Abstract:
Circuitry and method for dividing the frequency of an input clock signal for use in a prescaler of a digital frequency synthesizer. A flip flop is clocked on a first type of edge of the input clock signal, and provides an output for use as a divided clock signal. Feedback circuitry is clocked on the first type of edge of the input clock signal and provides a signal to a data input of the flip flop based on the inverse of the output of the flip flop. In a first mode the feedback circuitry is arranged to allow the signal provided to the data input of the flip flop to follow the inverse of the output of the flip flop so that each first type of edge causes the signal provided to the data input of the flip flop to toggle such that the output of the flip flop has a frequency which is substantially half of the frequency of the input clock signal, and wherein in a second mode the feedback circuitry is arranged to allow the signal provided to the data input of the flip flop to follow the inverse of the output of the flip flop with the exception that toggling of the signal provided to the data input of the flip flop is selectively prevented despite toggling of the output of the flip flop on an edge of the first type such that the output of the flip flop has a frequency which is at most a third of the frequency of the input clock signal. The feedback circuitry is arranged to operate independently from a second type of edge of the input clock signal in providing the signal to the data input of the flip flop, such that the output of the flip flop is independent from the duty cycle of the input clock signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of Indian patent application number 3126/ Del/2010, filed on Dec. 28, 2010, entitled FREQUENCY DIVISION OF AN INPUT CLOCK SIGNAL, which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to frequency division of an input clock signal. In particular, the present invention relates to circuitry for dividing the frequency of an input clock signal for use in a prescaler of a digital frequency synthesizer. 
         [0004]    2. Discussion of the Related Art 
         [0005]    Digital frequency synthesizer circuits, such as phase locked loop (PLL) digital frequency synthesizer circuits, are operable at high speeds and are typically used in digital mobile communication devices. A conventional PLL frequency synthesizer circuit includes a frequency divider which has a prescaler, such as a dual modulus prescaler. A prescaler is used to generate narrowly-spaced frequencies at frequencies which are too high to be reliably passed through a feedback system of a device. A dual modulus prescaler selectively frequency divides a frequency signal (e.g. an input clock signal from a voltage controlled oscillator (VCO)) by either a frequency dividing ratio k or a frequency-dividing ratio (k+x) to supply a frequency-divided signal. The modulus of a prescaler is its division ratio. A dual modulus prescaler can divide by two different factors, such as k and k+x. x may be positive or negative, and may for example be 1. A control input can be used to switch the factor used by the prescaler between k and k+x. 
         [0006]    One example of a dual modulus prescaler which is based on a shift register ring is shown in  FIG. 1   a.  The flip flops ( 102 ,  104 ,  106 ,  112  and  116 )  FIG. 1   a  are initialized to output high signals.  FIG. 1   b  shows a timing diagram of signals output from the flip flops  102 ,  104 ,  106  and  116  in the dual-modulus prescaler of  FIG. 1   a  when M is set to 0. It can be seen that when M is set to 0 the output signal has a frequency which is eight times less than the frequency of the input clock signal.  FIG. 1   c  shows a timing diagram of signals output from the flip flops  102 ,  104 ,  106 ,  112  and  116  in the dual-modulus prescaler of  FIG. 1   a  when M is set to 1. It can be seen that when M is set to 1 the output signal has a frequency which is nine times less than the frequency of the input clock signal. 
         [0007]    Another example of a dual modulus prescaler is shown in  FIG. 2   a  which uses cascaded divide-by-two stages ( 202 ,  204  and  206 ), as described in “High-Speed Architecture for a Programmable Frequency Divider and a Dual-Modulus Prescaler” by Patrik Larsson, IEEE Journal on Solid-State Circuits, VOL. 31, No. 5, May 1996. The one detector  208  gives a low output pulse when the M signal and the outputs of all divide-buy-two stages ( 202 ,  204  and  206 ) are high. This pulse is delayed one clock cycle and synchronized with the inverted input clock signal in the D flip-flop  212 . The input clock signal may, for example, be received from a voltage controlled oscillator (VCO). A low output from the D flip flop  212  will prohibit one negative pulse of the Clk signal from reaching the first divide-by-two stage  202 . After cancelling one clock pulse, it will take eight clock pulses before the one detector  208  output goes low, giving a division ratio of nine. By setting M low, the detector output is always high and the prescaler divides by eight. Therefore the circuit operates as an 8/9 prescaler, using M as the control signal, but can be easily modified to a 2 b /2 b +1 prescaler by adding or removing divide-by-two stages thereby extending or shortening the detector.  FIG. 2   b  shows a timing diagram of signals output from the divide-by-two stages  202 ,  204  and  206  and from the D flip flop  212  in the dual-modulus prescaler of  FIG. 2   a  when M is set to 0. It can be seen that when M is set to 0 the output signal has a frequency which is eight times less than the frequency of the input clock signal.  FIG. 1   c  shows a timing diagram of signals output from the divide-by-two stages  202 ,  204  and  206  and from the D flip flop  212  in the dual-modulus prescaler of  FIG. 1   a  when M is set to 1. It can be seen that when M is set to 1 the output signal has a frequency which is nine times less than the frequency of the input clock signal. 
       SUMMARY OF THE INVENTION 
