importance. The inventive bus system is therefore highly beneficial in data applications and telecommunications applications, for instance in telephone exchanges, switching centres, radio base stations or routers (a form of data switch). The inventive bus system can also be used to advantage in mobile equipment in the absence of an external power supply.  
           [0009]    The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a block diagram of a digital bus system, by way of an example of the present invention.  
         [0011]    [0011]FIG. 2 is a time diagram, which illustrates signals in the digital bus system.  
         [0012]    [0012]FIG. 3 is a constitutional diagram, which illustrates round-robin technology.  
         [0013]    [0013]FIG. 4 is a diagram that illustrates the used of a FIFO list in the digital bus system.  
         [0014]    [0014]FIG. 5 is a block diagram, which illustrates one embodiment of an arbitrator.  
         [0015]    [0015]FIG. 6 is a block diagram that illustrates a further embodiment of an arbitrator.  
         [0016]    [0016]FIG. 7 is a block diagram illustrating another embodiment of an arbitrator.  
         [0017]    [0017]FIG. 8 is a block diagram illustrating a further embodiment of an arbitrator.  
         [0018]    [0018]FIG. 9 is another block diagram, which illustrates an inventive digital bus system by way of example.  
         [0019]    [0019]FIG. 10 is a block diagram illustrating one embodiment of a clock unit.  
         [0020]    [0020]FIG. 11 is a block diagram illustrating another embodiment of a clock unit. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 1 is a block diagram illustrating an exemplifying embodiment of an inventive digital bus system  1 .  
         [0022]    The bus system  1  includes a data bus  3  having a clock signal line  3   a  and a data line  3   b . Alternatively, the data bus  3  may include one or more further data lines. The bus system  1  also includes an arbitrator  5 , which also has clock functions that are described in more detail further on. The bus system  1  includes a number (N) of transmitter units  9 . 1 - 9 .N connected to the data bus  3 . The system also includes a receiver  15  connected to the data bus  3 . Alternatively, one or more further receiver are connected to the bus  3 .  
         [0023]    The main function of the arbitrator  5  is to control the transmitter units  9 . 1 - 9 .N so as to avoid collisions on the data bus  3 , in other words to avoid two or more transmitters  9 . 1 - 9 .N from attempting to transmit on the data bus  3  simultaneously. The bus system  1  includes an arbitrator bus  7  which links the arbitrator  5  to arbitrator slaves  11 . 1 - 11 .N in the transmitter units  9 . 1 - 9 .N. In the case of the FIG. 1 embodiment, the arbitrator bus  7  includes four signal lines  7   a - 7   d  to which the arbitrator  5  and the arbitrator slaves  11 . 1 - 11 .N are connected. The arbitrator  5  is adapted to generate a first clock signal CLK 1  and a frame synchronising signal FS, which are laid out on the signal line  7   a  and  7   b  respectively. The first clock signal CLK 1  and the frame synchronising signal FS synchronise communication between the arbitrator  5  and the arbitrator slaves  11 . 1 - 11 .N. Each of the transmitter units  9 . 1 - 9 .N includes a transmitter  13 . 1 - 13 .N and each of the transmitters  13 . 1 - 13 .N is connected to the data bus  3  and to the corresponding arbitrator slave  11 . 1 - 11 .N. When one of the transmitters  13 . 1 - 13 .N, for instance the transmitter  13 . 1 , needs to send data over the data bus  3 , the transmitter  13 . 1  informs the corresponding arbitrator slave  11 . 1  to this effect. The arbitrator slaves  11 . 1 - 11 .N are adapted to generate an RTS signal (Request To Send), which is laid out on the signal line  7   c  in the example shown in FIG. 1. The arbitrator  5  is connected to the signal line  7   c  and adapted to receive the RTS signal, which includes information indicating which of the transmitter units  9 . 1 - 9 .N has requested to send data over the data bus  3 . The arbitrator  5  is adapted to evaluate the RTS signal and to generate a CTS signal (Clear To Send), which is laid out on the signal line  7   d  in the example shown in FIG. 1. The arbitrator slaves  11 . 1 - 11 .N are connected to the signal line  7   d  and adapted to receive the CTS signal, which controls which of the requesting transmitter units  9 . 1 - 9 .N may send data over the data bus  3 .  
