Abstract:
A device and method is described for the frequency division of an input clock signal, in which from the input clock signal at least two output clock signals are generated, with an output pulse frequency equal to an input pulse frequency divided by a given factor, whereby with a phase detector a phase difference is measured between the at least two output signals and each of the at least two output clock signals is either inverted or not inverted, as a function of the phase difference determined. A method of this type is particularly suitable for the demultiplexing of an input data signal, and can also be designed to be multi-step.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This Utility Patent Application claims priority to German patent application No. DE 103 45 163.3, filed on Sep. 29, 2003, which is incorporated herein by reference. 
   FIELD OF THE INVENTION  
   The present invention relates to a method and a device for the frequency division of a clock signal and the use of the method and device respectively for the demultiplexing of a data signal. A method and device of this type are used in particular in very high speed data receivers, in which data is received at a bit rate in the range of several Gigabits per second. 
   BACKGROUND  
   With high speed (e.g., several Gigabits per second) data receivers of this type, a received data signal is typically sampled with a clock signal, which has a frequency of the value of half the data rate of the data signal, in order in this way to create two data signals with half the data rate. This process is repeated in several steps, the data rate of the data signals being halved at each step. To achieve this, in each step clock signals with a corresponding clock frequency are required, which are likewise halved at every step. 
   Conventionally, these clock signals are generated by a central clock frequency divider. The consequence of this is that the clock signals need to be conveyed over broad paths to scanning units of the different steps, which at high speeds is difficult and leads to a circuit structure which is not entirely symmetrical, which can lead to imprecisions in the sampling of the data. 
   In addition to this, when demultiplexing the data, (i.e., when sampling the data with the individual clock signal in each case) output data signals are frequently generated which are displaced mutually by half a bit length. This displacement must be compensated for, for example by temporary storage during half a clock pulse cycle, in order nevertheless for a central clock signal generation to be used. In this situation, however, in practice an additional problem frequently arises in that the temporarily-stored data can be more strongly amplified in relation to the other data, which incurs undesirable dependencies of a bit failure rate and a clock recovery from a position of a bit in the data flow which is received in each case. In addition to this, temporary storage of this kind can also be difficult, depending on the circuit technology used. 
   Another formulation consists of using what is referred to as a distributed frequency divider structure, i.e., of dividing the clock pulses down parallel to the sampling devices. However, in view of the fact that, with the frequency dividers which are normally used, the initial state is determined at random, in this situation what is referred to as reset synchronization is required, in order for all the frequency dividers to start in a defined state. A reset synchronization of this kind is, however, very difficult to achieve at high speeds. 
   If, in addition to this, what is referred to as a half-rate arrangement with a quadrature oscillator is pursued, in which the oscillator runs at half the clock frequency of the data rate, and the four phases of the quadrature oscillator produced are used to sample the data signal, then as early as in the first step several frequency dividers must be used in parallel to divide down the individual clock phases of the oscillator. With a quadrature oscillator these clock phases are displaced mutually by a quarter pulse. This slight displacement, and the need for several frequency dividers which must start in the same start state, make reset synchronisation with a distributed structure practically impossible. Accordingly, a distributed frequency divider structure is normally not used at high speeds. 
   SUMMARY  
   The present invention provides a method and device suitable for frequency division, multiplexing, and demultiplexing a signal. In one embodiment, a method is used for the frequency division of an input clock signal, in which from the input clock signal at least two output clock signals are generated, with an output pulse frequency equal to an input pulse frequency divided by a given factor, whereby with a phase detector a phase difference is measured between the at least two output signals and each of the at least two output clock signals is either inverted or not inverted, as a function of the phase difference determined. A method of this type is particularly suitable for the demultiplexing of an input data signal, and can also be designed to be multi-step. 

   
     BRIEF DESCRIPTION OF THE DRAWING  
     The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles of the invention. Other embodiments of the present invention and many of the intended advantages of the present invention will be readily appreciated, as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. 
       FIG. 1  illustrates one embodiment of a combined frequency divider and demultiplexer cell. 
       FIG. 2  illustrates an embodiment according to the invention of a demultiplexing device making use of the cell from  FIG. 1 . 
       FIG. 3  illustrates a device with devices according to the embodiment of  FIG. 2  connected in step fashion serially. 
   

   DETAILED DESCRIPTION  
   In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as“top,”“bottom,”“front,”“back,”“leading,”“trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     FIG. 1  illustrates one embodiment of a demultiplexer basic cell  1 , which serves to sample an input data signal d and to simultaneously divide the frequency of an input clock signal t. To sample this input data signal d, it is fed to a data input D of a first D-flip-flop  2 , and the input clock signal t is fed correspondingly to a clock input of this D-flip-flop  2 . In this situation, in the present example, the input clock signal t has a clock frequency which is half as large as a data rate of the input data signal d. The data rate can be 10 GHz and the clock frequency 5 GHz, for example. Accordingly, a data signal d 0  is created, with half the data rate of the data signal d. 
   The input clock signal t is, in addition, fed to a clock input of a second D-flip-flop  3 . An inverted output Q of this second D-flip-flop  3  is connected back into a data input D of this D-flip-flop  3 . At a non-inverted output Q of the D-flip-flop  3 , a clock signal t 0  can then be tapped, which has half the clock frequency compared to the input clock signal t. An output state of the clock signal t 0  in this situation depends on the initial state of the inverted output Q of the second D-flip-flop  3 , and is therefore not determined in a defined manner. 