       [0008]    The prescalers of the prior art, such as those described above, which employ a clock pulse swallowing technique, work in half cycle operation of the clock signal. For example, it can be seen in  FIG. 2   a  that the D flip flop  212  is clocked using the falling edges of the input clock signal (due to the inverter  210 ), to thereby swallow one rising edge of the clock input using the transistor  216 . The rising edge of the clock signal is swallowed in the sense that it is blocked from reaching the Div 2 block  202 . It can be appreciated that the prescaler shown in  FIG. 2   a  relies on half clock cycle operation in order to function correctly. In other words it is necessary that the D flip-flop  212  is clocked using the falling edge of the clock signal rather than the rising edge, whilst other components (e.g. transistor  214 ) are clocked using the opposite edge, i.e. the rising edge. Therefore the timing of both rising and falling edges of the input clock signal affect the operation of the prescaler shown in  FIG. 2   a,  and as such the operation of the prescaler shown in  FIG. 2   a  is dependent upon the duty cycle of the input clock signal. 
         [0009]    Since the prescaler shown in  FIG. 2   a  relies upon the timing of both the rising edges and the falling edges of the input clock signal, the duty cycle of the input clock signal received from the VCO directly impacts the operation of the prescaler. In particular, the dependence of the prior art prescalers on the duty cycle of the input clock signal impacts on (i.e. limits) the maximum speed at which the prescaler can operate. It may be beneficial for a different mechanism to the clock swallowing mechanism of the prior art to be used in a prescaler so that it would be possible for the prescaler to operate independently of the duty cycle of the input clock signal. This can be advantageous in many different implementations, in particular when operating at the high frequencies that prescalers use in digital frequency synthesizer circuits in digital mobile communication devices. Embodiments of the present invention implement prescaler circuitry whose operation depends upon the frequency of the input clock signal, but does not depend upon the duty cycle of the input clock signal. 
         [0010]    In a first aspect there is provided circuitry for dividing the frequency of an input clock signal for use in a prescaler of a digital frequency synthesizer, the circuitry comprising: a flip flop configured to be clocked on a first type of edge of the input clock signal, and configured to provide an output for use as a divided clock signal; and feedback circuitry configured to be clocked on the first type of edge of the input clock signal and configured to: receive the output of the flip flop; provide a signal to a data input of the flip flop based on the output of the flip flop; and receive at least one control signal which determines a mode of the circuitry, wherein in a first mode the feedback circuitry is arranged to allow the signal provided to the data input of the flip flop to follow the inverse of the output of the flip flop so that each first type of edge causes the signal provided to the data input of the flip flop to toggle such that the output of the flip flop has a frequency which is substantially half of the frequency of the input clock signal, and wherein in a second mode the feedback circuitry is arranged to allow the signal provided to the data input of the flip flop to follow the inverse of the output of the flip flop with the exception that toggling of the signal provided to the data input of the flip flop is selectively prevented despite toggling of the output of the flip flop on an edge of the first type such that the output of the flip flop has a frequency which is at most a third of the frequency of the input clock signal, wherein the feedback circuitry is arranged to operate independently from a second type of edge of the input clock signal in providing the signal to the data input of the flip flop, the second type of edge being the opposite of the first type of edge of the input clock signal, such that the output of the flip flop is independent from the duty cycle of the input clock signal. 
         [0011]    The first mode is an unblocking mode in which the output from the flip flop has a frequency which is substantially half of the frequency of the input clock signal. The second mode is a blocking mode in which the data input to the flip flop is prevented from toggling at least once in a cycle. 
         [0012]    At least one embodiment removes the half cycle path in the prescaler and thus prevents blocking of the input clock signal on the clock input of the flip flop. This helps to avoid bad slopes, pulse width violations and voltage swings at the input of divide-by-two blocks. There is no logic in the path of the input clock signal, such that the input clock signal is not blocked. Furthermore, as described above, there is no half cycle operation, thus there is no dependency on the duty cycle of the input clock signal. There is no half cycle operation because the clocking of the circuitry is performed using only the first type of edge of the input clock signal (e.g. the falling edges). In other words, there is no clocking of the circuitry using the second type of edge of the input clock signal (e.g. the rising edges). Therefore, provided that the frequency of the input clock signal and the timing of the edges of the first type (e.g. falling edges) are not affected, altering the timing of the edges of the second type (e.g. rising edges) would not affect the operation of the circuitry (i.e. the operation of the circuitry is independent from the duty cycle of the input clock signal). 
         [0013]    The circuitry of embodiments operates as a divide-by-two block when it is operating in the first mode (e.g. when there is no block signal), but when it is operating in the second mode (e.g. when a block signal is received as one of the control signals), the signal applied to the data input of the flip flop is forcefully held to prevent toggling of the output of the circuitry. In this way, in the second mode the frequency of the divided clock signal output from the flip flop is 
         [0000]    
       