         [0024]    The arbitrator  5  is also adapted to generate a second clock signal CLK 2 , which is laid out on the clock signal line  3   a  of the data bus  3 . The receiver  15  and the transmitters  13 . 1 - 13 .N are connected to the signal line  3   a  and thus receive the second clock signal CLK 2 , which indicates the rate at which data is sent over the data bus  3 . In this respect, the transmitters  13 . 1 - 13 .N are adapted to synchronise their data transmissions over the data bus  3  in accordance with the data rate indicated by the received second clock signal CLK 2 , and the receiver  15  is correspondingly adapted to synchronise the reception of data in accordance with the second clock signal CLK 2 .  
         [0025]    [0025]FIG. 2 shows four time diagrams that illustrate respectively the first clock signal CLK 1 , the frame synchronising signal FS, the RTS signal and the CTS signal.  
         [0026]    The first clock signal CLK 1  is a standard clock signal in the form of a pulse train of rectangular pulses. The first clock signal CLK 1  has a predetermined frequency. The second clock signal CLK 2  resembles the first clock signal CLK 1  but does not necessarily have the same frequency as the first clock signal CLK 1 . The RTS signal has a frame structure in which a sequence of N number of frames F 1 -FN is constantly repeated. Each of the frames F 1 -FN is associated with one of the transmitter units  9 . 1 - 9 .N. Thus, the first frame F 1  is associated with the first transmitter unit  9 . 1 , the second frame F 2  is associated with the second transmitter unit  9 . 2 , and so on. The frames F 1 -FN in the RTS signal are synchronised with the first clock signal CLK 1 , and the time extension of the frames F 1 -FN corresponds to a time period of the first clock signal CLK 1 . The frame synchronising signal FS is illustrated in the second time diagram of FIG. 2. The frame synchronising signal FS is a pulse train of rectangular pulses that are synchronised with the first clock signal CLK 1  and recur with a time period corresponding to N periods of the first clock signal CLK 1 . The rectangular pulses of the frame synchronising signal FS indicate where the first frame F 1  in each frame sequence commences. In the case of the FIG. 2 example, the beginning of the frame Fl is indicated by a positive edge of the rectangular pulse. Each of the frames F 1 -FN of the RTS signal includes a binary information bit ( 0  or  1 ), which indicates whether or not the associated transmitter units  9 . 1 - 9 .N request permission to send data from the data bus  3 . If the information bit in one of the frames, for instance the frame F 1 , is one ( 1 ), the corresponding transmitter unit  9 . 1  requests permission to send data over the data bus  3 . In the FIG. 2 example, the transmitter units  9 . 1  and  9 .N thus request permission to send data, while remaining transmitter units  9 . 2 - 9 .N- 1  do not request such permission at that particular time. The CTS signal is illustrated in the last diagram of FIG. 2. The CTS signal has a frame structure that corresponds to the frame structure of the RTS signal. Each of the frames F 1 -FN in the CTS signal is associated with one of the transmitter units  9 . 1 - 9 .N. Thus, the first frame F 1  of the CTS signal is associated with the first transmitter unit  9 . 1 , the second frame F 2  of the CTS signal is associated with the second transmitter unit  9 . 2 , and so on. The CTS signal is synchronised to the first clock signal CLK 1 , and the frame synchronising signal FS indicates when the first frame Fl in the CTS signal is sent, in the same way as for the RTS signal. Each of the frames F 1 -FN in the CTS signal includes an information bit ( 0  or  1 ) that indicates which of the requesting transmitter units  9 . 1 - 9 .N may send data on the data bus  3  at that moment in time. In the FIG. 2 example, it is the transmitter unit N that may send data on the data bus  3 , which is indicated by virtue of the information bit in the frame FN of the CTS signal being a one ( 1 ).  
         [0027]    Naturally, there is an almost inexhaustible number of ways in which the arbitrator  5  may be designed to determine the order in which the requesting transmitter units  9 . 1 - 9 .N shall be allowed to send data over the data bus  3 . FIGS. 3 and 4 illustrate two of the most usual ways of deciding the order in which the requesting transmitter units may be allowed to send data over the data bus  3 .  
