Abstract:
A delay circuit having a delay element circuit composed of a plurality of series-connected first circuit elements being connected to a common power supply line and having a delay time varying correspondingly to a voltage of the common power supply line, the delay element circuit being adapted to receive an input signal and output an output signal obtained by delaying the input signal, and a PLL circuit including an oscillator circuit composed of a plurality of series-connected second circuit elements, which are equivalent to the first circuit elements, respectively, are connected to the common power supply line. The PLL circuit is adapted to oscillate the oscillator circuit at a predetermined frequency locked to a reference clock frequency by comparing a phase of the reference clock signal with a phase of an output frequency of the oscillator circuit and controlling the voltage of the power supply line according to a result of the comparison.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a delay circuit and, particularly, to a delay circuit having a delay time which is hardly influenced by variation of fabrication process thereof or a fabrication process of a semiconductor circuit internally including the delay circuit, hardly influenced by external condition such as environmental variation, etc., and can be set finely and precisely with respect to an input signal thereof. 
     2. Description of the Related Art 
     In the recent recordable/rewritable compact disk (CD-R/RW), the data write speed is increased at a double rate. In such CD-R/RW, it is usual that data, which is transferred from a host computer through an interface such as small computer system interface (SCSI) or ATPI is EFM-modulated and supplied to a laser controller. A laser light from a laser oscillator, which is regulated. for data write by the laser controller, is ON/OFF controlled by the EFM-modulated data to irradiate predetermined tracks of the compact disk (CD) to thereby write the data in the CD. 
     In such CD-R/RW or an optical disk such as recordable compact disk (CD-R), digital video disk random access memory (DVD-RAM), a circuit for precisely setting a delay time as short as several pico-seconds to several tens nano-seconds is required in writing data by means of a data write device. 
     Further, with increase of the clock speed of such as central processing unit (CPU), it has been required to precisely set a delay time as short as several pico-seconds to several tens nano-seconds in even a usual logic circuit. 
     In a prior art circuit for precisely setting a very short delay time, it has been usual, in order to absorb a variation of delay time of individual delay circuits or individual semiconductor circuits each including a delay circuit, which are fabricated through a fabrication process of the delay circuit or a fabrication process of the semiconductor circuit, that the delay time of each delay circuit or each semiconductor circuit including the delay circuit is set by an external voltage signal during the fabrication process since the delay time of the delay circuit is influenced by the fabrication process. Alternatively, as disclosed in Japanese Patent Application Laid-open No. H7-86888, a delay circuit is provided with a control terminal for controlling a delay time thereof and the delay time of the delay circuit is controlled by measuring a practical amount of delay of the delay circuit by a suitable measuring device and, on the basis of the measurement, applying a control signal to the control terminal of the delay circuit to set the delay time thereof appropriately. Alternatively, a delay time of each delay circuit is corrected by measuring an amount of delay thereof by forming a micro cell including an analog to digital (A/D) converter, a digital to analog (D/A) and a CPU, etc., and regulating the delay time by a processor on the basis of the measured delay time. 
     In any of the prior arts, in order to precisely set a very short delay time of a delay circuit with respect to an input signal, it is necessary to regulate or correct the delay time of the delay circuit by measuring a practical delay time thereof. However, since, in the delay time correction using the regulation of the external voltage, the delay circuit is easily influenced by change of external environment such as temperature change, secular change and power source voltage change, etc., there is a problem on accuracy of delay time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a delay circuit having a delay time, which is hardly influenced by the fabrication process of a delay circuit or a semiconductor device including the delay circuit, is hardly influenced by external condition such as environmental change and can be set precisely with respect to an input signal. 
     In order to achieve the above object, a delay circuit according to the present invention is featured by comprising a delay element circuit composed of a series circuit of a plurality of first circuit elements, which are connected to a common power supply line and have a delay time varying correspondingly to a voltage of the common power supply line, for receiving the input signal and outputting an output signal obtained by delaying the input signal, and a phase-locked loop (PLL) circuit including an oscillator circuit composed of a series circuit of a plurality of second circuit elements, which are equivalent to the first circuit elements, respectively, and connected to the common power supply line, for oscillating the oscillator circuit at a predetermined frequency locked to a reference clock signal frequency by comparing a phase of the reference clock signal with a phase of an output frequency of the oscillator circuit and controlling the voltage of the power supply line according to a result of the comparison. 
     Thus, according to the present invention, the delay element circuit is formed by the first circuit elements equivalent to the respective second circuit elements of the PLL circuit and the oscillation frequency of the PLL circuit is locked to the reference clock signal frequency. Therefore, it is possible to control the delay time of the second circuit elements of the delay element circuit measured from the signal input time thereto to the signal output time to a constant value, which is determined by the reference clock signal frequency, to thereby set the delay time of the first circuit elements. 
     It is usual that a clock generator circuit for generating the reference clock signal frequency, which is substantially constant and is hardy influenced by external condition such as temperature change, secular change and/or power source voltage, etc., is formed internally of an integrated circuit (IC). Therefore, by utilizing such construction of a delay circuit as that of the delay circuit of the present invention, a delay circuit can be realized as a stable circuit which is hardly influenced by external condition and is free from variation in delay time. 