         
           
             ( 
             
               1 
               
                 2 
                 + 
                 y 
               
             
             ) 
           
         
       
     
         [0000]    of the frequency of the input clock signal where y is the number of times during one cycle for which the data input of the flip flop is forcefully prevented from toggling. So, for example, where y is 1, toggling is prevented once during each cycle such that the frequency of the divided clock signal output from the flip flop is a third of the frequency of the input clock signal. 
         [0014]    The term “cycle” is used to mean the operations performed by the circuitry during one time period of the divided clock signal output from the flip flop. In other words, a “cycle” is a block of operations that are repeatedly performed by the circuitry during operation. 
         [0015]    Dedicated start up circuitry may be required in the prior art to place a prescaler in a functional state. Without such dedicated start up circuitry, prescalers of the prior art can become stuck and may not function correctly. As an example, the prescaler shown in  FIG. 2   a  can become stuck in the divide-by-three mode (i.e. when M=1) but not in the divide-by-two mode (when M=0). If the prescaler shown in  FIG. 2   a  is started up with M set to a high value, the output of each Div 2 block set to a high value and the output of the D flip flop set to a low value then the circuit is in a stuck state. The transistor  216  does not allow current to flow through it since the output of the D flip flop is low, such that a low signal is never received at the first Div 2 block  202 , so the outputs of all of the Div 2 blocks ( 202 ,  204  and  206 ) stay high such that the input to the D flip flop  212  from the one detector  208  stays low, such that, as described above, the transistor  216  does not allow current to flow through it. This means that the Div 2 blocks  202 ,  204  and  206  do not receive an edge of the clock signal (due to the blocking by transistor  216 ) and so the outputs of the Div 2 blocks  202 ,  204  and  206  do not change. 
         [0016]    Stuck states such as the one described above in relation to  FIG. 2   a  can occur because the input clock signal is blocked. In embodiments, there is no blocking to the input clock signal. In other words, the circuitry is arranged such that the input clock signal is received directly at the flip flop without being blocked. Since the input clock signal is directly received at the flip flop, dedicated start up circuitry is not required. Advantageously, the circuitry will self resolve itself during startup. Since dedicated start up circuitry is not required, the amount and the complexity of the hardware of the circuitry are reduced compared to prior art prescalers. As described in more detail below, some prescalers include a 50% duty cycle (50 D) module such that the divided clock signal output from the prescaler has a 50% duty cycle. Such 50 D modules may have more than one state in which they can get stuck (i.e. they have multiple stuck states). The circuitry of embodiments of the invention is particularly advantageous when used in a 50 D module since it avoids the need for start up circuitry for the multiple stuck states of a 50 D module. 
         [0017]    The concept may also be extended to a 50% duty cycle (50 D) prescaler to provide a high speed modular prescaler as compared to the prior art. For 50 D prescalers, toggling of the output is blocked depending on two least significant bits (LSBs), p 0  and p 1 , to achieve a desired frequency division of the input clock signal. The two bits, p 0  and p 1 , provide a binary representation of the amount of blocking to be applied to the input clock signal. The two p-bits can therefore be thought of as a binary counter. For example, in one 50 D module of a prescaler the input clock signal is divided by four if there is no extra blocking of the clock signal. The 50 D module may comprise two divide-by-two blocks, to output a clock signal which has a frequency which is a quarter of the frequency of the input clock signal. In an embodiment, the p-bits (p 0  and p 1 ) indicate how many times the clock signal is to be blocked or prevented from toggling in each cycle, such that:
       if p 1 =0 and p 0 =0, no blocking occurs and the output signal has a frequency which is ¼ of the frequency of the input clock signal;   if p 1 =0 and p 0 =1, the clock signal is blocked once per cycle, so that the output signal has a frequency which is ⅕ of the frequency of the input clock signal;   if p 1 =1 and p 0 =0, the clock signal is blocked twice per cycle, so that the output signal has a frequency which is ⅙ of the frequency of the input clock signal; and   if p 1 =1 and p 0 =1, the clock signal is blocked three times per cycle (two blocking structures may be required), so that the output signal has a frequency which is 1/7 of the frequency of the input clock signal.       
 
         [0022]    The output signal of the 50 D module has a 50% duty cycle. The use of a second divide-by-two block to provide the output from the module means that the output has substantially a 50% duty cycle. When dividing the input clock signal by an even number, n (e.g. where n is 4 or 6) then the duty cycle of the output divided clock signal has a 50% duty cycle, i.e. the high time is the same as the low time of the output signal over a period of the output signal. However, when the input clock signal is divided by an odd number then the output divided clock signal does not strictly have a 50% duty cycle. For example, dividing the frequency of the input clock signal by five results in the output clock signal having 2 cycles of HIGH TIME and 3 cycles of LOW TIME or vice-versa. In fact, when dividing the frequency of the input clock signal by an odd number n, the duty cycle, D, of the output divided clock signal from the 50 D module is given by 
         [0000]    
       
         
           
             D 
             = 
             
               
                 
                   n 
                   ± 
                   1 
                 
                 
                   2 
                    
                   n 
                 
               
               . 
             
           
         
       
     