         [0028]    [0028]FIG. 3 is a constitutional diagram that illustrates so-called round-robin technology. It is determined initially whether or not the first transmitter unit  9 . 1  requests permission to send data. If the first transmitter unit requests permission to send data, said first unit  9 . 1  is permitted to send data until it no longer requests permission to send data. When the first transmitter unit  9 . 1  does not request permission to send data, it is determined whether or not the second transmitter unit  9 . 2  requests permission to send data. If such is so, the second transmitter unit  9 . 2  is permitted to send data until said second transmitter unit  9 . 2  no longer requests permission to send data. The procedure is repeated in the same way for all of the remaining transmitter units  9 . 3 - 9 .N, and then begins again from the first transmitter unit  9 . 1 .  
         [0029]    [0029]FIG. 4 is a block diagram that illustrates how a FIFO list (First In First Out) can be used to organise the order in which requesting transmitter units shall be given access to the data bus  3 . At the top of the FIFO list in FIG. 4, three transmitter units  9 .N,  9 . 3  and  9 . 2  request permission to send data over the data bus  3 . The transmitter unit  9 .N is first in the FIFO list and is thus given permission to send data over the data bus  3 . Later, the transmitter unit  9 . 1  also requests permission to send data and the transmitter unit  9 . 1  is then placed last in the FIFO list. When the transmitter unit  9 .N has completed its transmission and therefore no longer requests permission to transmit data, the transmitter unit  9 .N is duly removed from the FIFO list. The transmitter unit  9 . 3  is now first in the FIFO list and may therefore send data over the data bus  3 . Thus, in the case of the FIFO list, the requesting transmitter units  9 .N,  9 . 3 ,  9 . 2  and  9 . 1  send data over the data bus  3  in the order in which the transmitter units  9 .N,  9 . 3 ,  9 . 2  and  9 . 1  have requested permission to send data.  
         [0030]    [0030]FIG. 5 is a block diagram illustrating an exemplifying embodiment of the arbitrator  5 . The arbitrator  5  includes a clock signal generator  21  which is adapted to generate the first clock signal CLK 1 . A signal generator  23  is adapted to receive the first clock signal CLK 1  and to generate the frame synchronising signal FS in response to the first clock signal CLK 1 . The arbitrator  5  includes an S/P converter  25  (Series-to-Parallel converter), which is adapted to receive the RTS signal. The S/P converter  25  is adapted to receive the serially incoming frames F 1 -FN of the RTS signal and to lay-out the frames F 1 -FN in parallel on a corresponding number (N) outputs. The S/P converter  25  is adapted to receive the first clock signal CLK 1  and the frame synchronising signal FS, which are used by the S/P converter  25  to synchronise correctly the receipt of the frames F 1 -FN of the RTS signal. A line or queue manager  27  is connected to the outputs of the S/P converter  25  and thus receives the frames F 1 -FN of the RTS signal in parallel. The line manager  27  is adapted to decide the order in which the requesting transmitter units may send data over the data bus  3 , this decision being made in relation to the received frames F 1 -FN. For example, the line manager  27  may be adapted to utilise the round-robin technique, a FIFO list, or some other system for determining the order in which the requesting transmitter units may send data. The line manager  27  is also adapted to generate the CTS signal frames F 1 -FN, which, as is known, indicate which of the transmitter units  9 . 1 - 9 .N may send data over the data bus  3  at that moment in time. The line manager  27  is adapted to lay-out the CTS signal frames F 1 -FN in parallel on a corresponding number (N) of line manager outputs. A P/S converter  29  (Parallel-to-Serial converter) is connected to the outputs of the line manager  27 . The P/S converter  29  is therewith adapted to receive the frames F 1 -FN of the CTS signal in parallel. The P/S converter  29  is also adapted to generate the CTS signal, by laying out the received frames F 1 -FN on an output in series. The P/S converter  29  is adapted to receive the first clock signal CLK 1  and the frame synchronising signal FS, which are used by the P/S converter  29  to correctly synchronise the frames F 1 -FN in the CTS signal.  
