Patent Publication Number: US-6667646-B2

Title: Small-sized digital generator producing clock signals

Description:
FIELD OF THE INVENTION 
     The invention relates to phase-locked loop type clock-signal generators that produce a high-frequency clock signal from a low-frequency clock signal. Among these generators, the invention relates more specifically to those using a digital oscillator producing clock signals whose period is proportional to a binary number received by the oscillator. 
     BACKGROUND OF THE INVENTION 
     A prior art generator  10  according to FIG. 1 has a comparator  12 , a binary number decoder  14  and a digital oscillator  20  that are series connected. An output of the oscillator  20  is connected to an input of the comparator  12 . The generator  10  provides a high-frequency signal CKHF (with a period PHF) from a low-frequency reference signal CKBF (with a period PBF). 
     The comparator  12  has two inputs to which the high-frequency signal CKHF and the reference signal CKBF are applied. The comparator  12  compares the period PHF of the clock signal CKHF with a desired period PHF 0 . The desired period PHF 0  is, for example, a multiple of the period PBF. At N serial outputs, the comparator  12  produces a number NR in the form of N=2 N  binary signals S( 1 ), . . . , S(N) representing the bits of NR. The number NR has the following characteristics: NR increases if PHF&lt;PHF 0 , NR decreases if PHF&gt;PHF 0 , otherwise NR is constant. 
     The binary signal decoder  14  has N inputs connected to the outputs of the comparator  12 . The decoder  14  provides decoded binary clock signals SD( 1 ), SD( 2 ), . . . , SD(2 N ) at 2 N  serial outputs. These decoded binary signals indicate the value of the number NR given by the counting circuit: S(NR+1)=1, and S(i)=0 for all values of i ranging from 1 to 2 N , and i≠NR+1. 
     The digital oscillator  20  receives the binary signals SD( 1 ), SD(N), . . . , SD (2 N ) and produces the clock signal CKHF at a serial output OUT. The oscillator  20  is shown in FIG.  2 . It comprises 2 N −1 elementary cells A( 1 ) to A(2 N −1) that are identical, and one cell B that is different from the cells A(j), with j being an integer ranging from 1 to 2 N −1. Each cell A(j), B comprises a data input e and a data output s. 
     The input e of the cell B is connected, first, to its output s by a switch INT( 1 ) controlled by the signal SD( 1 ) and, second, to the output of the cell A( 1 ). The input of each cell A( 1 ) to A(2 N −2) is connected, first, to the output s of the cell B by a switch INT( 2 ) to INT(2 N −1) controlled by the signal SD( 2 ) to SD(2 N −1) and, second, to the output of the next cell A( 2 ) to A(2 N −1). The input of the cell B is connected to the output of the cell A( 1 ), and the output of the cell A(2 N −1) is connected to the output of the cell B by a switch INT(2 N ) controlled by the signal SD(2 N ). The switch INT(i), with i ranging from 1 to 2 N , is closed when the signal SD(i) is active. If not it is open. 
     The output of the cell B is the output OUT of the oscillator  20  at which the high-frequency CKHF is given. The cell B comprises an odd number NB of series connected primary inverters, with NB≧1. The output of the first inverter is connected to the output of the cell B, and the input of the NB th  inverter is connected to the input e of the cell B. The cell B is equivalent to an inverter whose propagation times (downward propagation time TB 0  and upward propagation time TB 1 ≠TB 0 ) are greater than the propagation times of the primary inverters that form it. 
     Since all the cells A( 1 ) to A(2 N −1) are identical, only the first cell A(i=1) is described in detail in FIG.  2 . The first cell comprises an even number NA of identical primary inverters (in the example of FIG. 2, NA=2) series connected with a switch INTA(i=1) controlled by the signal SD(i=1) by an inverter. The input of the last inverter is connected to the input of the cell A( 1 ), and the output of the switch INTA( 1 ) is connected to the output of the cell A( 1 ). The switch INTA( 1 ) is open when SD( 1 ) is active. If not it is closed. 
     The switch INTA( 1 ) is necessary for isolating the primary inverters from one another. Without the presence of the switch INTA( 1 ), the output of an inverter of the cell A( 1 ) and the output of an inverter of the cell B would be short-circuited at the closing of the switch INT( 1 ). 
     A cell A( 1 ) to A(2 N −1) comprises a number NA of inverters. It is equivalent to a delay circuit. The cell transmits the signal that it receives at its input to its output, after a period of time TA 0  if the signal is equal to 0, or after a period of time TA 1  if the signal is equal to 1. The propagation times TA 0 , TA 1  are different from each other because the propagation time of a 0 in a switch INTA is different from the propagation time of a 1 in the same switch. 