         [0023]    There is a further problem with the prior art. If the programmable bits which determine the factor by which the input clock signal is to be divided are changed on the fly, then glitches may occur on the output clock signal. For example, in the prior art prescaler shown in  FIG. 2   a,  the timing of changes to the M signal (which provides a control signal, similar to the p-bits mentioned above) are not synchronized with the clock signal. Therefore, changes to M which are made on the fly (i.e. during operation of the prescaler) may occur at any phase throughout the cycle of the clock signal. As such, there will be a high probability that the input clock signal for the Div 2 block will be of high frequency leading to high frequency glitches at the output (which may be greater than the switching range) of the prescaler. 
         [0024]    Embodiments provide a solution to the high frequency glitches of the prior art described above. If the p-bits are changed on the fly, there will be a glitch (which can be greater than the operating range of prescaler) at the data input of the flip flop. However, the is input clock signal is received directly at the clock input of the flip flop such that the output of the flip flop is synchronized with the input clock signal. Therefore, although there may be glitches at the data input of the flip flop, there will not be an erroneous high frequency signal at the output of the flip flop, thus the prescaler can be easily placed as an output divider of a PLL with no additional hardware required. Intermediate frequencies may be seen at the output of the circuitry, but that is normally acceptable in most PLL applications even though, as explained above high frequencies may be seen at the data input of the flip flop. In this way, any glitches on the data input to the flip flop from the feedback circuitry will not be passed to the output of the flip flop as the flip flop is clocked by the input clock signal. This deals with the problem of high frequency glitches when the p-bits are changed on the fly without requiring additional circuitry dedicated to handling such glitches. 
         [0025]    In a second aspect, there is provided a modular frequency divider comprising a sequence of cells connected in series, the plurality of cells comprising: a first cell comprising circuitry according to the first aspect in which the feedback circuitry comprises a divide-by-two block for dividing the block output signal by two to thereby provide a final output signal; and at least one second cell comprising circuitry according to the first aspect in which the feedback circuitry is arranged to provide a block output signal, wherein the divided clock signal output from the flip flop of the circuitry of the first cell is arranged to provide the input clock signal for a first of the at least one second cell in the sequence, and wherein the block output signal from the first of the at least one second cell is arranged to provide one of the at least one control signals to the feedback circuitry of the first cell. 
         [0026]    In a third aspect, there is provided a method of dividing the frequency of an input clock signal for use in a prescaler of a digital frequency synthesizer, the method comprising: clocking a flip flop on a first type of edge of the input clock signal, to provide an output for use as a divided clock signal; using feedback circuitry to provide a signal to a data input of the flip flop based on the output of the flip flop; determining a mode of the feedback circuitry wherein in a first mode the signal provided to the data input of the flip flop is allowed to follow the inverse of the output of the flip flop so that each first type of edge causes the signal provided to the data input of the flip flop to toggle such that the output of the flip flop has a frequency which is substantially half of the frequency of the input clock signal, and wherein in a second mode the signal provided to the data input of the flip flop is allowed to follow the inverse of the output of the flip flop with the exception that toggling of the signal provided to the data input of the flip flop is selectively prevented despite toggling of the output of the flip flop on an edge of the first type such that the output of the flip flop has a frequency which is at most a third of the frequency of the input clock signal, wherein the feedback circuitry operates independently from a second type of edge of the input clock signal in providing the signal to the data input of the flip flop, the second type of edge being the opposite of the first type of edge of the input clock signal, such that the output of the flip flop is independent from the duty cycle of the input clock signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    For a better understanding of the present invention and to show how the same may be put into effect, reference will now be made, by way of example, to the following drawings in which: 
           [0028]      FIG. 1   a  shows a dual modulus 8/9 prescaler based on a shift register ring; 
           [0029]      FIG. 1   b  is a first timing diagram of signals in the prescaler of  FIG. 1   a;    
           [0030]      FIG. 1   c  is a second timing diagram of signals in the prescaler of  FIG. 1   a;    
           [0031]      FIG. 2   a  shows another dual modulus 8/9 prescaler with preprocessing of the clock signal; 
           [0032]      FIG. 2   b  is a first timing diagram of signals in the prescaler of  FIG. 2   a;    
           [0033]      FIG. 2   c  is a second timing diagram of signals in the prescaler of  FIG. 2   a;    
           [0034]      FIG. 3   a  shows a bit cell 2/3 architecture according to an embodiment; 
           [0035]      FIG. 3   b  is a timing diagram of signals in the bit cell 2/3 architecture of  FIG. 3   a  in one mode; 
           [0036]      FIG. 4   a  shows a 50 D bit cell architecture according to another embodiment; 
           [0037]      FIG. 4   b  is a timing diagram of signals in the 50 D bit cell architecture of  FIG. 4   a  in a first mode; 
           [0038]      FIG. 4   c  is a timing diagram of signals in the 50 D bit cell architecture of  FIG. 4   a  in a second mode; 
           [0039]      FIG. 4   d  is a timing diagram of signals in the 50 D bit cell architecture of  FIG. 4   a  in to a third mode; 
           [0040]      FIG. 4   e  is a timing diagram of signals in the 50 D bit cell architecture of  FIG. 4   a  in a fourth mode; and 
           [0041]      FIG. 5  shows a high speed 5-bit dual modulus prescaler architecture according to preferred embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    Embodiments will now be described by way of example only. 
         [0043]    One embodiment of circuitry for providing a 2/3 divider cell (which divides the frequency of an input clock signal by either two or three) is described herein with reference to  FIG. 3 . The circuitry shown in  FIG. 3   a  comprises a first flip flop  302 , a second flip flop  304 , a detector block  306  and block logic  308 . 