         [0031]    The arbitrator  5  in FIG. 5 also includes means for generating the second clock signal CLK 2 . The second clock signal CLK 2  is generated while taking into account the number of transmitter units  9 . 1 - 9 .N that request permission to send data over the data bus  3 . The second clock signal frequency, which controls the rate at which data is sent over the data bus  3 , decreases when a smaller number of transmitter units  9 . 1 - 9 .N request permission to send data over the data bus  3 . This means that the transmitter units  9 . 1 - 9 .N and the receiver  15  do not always operate at a high frequency adapted for a predetermined maximum traffic load in the digital bus system  1 , which, in turn, results in an average lower power consumption in the transmitter units  9 . 1 - 9 .N and the receiver  15 . This lower power consumption is also achieved without appreciably affecting transmitter unit waiting times in sending data over the data bus  3 . This is because the frequency of the second clock signal CLK 2  is adapted in relation to the number of transmitter units  9 . 1 - 9 .N that request permission to send data on the data bus  3 .  
         [0032]    The arbitrator  5  in FIG. 5 includes a signal generator  33 , which is adapted to generate a reference signal  34  in the form of a pulse train of rectangular pulses. The reference signal  34  has a predetermined frequency. A binary counter  35  is connected to the signal generator  33  and adapted to receive the reference signal  34 . The binary counter  35  is adapted to count the rectangular pulses of the reference signal  34  and to state the number of rectangular pulses counted binarily with a predetermined number of bits. The binary counter  35  of the FIG. 5 embodiment includes four (4) bits, although it may alternatively include a different number of bits, ranging from two bits and upwards. The first bit (the single digit bit) varies with the same frequency as the reference signal  34 . The second bit (the two digit bit) varies with a frequency corresponding to half the frequency of the reference signal  34 . The third bit (the four digit bit) varies with a frequency corresponding to a fourth of the reference signal frequency. The fourth bit (the eight digit bit) varies with a frequency that corresponds to an eighth of the reference signal frequency. A controllable selector  37  is connected to the binary counter  35  and functions to receive the four bits from said binary counter  35 . The selector  37  is adapted to enable one of the bits received to be selected and applied to an output of the selector  37 . In this case, the bit selected in this manner constitutes the second clock signal CLK 2 . A control circuit  39  is connected to the selector  37  and functions to control which of the bits is selected by the selector  37 . The arbitrator  5  in FIG. 5 also includes an adder  31  connected to the outputs of the S/P converter  25 . The adder  31  functions to add together the information bits in the RTS signal frames F 1 -FN, thereby obtaining a sum M which denotes the number of transmitter units  9 . 1 - 9 .N that request permission to send data over the data bus  3  at that particular moment in time. The control circuit  39  is connected to the adder  31  and functions to receive from the adder  31  information relating to the sum M. The control circuit  39  is designed to control the selector  37  in accordance with the sum M, in other words in accordance with the number of requesting transmission units. In this respect, the control circuit  39  is adapted to compare the sum M with a number of stored threshold values that denote the values of the sum M for which the different bits from the binary counter  35  shall be selected. In a concrete example, the number of transmitter units  9 . 1 - 9 .N is  14  and the frequency of the reference signal  34  is 32 MHz. The threshold values may, for instance, be set to twelve (12), eight (8) and three (3). When the sum M lies in the intervals [ 13 ,  14 ], the first bit from the binary counter  35  is chosen to constitute the second clock signal CLK 2 , which therewith obtains the frequency 32 MHz. When the sum M lies in the interval [ 9 ,  12 ], the second bit from the binary counter  35  is chosen to constitute the second clock signal CLK 2 , which therewith obtains the frequency 16 MHz. When the sum M lies in the interval [ 4 ,  8 ], the third bit from the binary counter  35  is chosen to constitute the second clock signal CLK 2 , which therewith obtains the frequency 8 MHz. When the sum M lies in the interval [ 0 ,  3 ], the fourth bit from the binary counter  35  is selected to constitute the second clock signal CLK 2 , which therewith obtains the frequency 4 MHz.  