     The clock signal generator  10  works as follows. The comparator  12  provides a number NR ranging from 0 to 2 N −1 and produces an associated signal SD(NR+1)=1, with the other signals SD(i≠NR+1) being all zero signals. The switch INT(NR+1) controlled by the signal SD(NR+1) closes and the switch INTA(NR+1) opens. NR cells A are selected to form, with the cell B, a looped chain of cells A, B in the oscillator  20 . 
     The propagation time of a 0 between the input and the output of the chain of cells is equal to T 10 =NR*TA 0 +TB 1 , and the propagation time of a 1 between the input and output of the chain of cells is equal to T 11 =NR*TA 1 +TB 0 . The period PHF of the high-frequency signal CKBF obtained at the output OUT of the oscillator is equal to PHF=T 10 +T 11 =NR*(TA 1 +TA 0 )+(TB 1 +TB 0 ). It is therefore proportional to NR. 
     If the period obtained PHF is smaller than the desired period PHF 0 , then the number NR is increased by the comparator  12  to increase the number of cells A in the chain of cells, and thus increase the period PHF of the signal CKHF. Conversely, if the signal obtained PHF is greater than the desired period PHFO, then the number NR is diminished by the comparator  12  to diminish the number of cells A in the loop, and thus reduce the period of CKHF. 
     In other words, in the oscillator  20 , the total number of cells, and therefore the total number of inverters in the chain of cells, varies as a function of the number NR given by the comparator  12 , and the period PHF of the clock signal obtained is proportional to NR. The total number of inverters in the chain, however, must be an odd number in order that there may be oscillations. 
     Furthermore, when NR increases (or decreases) by one, a cell A is added (or eliminated) in the chain. It may be recalled that a cell A comprises an even number NA of inverters and a switch INTA. 
     The minimum variation of the period of a signal CKHF produced by the oscillator  20  defines the uncertainty on the period of the signal CKHF, in other words, the precision of the oscillator. For the oscillator  20 , the uncertainty on the period is therefore equal to TA 0 +TA 1 , with TA 0  and TA 1  being the propagation time of a 0 and a 1 in a cell A comprising an even number of inverters and a switch INTA. 
     By way of an indication, exemplary embodiments of switches INT( 1 ), . . . , INT(2 N ) and primary inverters used in the cells A( 1 ), . . . , A(2 N −1), B are shown in FIGS. 3 a  and  3   b . When a switch has to be series connected with an inverter, as is the case in the cells A( 1 ), . . . , A(2 N −1), then these two elements are preferably made in the form of a single circuit like that of FIG. 3 c , which shows an inverter switch. 
     A first problem of signal generators such as that of FIG. 1 is the large size of these circuits. The decoders include a set of at least 2 N  logic gates, with at least one per binary signal SD(i) being produced. The decoders thus become bulky very soon when the number n rises. 
     A second problem of existing generators is related to the construction of inverter-based oscillators. All the cells A( 1 ), . . . , A(2 n −1), B are physically linked to the output point S of the oscillator  20 , directly or by a switch. Thus, the parasitic capacitance at this point is very high, due to the cells themselves, the switches and the connecting wires between the elements of the circuit. This capacitance leads to parasitic oscillations, for example, when two switches placed side by side switch simultaneously. These oscillations can result in errors in the oscillator  20 , for example, when there is an unwanted conversion of a 0 to a 1. 
     A third problem lies in the fact that the cycle ratio of the signal CKHF obtained, equal to the time during which the signal is in the high state divided by the period of the signal, is different from ½. 
     Due to the presence of a switch INT(i) and an even number NA of inverters in the cell A(i), the propagation time T 10  of a 0 and the propagation time T 11  of a 1 in the chain of cells are substantially different from each other. Thus, the cycle ratio of the signal CKHF obtained, which is equal to T 11 /PHF=T 11 /(T 10 +T 11 ), is different from ½. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, an object of the invention is to provide a small-sized clock signal generator while preferably eliminating the decoder, which is generally used in this type of circuit. 
     Another object of the invention is to provide a clock signal generator having a parasitic capacitance distributed throughout the circuit, which is no longer concentrated at the output of the oscillator. 
     Yet another object of the invention is to provide a generator that produces substantially perfect clock signals, and preferably having a cycle ratio equal to ½. 
     These and other objects, advantages and features according to the invention are provided by a generator comprising an oscillator producing a clock signal from N logic signals representing an N-bit control number, with N being an integer greater than 1. 