         [0044]    The inverse of the input clock signal  CLK  is coupled to the clock inputs of the first and second flip flops  302  and  304 . The output of the block logic  308  is coupled to the data input of the first flip flop  302 . The output of the first flip flop  302  is provides an output signal, denoted “Divided clk output” in  FIG. 3   a.  The output of the first flip flop  302  is also coupled to a first input of the logic block  308 . The output of the first flip flop  302  is also connected to a first input of the detector block  306 . A block input signal is coupled to a second input of the detector block  306 . The output of the detector block  306  is coupled to the data input of the second flip flop  304 . The output of the second flip flop  304  provides a block output signal. The output of the second flip flop  304  is also coupled to a second input of the logic block  308 . A programmable control bit denoted “P-bit” in  FIG. 3   a  is coupled to a third input of the block logic  308 . A signal providing an indication of other “higher” programmable bits may be input to the detector block  306 . This signal (denoted “higher P-bit” in  FIG. 3   a ) is used when the cell shown in  FIG. 3   a  is part of a series of cells as described in more detail below in relation to  FIG. 5 . 
         [0045]    The circuitry shown in  FIG. 3   a  other than the first flip flop  302  can be thought of as feedback circuitry, as shown in dotted box  303 . In operation the feedback circuitry acts such that the block logic  308  provides either an inverted version of the output of the first flip flop  302  or the block output signal to the data input of the first flip flop  302  in dependence upon the value of the P-bit and the block input signal. The signal output from the first flip flop  302  (Divided clk output) is a signal which has a frequency which is either two or three times less to than the frequency of the input clock signal CLK depending upon the value of the P-bit. 
         [0046]    The block logic  308  is arranged to operate such that if the P-bit is low, or if the block output signal is high, then the signal provided to the data input of the first flip flop  302  follows an inverted version of the output of the first flip flop  302 . However, the block logic  308  is arranged to operate such that if the P-bit is high and the block output signal is low then the signal provided to the data input of the first flip flop  302  is low. The detector block  306  is arranged to operate such that if the block input signal is low then the output of the detector block  306  is high, whereas if the block input signal is high then the output of the detector block  306  is the inverse of the signal output from the first flip flop  302 . When the circuitry shown in  FIG. 3   a  is operated in isolation the block input signal is arranged to be high (e.g. the block input signal may be connected to the supply voltage DVDD). However, when the circuitry shown in  FIG. 3   a  is implemented in a cell which is a part of a series of cells as shown in  FIG. 5  and described in more detail below, the block input signal is brought high or low in accordance with a block output signal from another cell in the series. In this way, as described in more detail below, a cell including the circuitry shown in  FIG. 3   a  can take account of the output of other cells to thereby divide the input clock signal by more than three. 
         [0047]    When the P-bit is low then the bit cell shown in  FIG. 3   a  provides an output on the Divided clk output which has a frequency which is half of that of the input clock signal CLK. This is because when the P-bit is low the block logic  308  acts to provide to the data input of the first flip flop  302  an inverted version of the output from the first flip flop  302 . The precise mechanism for providing the inverted version of the output of the first flip flop  302  to the data input of the first flip flop  302  when the P-bit is low may be implemented in different ways by the block logic  308  as would be apparent to a person skilled in the art. In this way, when the output from the first flip flop  302  is low, the input to the data input of the first flip flop  302  is brought high by the block logic  308 . This results in the output of the first flip flop  302  being brought high (such that the Divided clk output signal is brought high) on the next falling edge of the input clock signal CLK. Therefore, the input to the data input of the first flip flop  302  is brought low by the block logic  308 . This results in the output of the first flip flop  302  being brought low (such that the Divided clk output signal is brought low) on the next falling edge of the input clock signal CLK. Therefore, as described above, when the P-bit is low the bit cell shown in  FIG. 3   a  provides the Divided clk output which has a frequency which is half of that of the input clock signal CLK. 
         [0048]    However, when the P-bit is high then the Divided clk output signal provided by the bit cell shown in  FIG. 3   a  has a frequency which is a third of that of the input clock signal CLK, as explained below with reference to  FIG. 3   b  which shows a timing diagram of the Divided clk output signal, the block output signal and the output of the block logic  308 . The Divided clk output goes high on a falling edge of the input clock signal. Initially, the block output signal is high so the block logic  308  acts to provide the inverse of the Divided clk output signal to the data input of the first flip flop  302 . Therefore on the second falling edge of the input clock signal CLK the output of the first flip flop  302  is brought low. When the output of the first flip flop  302  is high after the first falling edge of the input clock signal CLK, since the block input signal is high then the output of the detector block  306  goes low. Therefore on the second falling edge of the input clock signal the block output signal from the second flip flop  304  goes low. As described above, when the block output signal is low and the P-bit is high then the block logic  308  acts to provide a low signal to the data input of the first flip flop  302 . Therefore on the third falling edge of the input clock signal CLK the output of the first flip flop  302  remains low. However, since after the second falling edge of the input clock signal CLK the output of the first flip flop  302  is low, the output of the detector block  306  is brought high. Therefore, on the third falling edge of the input clock signal CLK the block output signal from the second flip flop  304  is brought high. Therefore after the third falling edge of the input clock signal CLK the circuitry shown in  FIG. 3   a  is in the same state as before the first falling edge of the input clock signal CLK shown in  FIG. 3   b,  and the operation repeats such that the Divided clk output has a frequency which is a third of the frequency of the input clock signal CLK. 