         [0033]    Power consumption can be further reduced, by arranging for the arbitrator  5  to switch off the second clock signal CLK 2  completely when none of the transmitter units  9 . 1 - 9 .N requests permission to send data (M=0). For instance, the selector  37  may be designed to refrain from selecting any of the bits from the binary counter  35  in response to a command from the control circuit and applies no signal on the output of the selector  37  instead. Alternatively, the arbitrator  5  may be designed to enable the signal generator  33  to be switched off in response to a command from the control circuit  39 .  
         [0034]    The binary counter  35  forms in combination with the selector  37  a simple and inexpensive type of frequency divider which divides down the frequency of the reference signal  34  by 2 n (n=0,1,2,3). Alternatively, the arbitrator  5  may include, instead, a more advanced type of frequency divider for frequency modification of the reference signal  34 . Naturally, a frequency multiplier may be used in a similar way instead, such as to modify the frequency of the reference signal  34  in relation to the sum M.  
         [0035]    In the case of the FIG. 5 embodiment, the frequency of the reference signal  34  corresponds to a maximum frequency of the second clock signal CLK 2 . The frequency of the reference signal  34  is adapted with respect to a predetermined maximum traffic load in the digital bus system  1 , and the receiver  15  and the transmitters  13 . 1 - 13 .N are respectively adapted so as to handle the receipt of respective transmitted data at the rate indicated by the reference signal  34 . However, the frequency of the reference signal  34  may alternatively be set to a higher value than that for which the receiver  15  is intended. A receiving buffer (not shown) of the receiver  15  will then have a size that is adapted with respect to the frequency of the reference signal  34  and a probability distribution as to the lengths of time that data will be sent at the maximum data rate given by the reference signal  34 . This enables short data bursts to be sent at a rate which exceeds the rate at which data can be sent on the data bus  3  over a long time period.  
         [0036]    So that data sent over the data bus  3  will not be lost, the control circuit  39  is designed to select suitable time points at which the frequency of the second clock signal CLK 2  is changed. In the example shown in FIG. 5, the control circuit  39  is designed to receive the first clock signal CLK 1 , the frame synchronising signal FS and the CTS signal for correctly selecting said suitable time points. In this regard, it is preferred that the control circuit  39  is designed to change the frequency of the second clock signal CLK 2  in between the transmission of data by two of said transmitter units  9 . 1 - 9 .N over the data bus  3 .  
         [0037]    Alternatively, it is possible, however, to change the frequency of the second clock signal CLK 2  while sending data over the data bus  3 . In this case, the control circuit  39  is designed to ensure that the change of frequency from an original frequency to a new frequency is glitch-free, in other words to ensure that the frequency will not temporarily exceed either the original frequency or the new frequency during said frequency change.  
         [0038]    [0038]FIGS. 6 and 7 are block diagrams that illustrate variations of the embodiment of the arbitrator  5  shown in FIG. 5. In the FIG. 6 variation, the reference signal  34  is also used as the first clock signal CLK 1 , therewith enabling the exclusion of the clock signal generator  21 . In the FIG. 7 variation, the second clock signal CLK 2  is also used as the first clock signal CLK 1 , meaning that the clock signal generator  21  can be excluded and the power consumption in the bus system  1  further reduced. The embodiments illustrated in FIGS. 6 and 7 are the same as the embodiment in FIG. 5 in other respects.  