   This demultiplexer basic cell has the advantage that all the signal lines are symmetrical, as a result of which delays in a demultiplexer tree can be kept the same. This demultiplexer basic cell can in principle also be used in devices other than those described hereinafter. 
   In principle, however, other means are also conceivable for a demultiplexer basic cell  1  of this kind for the sampling of the input data signal d and for the frequency division of the input clock signal t, and other division factors than the factor two are also possible. 
     FIG. 2  illustrates a demultiplexer  7  according to the invention, which comprises essentially two demultiplexer basic cells  1   a ,  1   b , which are made up in the same way as the demultiplexer basic cell  1  from  FIG. 1 . In this situation, the input data signal d and the input clock signal t are fed unaltered to a demultiplexer basic cell  1   a , while the input clock signal is fed in inverted form to the demultiplexer basic cell  1   b . As an alternative, the D-flip-flops of the demultiplexer basic cell  1   b  can also be controlled by a falling flank of the input clock signal t, while the D-flip-flops of the demultiplexer basic cell  1   a  are controlled by a rising flank of the input clock signal t. The effect of this is that, for example, the output data signal d 1  emitted by the demultiplexer basic cell  1   a  comprises only even bits of the input data signal d, while the output data signal d 2  emitted by the demultiplexer basic cell  1   b  correspondingly contains only odd bits. 
   Accordingly, an output clock signal t 2  generated by the demultiplexer basic cell  1   a  and an output clock signal t 1  generated by the demultiplexer basic cell  1   b  are offset in relation to one another. 
   As illustrated in  FIG. 2 , in this situation the output clock signal t 1  is allocated to the output data signal d 1 , and the output clock signal t 2  is allocated to the output data signal d 2 , which is achieved by a simple crossing of the lines. This is due to the fact that, for a multi-step demultiplexer structure, as described hereinafter, the pulse flanks of the output clock signals t 1  and t 2  should lie in the middle of the bits of the output data signals d 1  and d 2  allocated to them, in order, in a subsequent step, to guarantee reliable sampling. 
   As already explained by reference to  FIG. 1 , the initial state of the output clock signals t 1  and t 2  is not defined. In order to compensate for this, a phase detector  5  is provided, which compares the phase position of the output clock signals t 1  and t 2  and transfers a phase signal determined in this way to a control unit  6 . Depending on the phase position, the control unit  6  controls Exclusive-Or gates  42  and  44  with actuating signals s 1  and s 2 , to which the clock signals t 1  and t 2  are fed. Accordingly, if required, one or both of the clock signals can be inverted in order to guarantee a desired phase position. 
   Exclusive-or gates  41  and  43  of this type are preferably also present in the lines for the output data signals d 1  and d 2 , to which a zero signal is fed, so that they allow the output data signals through unchanged. As a result of this measure, a matching of the run time of the output clock signals t 1 , t 2 , and of the output data signals d 1 , d 2 , is achieved. 
   Illustrated in  FIG. 3  is a multi-step demultiplexer according to one embodiment of the invention. The first step S 1  in this situation comprises a demultiplexer  7 , whereby in each case an output data signal d 1 , d 2 , and, allocated to these, output clock signals t 1 , t 2 , are conducted to two further demultiplexers  7  in the second step S 2 . The data signals d 3  to d 6  and clock signals t 3  to t 6  that are generated in this way are conducted to four further demultiplexers  7  in the third step S 3 , so that finally eight output data signals d 7  to d 14  and eight output clock signals t 7  to t 14  are generated. A device of this type can of course also contain only two steps, or may comprise more than three steps. 
   The phase detection illustrated in  FIG. 2  can be extended with a structure of this kind, in that the phase differences determined by the phase detectors  5  are fed together to the control means  6 , and additional phase detectors between pulse outputs of different demultiplexers  7  of a step are also conceivable. The control means  6  then determine the phase position of the different output clock signals in relation to one another, for example by means of a logic circuit, and control the Exclusive-Or gates of the different demultiplexers  7  accordingly, in order to set a desired phase relationship. 
   In the event of a clock structure distributed in this manner being required for applications other than the demultiplexing of data, this can be achieved simply by omitting the path for the data in  FIGS. 1 to 3 . 
   A clock frequency divider structure of this type can be combined in particular with a quadrature oscillator to generate the input clock signal t. 
   A particular advantage of the device according to the invention lies in the fact that all the data paths are structured in an identical manner and the frequency division is made substantially easier. Likewise, no delays are required, as described in the introduction to the description. The additional elaboration of the circuit in which this incurs, in that for each output data signal a separate clock signal is generated, is relatively small, and is also compensated for by doing away in part with the temporary storage. 
   It is therefore possible, without any reset synchronization circuit and with little effort, to ensure the correct operation of a distributed frequency divider structure in a demultiplexer tree. In addition to this, in the last step, the path for the frequency division can be done away with, if the pulses are no longer required. The temporal arrangement of the individual data bits next to one another is effected in this situation at slower pulse rates than the input pulse frequency, with the result that difficulties with the temporary storage of a bit can be avoided. 
   Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.