     According to the invention, the oscillator comprises N+1 components. Each of the N most significant components are assigned a place value i, with i ranging from 1 to N. The least significant component gives the clock signal, and at least one component (C(i)) with a place value i greater than 1 comprises first and second arms. The first arm comprises, connected in series, a cell and a first switch controlled by the logic signal with a place value i. The first switch is open when the logic signal with the place value i is active. The second arm comprises, connected in series, cells and a second switch controlled by the logic signal with a place value i. The second switch is closed when the logic signal with the place value i is active. 
     Thus, the oscillator of the generator of the invention makes direct use of the N binary signals produced by the comparator as will be seen more clearly below. The decoder of the prior art circuit is therefore eliminated. 
     Furthermore, every common point of the oscillator is connected to a small number of components. For example, the input of the component with a place value I is connected only to the two arms of the component with the place value I, and to the two arms of the component with the place value I+1. Consequently, the parasitic capacitance of the oscillator is thus distributed throughout the elements of this circuit, and there is no longer any excessively capacitive point, which is a source of error. 
     Preferably, the second arm of the component with the place value i comprises a number of cells equal to 2 i +1. This number is an odd number. Consequently, two adjacent cells of the second type constantly receive opposite logic signals and the cycle ratio of the high-frequency signal, obtained with a generator of the invention, is equal to ½, as shall be seen more clearly below. 
     Preferably, the least significant component comprises an even number of inverters if N is an odd number, or an odd number of inverters if N is an even number. Preferably again, each cell comprises an odd number of series connected inverters. Consequently, regardless of the value of the number NR, the total number of inverters in the chain of cells is always an odd number. The oscillations are therefore always possible. Each cell may also comprise a switch series connected with the inverters. The output of each cell can thus be isolated from the output of any other cell if necessary. 
     According to a preferred embodiment, the oscillator also comprises means to apply a precharging signal to the cells of the first and second arms of the component with the place value i. For the component with the place value i, the precharging signal is either the clock signal or the signal applied to an input of the component with a place value i. 
     A generator according to the invention comprises, in addition to an oscillator such as the one described above, a comparator to compare the period of the clock signal with a desired period and provide an N-bit control number in the form of N logic signals. The control number varies as follows. The control number increases if the period of the clock signal is smaller than the desired period. The control number decreases if the period of the clock signal is greater than the desired period, and if not, the control number is constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more clearly and other features and advantages shall appear from the following description, made with reference to the appended drawings, of which: 
     FIG. 1 is a functional block diagram of a clock signal generator according to the prior art; 
     FIG. 2 is a detailed block diagram of the oscillator illustrated in FIG. 1; 
     FIGS. 3 a  to  3   c  are detailed schematic diagrams of different embodiments of a cell within the oscillator illustrated in FIG. 2; 
     FIG. 4 is a detailed schematic diagram of an oscillator according to the present invention; and 
     FIG. 5 is a detailed schematic diagram showing another embodiment of the oscillator illustrated in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An oscillator according to the invention is shown in FIG.  4 . The oscillator is used in a clock signal generator (not shown) comprising one comparator and the oscillator  40 . The comparator is identical to that of FIG. 1, and provides the oscillator  40  binary signals S( 1 ) to S(N) representing an N-bit number NR. 
     The oscillator  40  comprises N+1 components C( 0 ) to C(N). Each component comprises a data input e and a data output s. The components C( 1 ) to C(N) also have a control input to which the signal S( 1 ) to S(N) is applied. 
     The components C( 0 ) to C(N) are series connected. The input e of the components C( 0 ) to C(N−1) is connected respectively to the output s of the components C( 1 ) to C(N), and the input e of the component C(N) is connected to the output s of the component C( 0 ) which is the output OUT of the oscillator  40 . 
     Each component C(i), with a place value i ranging from 1 to N, has two parallel-connected arms between the input e and the output s of the component C(i). For each component C(i), the first arm comprises a cell F series connected with a switch INTC 1 (i) and is controlled by the signal S(i) provided via an inverter. The switch INTC 1  is open if the signal S(i) is active, for example, equal to 1. If not, it is closed. 
     For each component C(i), the second arm has a number NC(i)=1+2 i  of cells F series connected with a switch INTC 2 (i) controlled by the signal S(i). The switch INTC 2 (i) is closed if the signal S(i) if active. If not, it is open. All the cells F of all the components C( 1 ) to C(N) are identical. Each of them includes an odd number NF of primary inverters, series connected between the input and the output of the cell F. In the example of FIG. 4, NF is equal to 1. 