         [0039]    [0039]FIG. 8 is a block diagram illustrating an alternative embodiment of the arbitrator  5 . The embodiment shown in FIG. 8 has significant similarities with the embodiment shown in FIG. 5, and hence only the differences between the two embodiments will be described in more detail. The embodiment shown in FIG. 8 differs from the embodiment shown in FIG. 5 by virtue of the arbitrator  5  in FIG. 8 including a digitally controlled oscillator (DCO)  36  to generate the second clock signal CLK 2 . The oscillator  36  is connected to the control circuit  39 , which is adapted to control the oscillator  36  in relation to the value of the sum M, which denotes the number of transmitter units  9 . 1 - 9 .N that have requested permission to send data over the data bus  3 . The control circuit  39  is adapted to control the oscillator  36  so that the frequency of the second clock signal CLK 2  will depend on the sum M in a manner such that the frequency f of the second clock signal CLK 2  will decrease as the sum M decreases. In other words, if M 1  and M 2  signify two different values of the sum M and f(M 1 ) and f(M 2 ) signify the corresponding frequencies, then f(M 2 )&lt;f(M 1 ) will apply when M 2 &lt;M 1 . The frequency of the second clock signal CLK 2  can be varied by the oscillator  36  in relation to the sum M in a finer way than is possible with the arbitrator  5  of the FIG. 5 embodiment. In principle, the frequency of the second clock signal CLK 2  can be given a unique value for each value (M=0,1,2, . . . ,N) of the sum M. In turn, this enables the frequency of the second clock signal CLK 2  to be changed at the same time as data is sent over the data bus  3 , without the risk of data being lost. For example, data can be sent continuously over the data bus  3 , therewith leading to more effective utilisation of communications resources in the digital bus system  1 . The frequency of the second clock signal CLK 2  may, of course, be varied in relation to the sum M in different ways. For instance, the frequency of the second clock signal CLK 2  may be varied linearly in dependence of the sum M.  
         [0040]    In an alternative embodiment of the arbitrator  5  shown in FIG. 8, the second clock signal CLK 2  may also be used as the first clock signal CLK 1 , in a similar manner to that described in FIG. 7. This means that the clock signal generator  21  can be excluded from FIG. 8, and that power consumption can be further reduced.  
         [0041]    [0041]FIG. 9 is a block diagram illustrating another exemplifying embodiment of an inventive digital bus system referenced  1   a . Many features of the bus system construction illustrated in FIG. 9 are the same as in the bus system  1  illustrated in FIG. 1. However, the bus system  1   a  differs from the bus system  1  insofar as it does not include an arbitration function. Instead, the transmitters  13 . 1 - 13 .N in the transmitter units  9 . 1 - 9 .N are equipped with circuits (not shown) for detecting collisions on the data bus  3 . If one of the transmitters  9 . 1 - 9 .N attempts to send data over the data line  3   b  of the data bus  3  and detects a collision, the transmitter waits for a randomly selected time period before making a fresh attempt to send data. The bus system  1  a includes a clock unit  5   a , which is adapted to generate a second clock signal CLK 2 , which is laid out on the clock signal line  3   a  and which indicates a rate at which data is sent over the data bus  3 . The frequency of the second clock signal CLK 2  is based on how often collisions occur on the data bus  3 . To enable collision information to be fetched from the transmitter units  9 . 1 - 9 .N, the digital bus system  1   a  includes an information bus  7 . 1  that has three signal lines  7   a ,  7   b  and  7   c . The information bus  7 . 1  interlinks the clock unit  5   a  with slaves  11 . 1   a - 11 .Na in the transmitter units  9 . 1 - 9 .N. The clock unit  5   a  is adapted to generate a first clock signal CLK 1  and a frame synchronising signal FS, these signals being applied on respective signal lines  7   a  and  7   b . When the transmitters  13 . 1 - 13 .N have detected collisions on the data bus  3 , they send information concerning these collisions to the slaves  11 . 1   a - 11 .Na, which, in turn, send information concerning collisions that have occurred to the clock unit with the aid of a collision indicator signal (CIS). The clock unit  5   a  is adapted to receive the CIS signal via the signal line  7   c . The frame structure of the CIS signal is similar to the frame structure of the, e.g., RTS signal in the bus system  1 . The CIS signal is synchronised with the aid of the first clock signal CLK 1  and the frame synchronising signal FS. The frames of the CIS signal include information as to whether the transmitters have been subjected to a collision in the latest attempt to send data over the data bus  3 . An information bit in the form of a one (1) in the frames indicates that the corresponding transmitter was subjected to a collision in its latest attempt to send data over the data bus  3 , while an information bit in the form of a zero (0) in the frame correspondingly indicates that no collision occurred in the latest attempt to send data.  