         [0049]    It can therefore be seen that the circuitry shown in  FIG. 3   a  acts as a divide-by-two block when the P-bit is low. However, when the P-bit is high then the circuitry shown in  FIG. 3   a  acts as a divide-by-three block. Therefore the circuitry shown in  FIG. 3   a  acts as a 2/3 divider cell, which can be used in a prescaler. 
         [0050]    It is clear from  FIG. 3   b  that all of the transitions occur on falling edges of the input clock signal CLK. There is no half cycle operation of the circuitry shown in  FIG. 3   a.  Therefore the output of the circuitry shown in  FIG. 3   a  does not depend upon the duty cycle of the input clock signal CLK. This allows the circuitry to operate at higher maximum speeds. This can be particularly advantageous when the circuitry is used in a prescaler of a digital frequency synthesizer in digital mobile communication devices. The critical path is one flip flop delay, one combo logic delay and the delay for the set up of the next flip flop. 
         [0051]    Although  FIGS. 3   a  and  3   b  show the operation of the circuitry being dependent on the falling edges of the input clock signal CLK, it would be apparent to a skilled person that the circuitry could be modified to be dependent upon the rising edges of the input clock signal rather than the falling edges of the input clock signal. Provided that only one type of edge (i.e. either rising edges or falling edges, but not both) is used by the circuitry shown in  FIG. 3   a  then the output of the circuitry will not be dependent upon the duty cycle of the input clock signal. 
         [0052]    In the circuitry shown in  FIG. 3   a,  there is no blocking of the input clock signal. The input clock signal is received directly at the flip flops  302  and  304  without being blocked. Since the input clock signal is directly received at the flip flops, dedicated start up circuitry is not required. Advantageously, the circuitry will self resolve itself during startup, thereby avoiding stuck states which may occur in the prior art prescalers. Since dedicated start up circuitry is not required, the amount and the complexity of the hardware of the circuitry may be reduced as compared to prescalers of the prior art. 
         [0053]    Furthermore, if the programmable P-bit is changed on the fly (i.e. during operation of the circuitry shown in  FIG. 3   a ) then glitches are not produced on the Divided clk output or on the block output signal. Changing the P-bit on the fly may cause intermediate frequencies to be received at the data input of the flip flop  302 . However, since the first flip flop  302  is clocked using the input clock signal, the output of the first flip flop  302  does not contain the higher frequencies than the range of the prescaler. The delay between changing the P-bit and the logic block  308  reaching a steady state is the delay in the logic block  308  plus the delay of the second flip flop  304  plus the setup time of the first flip flop  302 , which is advantageously less than the period of the input clock signal CLK. Therefore, the glitches are not passed from the data input of the first flip flop  302  to the output of the first flip flop  302 . Therefore, no additional circuitry is required to allow the P-bit to be changed on the fly. 
         [0054]    It will be appreciated that the circuitry shown in  FIG. 3(   a ) when placed in series as explained below with reference to  FIG. 5  outputs a clock signal (the block output signal) at the output of the second Flip Flop  304  which may not have 50% duty cycle. For example, in the case of divide by 5, we will have one time period of the input clock signal CLK of HIGH time and four time periods of the input clock signal CLK of LOW time (or vice versa), which means a duty cycle of 20% (or 80%). So, as described later with reference to  FIG. 5  the first bit cell (the left most) includes circuitry as described below in relation to  FIG. 4   a  for producing a final output signal with a duty cycle closer to 50%. 
         [0055]      FIG. 4   a  shows modified circuitry to that shown in  FIG. 3   a  which outputs a clock signal having substantially a 50% duty cycle. It can be advantageous to output a clock signal having a 50% duty cycle (a 50 D signal) because some other components (to which the output clock signal may be provided) may be sensitive to variations in the duty cycle of the clock signal. 
         [0056]      FIG. 4   a  shows a first flip flop  402 , a second flip flop  404 , a detector block  406  and block logic  408 , which correspond to, and are configured correspondingly to, the first flip flop  302 , second flip flop  304 , detector block  306  and block logic  308  shown in  FIG. 3   a.  Therefore, the first flip flop  402  is clocked using the inverted input clock signal  CLK . The output of the block logic  408  is coupled to the data input of the first flip flop  402 . The first flip flop  402  provides a Divided clk output signal. The Divided clk output signal is coupled to the block logic  408 . The Divided clk output signal is also coupled to the detector block  406 . The block input signal is coupled to the detector block  406 . The output of the detector block  406  is coupled to the data input of the second flip flop  404 . The second flip flop  404  is clocked using the inverted input clock signal  CLK . A signal providing an indication of other “higher” programmable bits may be input to the detector block  406 . This signal (denoted “higher P-bit” in  FIG. 4   a ) is used when the cell shown in  FIG. 4   a  is part of a series of cells as described in more detail below in relation to  FIG. 5 . 
         [0057]    The circuitry of  FIG. 4   a  also includes a third flip flop  410  and a divide-by-two block  412 . 
         [0058]    The output (B 1 ) of the second flip flop  404  is coupled to the data input of the third flip flop  410 . The output of the second flip flop  404  is also coupled to the block logic  408  and to the divide-by-two block  412 . The inverse of the input clock signal  CLK  is coupled to the clock input of the third flip flop  410 . The output (B 2 ) of the third flip flop  410  is coupled to the block logic  408 . 
         [0059]    The output of the divide-by-two block  412  provides a final output signal. The output of the divide-by-two block  412  is also coupled to the block logic  408 . The block logic  408  is arranged to receive two programmable control bits (P 0  and P 1 ). 
         [0060]    The circuitry shown in  FIG. 4   a  other than the first flip flop  402  can be thought of as feedback circuitry, as shown in dotted box  403 . In operation the feedback circuitry acts such that the block logic  408  provides either an inverted version of the output of the first flip flop to  402  or a block output signal (B 1  or B 2 ) to the data input of the first flip flop  402  in dependence upon the value of the P-bits and the block input signal. 
         [0061]    The circuit shown in  FIG. 4   a  will alter the frequency of the Divided clk output signal from the first flip flop  402  (and thereby also alter the frequency of the signal on the final output from the divide-by-two block  412 ) in dependence upon the values of P 0  and P 1  as described below. 