     The component C( 0 ) comprises a number ND of series connected inverters. It is known that there must necessarily be an odd total number of inverters in the chain in order that the oscillations may be possible. The number ND of inverters in the component C( 0 ) has the following characteristics: ND is an odd number if N is an even number, and ND is an even number if N is an odd number. The propagation time of a 0 in the component C( 0 ) is equal to TD 0 , and the propagation time of a 1 is equal to TD 1 . 
     The oscillator  40  of FIG. 4 works as follows. Depending on the number NR given by the comparator and on the corresponding signals S( 1 ), . . . , S(N), one of the switches INTC 1 (i), INTC 2 (i) of a same component C(i) is closed and the other is open, thus forming a variably long chain of cells F (and therefore of inverters) series connected with the component C( 0 ). 
     The total number of cells F in the chain thus varies between N and N−2+2 N+1 , according to the value of the number NR. Whenever the number NR is incremented or decremented by 1, the signals S( 1 ), . . . , S(N) produced by the comparator are modified accordingly. The state of the switches of the components C( 1 ), . . . , C(N) is modified accordingly, and the total number of cells F in the chain is increased or reduced by 2. 
     This can be seen clearly in the following example in which it is assumed that, initially, NR is equal to 1. The signals S( 2 ) to S(N) are equal to 0, and S( 1 ) is equal to 1. The switches INTC 1 (i) are closed and the switches INTC 2 (i) are open for all values of i greater than or equal to 2. However, the switch INTC 2 ( 1 ) is closed and the switch INTC 1 ( 1 ) is open. The chain of cells, in this case, includes the first arms of the components C( 2 ) to C(N), the second arm of the component C( 1 ) and the component C( 0 ). The chain of cells therefore comprises (N−1)*1+3 N+2 cells F. 
     If NR is increased by 1, then the signals S( 1 ), S( 3 ) to S(N) are equal to 0 and the signal S( 2 ) is equal to 1. The switches INTC 1 ( 1 ), INTC 1 ( 3 ) to INTC 1 (N) are open and the switches INTC 2 ( 1 ), INTC 2 ( 3 ) to INTC 2 (N) are closed. However, the switch INTC 1 ( 2 ) remains open and the switch INTC 2 ( 2 ) is closed. The chain of inverters, in this case, includes (N−2)*1+5+1=N+4 cells F. 
     Thus, by varying the number NR by 1, the number of cells F in the chain has clearly increased by 2. The period PHF of the clock signal obtained is increased by TF 0 +TF 1 , with TF 0  being the descending time and TF 1  being the rising time in a cell F. 
     The oscillator according to the invention therefore works similarly to the known oscillator of FIG.  2 . The number of cells in the looped chain, and therefore the period PHF of the high-frequency signal CKHF at the output of the oscillator  40 , varies linearly with NR. It may be recalled that NR is given by the comparator in the form of N binary signals S(l) to S(N). NR is increased or decreased as a function of the difference between the period PHF of the signal CKHF and a desired value PHF 0 . 
     The oscillator  40  according to the invention makes direct use of the binary signals produced by the comparator  12 . Consequently, a generator according to the invention does not include a decoding circuit. A generator according to the invention using an oscillator  40  such as the one of FIG. 4 therefore effectively has a reduced size as compared with a generator such as the one illustrated in FIG.  1 . 
     As can be seen in FIG. 4, at most two outputs of cells (and hence inverters) and two outputs of switches are connected at the same point of the circuit. The parasitic capacitance is thus distributed on all the elements of the oscillator  40  and there are no longer any excessively capacitive points. 
     The cycle ratio of the oscillator according to the invention is also improved as compared with that of the prior art oscillator  20 . The cells F have different rising times TF 1  and descending times TF 0  because the inverter or inverters (whose number NF is an odd number) that form them have different rising and falling times. On the contrary, for any number NR, the chain comprises N+2*NR cells F and the component C( 0 ) has two adjacent cells F which always receive opposite signals 0 or 1 at their input. 
     The propagation time of a 0 in the oscillator  40  is equal to T 20 =(TF 0 +TF 1 )*(N+2*NR)/2+TD 1 , and the propagation time of a 1 is equal to T 21 =(TF 0 +TF 1 ) *(N+2*NR)/2+TD 0 . Since the terms TD 0 , TD 1  are much smaller than the terms (TF 0 +TF 1 )*(N+2*NR)/2, (TF 0 +TF 1 ) *(N+2*NR)/2, the propagation times T 20 , T 21  are equal. Consequently, the period PHF of the signal CKHF is equal to 2*T 21  and its cycle ratio is equal to ½. 