         [0042]    [0042]FIG. 10 is a block diagram of an exemplifying embodiment of the clock unit  5   a . The construction of the clock unit  5   a  in FIG. 10 corresponds essentially to the arbitrator  5  in FIG. 5. However, because the bus system  1   a  does not include an arbitrator function, the clock unit  5   a  in FIG. 10 will neither include the line manager  27  nor the P/S converter  29 . Moreover, the S/P converter  25  is adapted to receive the CIS signal instead of the RTS signal. Thus, the adder  31 , which is connected to the outputs of the S/P converter  25 , is adapted to generate a sum M 1  by adding together the information bits in the frames of the CIS signal. The sum M 1  therefore corresponds to the number of transmitter units  9 . 1 - 9 .N that have newly detected collisions in attempting to send data over the data bus  3 . The more collisions that have been detected, the more transmitters  13 . 1 - 13 .N that attempt to send data over the data bus  3 . The sum M 1  thus gives an indirect indication of the number of transmitter units that need to send data over the data bus  3 . The control circuit  39  is adapted to control the frequency of the second clock signal CLK 2  in relation to the sum M 1  in a similar manner as the frequency of the second clock signal CLK 2  of the FIG. 5 embodiment is varied in relation to the sum M. The embodiment of the clock unit  5   a  in FIG. 10 can be varied in different ways. For example, the reference signal  34  or the second clock signal CLK 2  can be used as the first clock signal CLK 1  in a similar way as in the embodiments of the arbitrator  5  in FIGS. 6 and 7.  
         [0043]    [0043]FIG. 11 is a block diagram illustrating a further exemplifying embodiment of the clock unit  5   a . The construction of the clock unit  5   a  in FIG. 11 corresponds essentially to the arbitrator  5  in FIG. 8. However, because the bus system  1   a  does not include an arbitrator function, the clock unit  5   a  in FIG. 11 does not include the line manager  27  or the P/S converter  29 . Moreover, the S/P converter  25  is adapted to receive the CIS signal instead of the RTS signal. The adder  31 , which is connected to the outputs of the S/P converter  25 , is thus adapted to generate a sum M 1  by adding together the information bits in the frames of the CIS signal. The sum M 1  thus corresponds to the number of transmitter units  9 . 1 - 9 .N that have newly detected collisions when attempting to send data over the data bus  3 . The more collisions that are detected, the more transmitters  13 . 1 - 13 .N that have attempted to send data over the data bus  3 . The sum M 1  thus indicates indirectly how many of the transmitter units need to send data over the data bus  3 . The control circuit  39  is adapted to control the frequency of the second clock signal CLK 2  in relation to the sum M 1 , in a similar manner as the frequency of the second clock signal CLK 2  in the FIG. 8 embodiment is varied in relation to the sum M. The embodiment of the clock unit  5   a  in FIG. 11 can be varied in different ways. For example, the second clock signal CLK 2  can be used as the first clock signal CLK 1  in a similar manner as in the embodiments of the arbitrator  5  shown in FIG. 7.  
         [0044]    Normally, the digital bus systems  1  and  1   a  are constructed for a given number (N) of transmitter units  9 . 1 - 9 .N and the performance of the bus systems  1  and  1   a  is adapted to handle this number. However, the manner in which the second clock signal CLK 2  is generated in accordance with the invention causes the digital bus systems  1  and  1   a  to function effectively even when the bus systems  1  and  1   a  include a smaller number of transmitter units instead (say N-K). The digital bus systems  1  and  1   a  are thus more flexible, since they can be used beneficially with different numbers of transmitter units.  
         [0045]    The embodiments of the arbitrator  5  shown in FIGS.  5  to  8  inclusive, and the embodiments of the clock unit  5   a  in FIGS. 10 and 11, can be constructed with different circuit technologies. For example, there can be used programmable circuits, such as FPGA circuits (Field Programmable Gate Array). Alternatively, there may be used instead ASIC circuits (Application-Specific Integrated Circuit), or ASIC circuits in combination with programmable circuits.  
         [0046]    It will be understood that all technical applications considered appropriate by the person skilled in this art may be used. The invention is particularly beneficial with respect to technical applications in which a large number of transmitter units are used, for instance in data and telecommunications applications. The invention can also be applied beneficially with mobile equipment without an external power supply, with which there is, of course, a need to keep down power consumption.