         [0062]    If p 0 =p 1 =0 (i.e. p 0  and p 1  are “low”) then the final output signal has a clock frequency which is a quarter of the frequency of the input clock signal CLK, and has a 50% duty cycle.  FIG. 4   b  shows a timing diagram of the Divided clk output signal, B 1 , B 2  and the final output signal. The block logic  408  is arranged such that when P 0 =P 1 =0 then the output of the block logic  408  to the data input of the first flip flop  402  follows the inverse of the output of the first flip flop  402 . Therefore when P 0 =P 1 =0, the circuit shown in  FIG. 4   a  operates in the same way as  FIG. 3   a  operates when the P-bit is low. In other words, the Divided clk output signal has a frequency which is half of the frequency of the input clock signal CLK. This means that as shown in  FIG. 4   b  the signal B 1  also has a frequency which is half of the frequency of the input clock signal CLK. The divide-by-two block  412  operates to divide the frequency of the B 1  signal by two. Therefore, as shown in  FIG. 4   b  the final output signal from the divide-by-two block  412  has a frequency which is a quarter of the frequency of the input clock signal. 
         [0063]    The block logic  408  is arranged such that if p 1 =0 and p 0 =1 then the input clock signal is blocked once per cycle, such that the signal provided as the final output from the divide-by-two block  412  has a clock frequency which is a fifth of the frequency of the input clock signal CLK, and has a duty cycle which is near to 50%.  FIG. 4   c  shows a timing diagram. of the Divided clk output signal, B 1 , B 2  and the final output signal when P 1 =0 and P 0 =1. The block logic  408  is arranged such that in the mode when P 0  is 1 (and P 1  is 0) the output of the block logic  408  is held low when B 1  and the final output signals are low, but otherwise the output of the block logic  408  follows the inverse of the output of the first flip flop  402 . This can be seen in  FIG. 4   c  in that the output of the first flip flop  402  (Divided clk output) changes on every falling edge of the input clock signal except when the B 1  and final output signals are both low. This results in the Divided clk output signal having an average frequency which is 2.5 times less than the frequency of the input clock signal. Actually, as shown in  FIG. 4   c  the frequency of the divided clk signal alternates between being a half of the frequency of the input clock signal and a third of the frequency of the input clock signal, such that five periods of the input clock signal has a duration equal to two periods of the Divided clk output signal. The final output signal therefore has a frequency which is five times less than the frequency of the input clock signal CLK. 
         [0064]    The block logic  408  is arranged such that if p 1 =1 and p 0 =0 then the input clock signal is blocked twice per cycle, such that the signal provided as the final output from the divide-by-two block  412  has a clock frequency which is a sixth of the frequency of the input clock signal CLK, and has a 50% duty cycle.  FIG. 4   d  shows a timing diagram of the Divided clk output signal, B 1 , B 2  and the final output signal when P 1 =1 and P 0 =0. The block logic  408  is arranged such that in the mode when P 1  is 1 (and P 0  is 0) the output of the block logic  408  is held low when B 1  is low, but otherwise the output of the block logic  408  follows the inverse of the output of the first flip flop  402 . This can be seen in  FIG. 4   d  in that the output of the first flip flop  402  (Divided clk output) changes on every falling edge of the input clock signal except when the B 1  signal is low. This results in the Divided clk output signal having a frequency which is three times less than the frequency of the input clock signal. The final output signal therefore has a frequency which is six times less than the frequency of the input clock signal CLK. 
         [0065]    The block logic  408  is arranged such that if p 1 =1 and p 0 =1 then the input clock signal is blocked three times per cycle, such that the signal provided as the final output from the divide-by-two block  412  has a clock frequency which is a seventh of the frequency of the input clock signal CLK, and has a duty cycle which is near to 50%.  FIG. 4   e  shows a timing diagram of the Divided clk output signal, B 1 , B 2  and the final output signal when P 1 =1 and P 0 =1. The block logic  408  is arranged such that in the mode when P 0  is 1 and P 1  is 1 the output of the block logic  408  is held low when B 1  is low or when B 2  and the final output signals are both low, but otherwise the output of the block logic  408  follows the inverse of the output of the first flip flop  402 . This can be seen in  FIG. 4   e  in that the output of the first flip flop  402  (Divided clk output) changes on every falling edge of the input clock signal except when the B 1  signal is low or when both the B 2  and final output signals are low. This results in the Divided clk output signal having an average frequency which is 3.5 times less than the frequency of the input clock signal. Actually, as shown in  FIG. 4   e  the frequency of the divided clk signal alternates between being a third of the frequency of the input clock signal and a quarter of the frequency of the input clock signal, such that seven periods of the input clock signal has a duration equal to two periods of the Divided clk output signal. The final output signal therefore has a frequency which is seven times less than the frequency of the input clock signal CLK. 
         [0066]    As described above, a “cycle” has a duration which is equal to the time period of the final output from the divide-by-two block  412 . The precise implementation of the block logic  408  is not described in detail here, but a skilled person would be aware of a number of different arrangements which would operate as described above such that the output of the block logic  408  changes in accordance with the changes to the inputs to the block logic  408  as described above. 
         [0067]    The use of the divide-by-two block  412  to provide the final output signal from the module shown in  FIG. 4   a  means that the final output has substantially a 50% duty cycle. When dividing the input clock signal by an even number, n (e.g. where n is 4 or 6) then the duty cycle of the final output signal has a 50% duty cycle, i.e. the high time is the same as the low time of the output signal over a period of the final output signal. This can be seen in  FIGS. 4   b  and  4   d.  However, when the input clock signal is divided by an odd number then the final output signal does not strictly have a 50% duty cycle. For example, dividing the frequency of the input clock signal by five results in the final output signal having 2 cycles of HIGH TIME and 3 cycles of LOW TIME or vice-versa (as shown in  FIG. 4   c ). In fact, when dividing the frequency of the input clock signal by an odd number n, the duty cycle, D, of the final output signal from the divide-by-two block  412  is given by 
         [0000]    
       