     Improvements in the oscillators  40  may also be made. In particular, the components C( 1 ) to C(N) may be modified in accordance with FIG.  5 . The NC(i)=1+2 i  cells F of the second arm of the component C(i) are assembled in groups of N 0  cells F, with N 0  being an even number. The first group comprises N 1  cells F and the last group comprises N 2  cells F. 
     One of the groups must necessarily comprise an odd number of cells F since the second arm comprises an odd number of cells F. In the example of FIG. 5, N 1 =N 0  and N 2  is an odd number. For example, for i greater than 2, it is possible to choose N 1 =N 0 =4 and N 2 =5 or else N 1 =N 0 =2 and N 2 =3. 
     The groups are series connected with the switch INTC 2 (i) between the input e and the output s of the component C(i). The common points between two groups are also connected together to the input e of the component C(i) by a switch INTC 3 (i) which is controlled by the signal S(i) provided via an inverter. The switch INTC 3 (i) is open when the signal S(i) is active. 
     This improvement of the components C(i) is used into account the difference between the propagation times in the first arm of the component C(i), which includes a single cell F, and the propagation times in the second arm of the component C(i), which comprises NC=1+2 i  cells F. 
     The propagation time T 2 (i), of a 0 or a 1, in the second arm is far lengthier than the propagation time T 1  in the first arm. This is obviously due to the difference in the number of cells F in the two arms. This difference between the propagation times T 1 , T 2 (i) may lead to transmission errors, especially during the switching of the switches INTC 1 (i), INTC 2 (i) of a same component C(i), which switch over simultaneously. 
     The added switch INTC 3 (i) precharges the cells F of the second arm, which is inactive when the first arm is active (switch INTC 1 (i) is closed and switch INTC 2 (i) is open). In other words, the switch INTC 3 (i) enables the application, to the inputs of the cells F of the second arm, of the signal given by the component C(i+1). This signal would normally be applied if the second arm were active (switch INTC 2 (i) is closed and switch INTC 1 (i) is open). 
     Thus, the signal present at the input of the switch INTC 2 (i) has the same logic value as the one present at the input of the switch INTC 1 (i). If a switching of these two switches occurs, due to a variation in the number NR, then the switching takes place without risk of error. The correct signal then appears at the output s of the component C(i). 
     Variations of the component C(i) of FIG. 5 may also be planned. Since all the components C( 0 ) to C(N) are series connected, the order in which they are connected matters little. It is thus quite possible to connect the cell C( 0 ) between two adjacent cells C(i), C(i+1), or else to mix the order of the components C( 1 ) to C(N). In the same way, the signal CKHF may be obtained at the output of any of the components C( 0 ) to C(N) since they are series connected. 
     It is possible to choose N 2 =N 0  and N 1  as odd numbers. For example, for i greater than 2, it is possible to choose N 2 =N 0 =4 and N 1 =5 or else N 2 =N 0 =2 and N 1 =3. In this case, an inverter I is added and is series connected with the switch INTC 3 (i). Since N 1  is an odd number, it is necessary to invert the signal present at the input e of the component C(i) before applying it to the groups of N 0  cells F in order to obtain the right logic value at the input of the switch INTC 2 (i). 
     It is also possible to replace the least significant cell F of a group of cells by a cell F′ comprising the same number NF of inverters, and a switch INTF that are series connected. A cell F′ of this kind is described in detail by way of an example in FIG.  5 . The switch INTF is controlled by the signal S(i), and is closed when the signal S(i) is active. If not, it is open. 
     Thus, the output of an inverter of the cell F′ is never connected to the input e of the component C(i), namely to the output of an inverter of the component C(i+1). The switches INTC 3 , INTF are never simultaneously closed. 
     It is also possible to replace all the cells F by cells F′. In this case, the switches INTF of the most significant cells of the same group of cells F′ are kept permanently closed. Only the switch INTF of the least significant cell F′ of a same group is controlled by the signal S(i). 
     In this case, the circuit is simpler to make because it comprises a single type of cells F′. The general precision of the circuit is furthermore improved because the switching step, namely the variation in the period PHF when NR varies by 1 is identical regardless of the value of NR. 
     It is again possible to connect the switch INTC 3  (or the inverter I as the case may be) not to the input e of the component C(i), but directly to the output OUT of the oscillator  40 . In this case, an inverter I is used either for the even place value components or for the odd place value components to apply a signal of appropriate value to the input of the cells F. 
     The signal CKHF is then applied as a precharging signal to the cells F (or F′), instead of the signal given by the component C(i+1). Other variations may be contemplated. What is essential is to precharge the cells F (or F′) of the second arm of each component C(i) so that the signals that appear at the input of the switches INTC 1 , INTC 2  are identical (in particular, with the same phase).