         
           
             D 
             = 
             
               
                 
                   n 
                   ± 
                   1 
                 
                 
                   2 
                    
                   n 
                 
               
               . 
             
           
         
       
     
         [0068]    The critical path of the circuitry shown in  FIG. 4   a  is one flip flop delay, two combo delays and setup of the next flip flop. 
         [0069]      FIG. 5  shows an example of how multiple divider cells can be combined to provide a high speed 5-bit dual modulus prescaler.  FIG. 5  shows three divider cells  502  to  506 , a clock input pin  508  for receiving the input clock signal and an output pin  510  for providing the output of the 5-bit dual modulus prescaler. In this way the modular frequency divider shown in  FIG. 5  comprises a sequence of divider cells connected in series. 
         [0070]    The first divider cell  502  is a SOD bit cell such as that shown in  FIG. 4   a  and described above. The second and third divider cells  504  and  506  are 2/3 bit cells such as that shown in  FIG. 3   a  and described above. 
         [0071]    The input clock signal is received at the clock input of the first divider cell  502  from the input pin  508 . The control bits p 0  and p 1  are received at the first divider cell  502 . The values of p 0  and p 1  determine whether the first divider cell divides the input clock signal by 4, 5, 6 or 7. In this sense the control bits p 0  and p 1  act as a binary representation of x where the input clock frequency is divided by (4+x) by the first divider cell  502 . 
         [0072]    The second divider cell  504  receives the Divided clk output signal from the first divider cell  502  which is used as the input clock signal in the second divider cell  504 . The second divider cell  504  also receives a programmable bit P 2  for use as the P-bit described above and shown in  FIG. 3   a.  The block output signal shown in  FIG. 3   a  is output from the second divider cell  504  and used as the block input signal for the first divider cell  502 . 
         [0073]    The third divider cell  506  receives the Divided clk output signal from the second divider cell  504  which is used as the input clock signal in the third divider cell  506 . The third divider cell  506  also receives a programmable bit P 3  for use as the P-bit described above and shown in  FIG. 3   a.  The block output signal shown in  FIG. 3   a  is output from the third divider cell  506  and used as the block input signal for the second divider cell  504 . A supply voltage is fed into the third divider cell  506  and is used as the block input signal in cell  506  as shown in  FIG. 3   a.    
         [0074]    The modular frequency divider shown in  FIG. 5  operates as follows. The Block  2  signal is generated by the third cell  506 , and this signal propagates up the sequence of cells being re-clocked by each cell along the way. An active block input signal enables a cell to divide by more than the minimum dividing factor for that cell (the minimum dividing factor is two for cells  504  and  506 , and the minimum dividing factor for cell  502  is 4, as described above). 
         [0075]    It will be clear to a person skilled in the art that the architecture shown in  FIG. 5  could be extended or reduced by adding or removing divider cells as appropriate to provide a prescaler which can divide the input clock signal by different amounts. 
         [0076]    The first cell  502  divides the frequency by 4, 5, 6 or 7 depending on the values of P 0  and P 1 . The second and third cells  504  and  506  divide the frequency of their input clock signals by 2 or 3 depending upon the values of P 2  and P 3 . 
         [0077]    A higher P-bit_ 1  signal is received at the first cell  502  and is used to provide an indication of the value of the P-bits which are supplied to the subsequent cells in the sequence. Therefore the higher P-bit_ 1  signal provides an indication of the values of P 2 , P 3  and P 4 . In one embodiment the higher P-bit_ 1  signal is the output of an OR operation applied to P 2 , P 3  and P 4 . In detector block  406  of cell  502 , if the higher p-bit_ 1  signal is low indicating that none of p 2 , p 3  or p 4  are HIGH, then the block input signal (block 1 ) to cell  502  will always be HIGH which means the output of the detector block  406  in cell  502  is the inverse of the divided clk signal output from the first flip flop  402  in cell  502 , such that the final output signal will have a frequency which is either 4, 5, 6 or 7 times less than the frequency of the to input clock signal depending on solely p 0  and p 1 . However, if any of p 2 , p 3  and p 4  are HIGH (such that the higher P-bit_ 1  signal is high), then the block input signal may be LOW depending on the values of p 2 , p 3  and p 4 , such that the output of the detector block  406  in cell  502  will be dependent upon the divided clk signal output from the first flip flop  402  in cell  502  and upon the block input signal received from cell  504 . This enables the final output signal to have a frequency which equal to the frequency of the input clock signal divided by any number between  4  and  31  depending on the values of the p-bits (p 0  to P 4 ). 
         [0078]    Similarly, a higher P-bit_ 2  signal is received at the second cell  504  and is used to provide an indication of the value of the P-bits which are supplied to the subsequent cells in the sequence. Therefore the higher P-bit_ 2  signal provides an indication of the values of P 3  and P 4 . In one embodiment the higher P-bit_ 2  signal is the output of an OR operation applied to P 3  and P 4 . In detector block  306  of cell  504 , if the higher p-bit_ 2  signal is DEACTIVATED that is neither p 3  nor p 4  are HIGH, then the block input signal (block 2 ) to cell  504  will always be HIGH which means the output of the detector block  306  (block 1 ) in cell  504  is the inverse of the divided clk signal output from the first flip flop  302  in cell  504 , and is either 2 or 3 times less than the frequency of the input clock signal received at the cell  504  from the cell  502  depending on solely on p 2 . However, if either p 3  or p 4  is HIGH (such that the higher P-bit_ 2  signal is high), then the block input signal may be LOW depending on the values of p 3  and p 4 , such that the output of the detector block  306  in cell  504  will be dependent upon the divided clk signal output from the first flip flop  302  in cell  504  and upon the block input signal received from cell  506 . 
         [0079]    Similarly, the P 4  bit is received at the third cell  506  and is used as the higher P-bit signal in cell  506 . 
         [0080]    The result is that the values of P 0 , P 1 , P 2 , P 3  and P 4  control the frequency of the signal output to pin  510 . The minimum factor that the input clock signal can be divided by is four and that is when P 0 =P 1 =P 2 =P 3 =P 4 =0, such that the cell  502  does not take account of the blocking performed by cells  504  or  506  (because higher P-bit_ 1  is low) and the cell  502  divides the frequency of the input clock signal by four. With P 2 , P 3  and P 4  low, no blocking is performed by the cells  504  and  506 . The frequency of the final output signal to pin  510  is also a quarter of the frequency of the input clock signal when the values of the P-bits P 0  to P 4  are (00100 respectively). 
         [0081]    The maximum factor that the input clock signal can be divided by is thirty one and that is when P 0 =P 1 =P 2 =P 3 =P 4 =1. This is achieved because here each cell ( 502 ,  504  and  506 ) is in the blocking mode. In this case there are four divide-by-two blocks enabled. The first divide-by-two block blocks the input clock cycle once, which becomes twice in second divide-by-two block, four times in third divide-by-two block and eight times in last divide-by-two block. So the division of the frequency of the input clock signal is given by 2*2*2*2+1+2+4+8=31. In other words, all the blocking operations lead to addition of cycles which when passed through the individual bit cells enable division of the frequency of the clocks signal. 
         [0082]    In summary, there is described above a high speed prescaler with no block in the clock path. No dedicated start up circuitry is required as the architecture self resolves itself. Changing the programmable bits (the p-bits) on the fly will lead to only intermediate frequencies in transit, with no high frequency output glitches being seen. The architecture may be advantageously employed in PLL&#39;s due to (i) increased speed (due to lack of dependency on duty cycle of the input clock signal), (ii) no requirement for dedicated startup circuitry, and (iii) no glitches occurring on the output when the p-bits are changed on the fly. 
         [0083]    The block logic  308  and  408  described above may be implemented in many ways. In preferred embodiments, the logic blocks  308  and  408  comprise a plurality of transistors (e.g. mosfet transistors) arranged such that the logic blocks  308  and  408  act as described above. It would be apparent to a person skilled in the art that a number of different arrangements may produce logic blocks that operate as described above. Similarly, the detector block  306  and  406  described above may be implemented in many ways. In preferred embodiments, the detector blocks  306  and  406  comprise logic gates arranged such that the detector blocks  306  and  406  act as described above. It would be apparent to a person skilled in the art that a number of different arrangements may produce detector blocks that operate as described above. 
         [0084]    While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appendant claims.