     Particularly, when a crystal oscillator, etc., is utilized as the clock generator circuit for generating the reference clock, it is possible to realize a delay circuit which does not require a delay time regulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram of a delay circuit according to an embodiment of the present invention; 
     FIG. 2 is a circuit diagram of a delay circuit according to another embodiment of the present invention; 
     FIG. 3 a  shows a layout of a delay element portion and a voltage-controlled oscillator (VCO); 
     FIG. 3 b  shows a cell layout of a VCO and delay element circuit; and 
     FIG. 3 c  shows a wiring line of the VCO and the delay element circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a delay circuit  10  according to the present invention comprises a PLL circuit  1 , a delay element circuit  11  composed of a plurality of series-connected inverters  2   b , a reference clock generator circuit  12  and a controller  13 . In this embodiment, the reference clock generator circuit  12  is constructed with a crystal oscillator, etc., and an oscillation frequency thereof is hardly influenced by variation of external condition. 
     The PLL circuit  1  as a whole constitutes an oscillator circuit, which oscillates at a frequency (or phase) locked to a frequency (or phase) of a reference clock signal CLK from the reference clock generator circuit  12 , and outputs a power source voltage signal for setting a delay time of each inverter element of the delay element circuit  11 . The PLL circuit  1  comprises a voltage-controlled oscillator (VCO)  2  composed of a series-connected circuit of an odd number of inverters  2   a , a 1/n frequency divider  3 , where n is an integer, a phase comparator circuit  4 , a charge pump  5 , a low-pass filter (LPF)  6 , a voltage follower  7  and a 1/m frequency divider  8 , where m is an integer. The voltage signal applied to the respective inverters  2   b  of the delay element circuit  11  as a control voltage Vs is also applied to the respective inverters  2   a  of the VCO  2 . 
     In this embodiment, the VCO  2  takes in the form of a ring oscillator formed by feeding back an output of the series-connected inverter circuit thereof to an input thereof. The series-connected inverters  2   b  of the delay element circuit  11  are integrated together with the inverters  2   a  of the VCO  2  in an IC. Thus, the inverters  2   a  of the VCO  2  are provided as a circuit substantially equivalent to the inverters  2   b  of the delay element circuit  11 . Since the inverters  2   a  and the inverters  2   b  are supplied with the control voltage Vs and the operation delay times of each of the inverters  2   a  and each of the inverters  2   b  are changed correspondingly to the value of the control voltage Vs, the operation delay time of the inverter  2   a  becomes equal to that of the inverter  2   b.    
     The control voltage Vs as the common power source voltage of the inverters  2   a  and  2   b  controls the oscillation frequency of the VCO  2  of the PLL circuit  1  such that the oscillation frequency coincides with the frequency of the clock signal CLK of the reference clock generator circuit  12  or the frequency of the clock signal CLK multiplied by a predetermined coefficient value. That is, in the PLL circuit  12 , the output frequency of the VCO  2  is divided by n by the 1/n frequency divider  3  and supplied to one input of the phase comparator circuit  4 , in which it is compared in phase with the reference clock CLK divided by m by the 1/m frequency divider  8  and supplied to the other input of the phase comparator circuit  4 . 
     The phase comparator circuit  4  sends a charge up signal CU, which is in “H” level for a time period corresponding to a phase difference between a rising of the clock signal CLK and a rising of the input signal to the VCO  2 , to a gate of a P channel MOS transistor Q 1  of the charge pump  5  on the current discharge side thereof through an inverter  4   a  to turn the P channel MOS transistor Q 1  ON. In this case, a charge down signal CD from the phase comparator  4  becomes “L”, to turn an N channel MOS transistor Q 2  on the current sink side of the charge pump  5  OFF. 
     Further, the phase comparator circuit  4  sends the charge down signal CD, which is in “H” level for a time period corresponding to a phase difference between a rising of the input signal on the VCO  2  and a rising of the clock signal CLK, to a gate of the N channel MOS transistor Q 2  of the charge pump  5  on the current sink side thereof to turn the N channel MOS transistor Q 2  ON. In this case, the charge up signal CU becomes “L” to turn the P channel MOS transistor Q 1  OFF. 
     The output signal of the charge pump  5  is supplied to the LPF  6  and, after it is smoothed thereby, inputted to the voltage follower  7 . Thus, the control voltage Vs for controlling the oscillation frequency of the VCO  2  of the PLL circuit  1  such that the oscillation frequency coincides with the frequency of the clock signal CLK of the reference clock generator circuit  12  or a predetermined ratio of the frequency of the clock signal CLK is generated by the voltage follower  7 . 
     As described, in the PLL circuit  1 , the VCO  2  is driven by the output of the voltage follower  7  as the control voltage Vs for determining the operating voltage of the inverters. Thus, the oscillation frequency (or phase) of the VCO  2  is controlled so that the oscillation frequency is locked to the frequency of the clock signal CLK. 
     As a result, the oscillation frequency of the PLL circuit  1  is controlled such that it is locked to a value calculated by using, as a coefficient, the predetermined ratio determined by the frequency dividing ratios 1/n and 1/m of the frequency dividers  3  and  8  to the frequency of the reference clock generator circuit  12 , which is hardly influenced by the external condition such as variation of temperature and/or power source voltage of the device, etc. In this case, the delay time of operation of each of the inverters  2   a  of the VCO  2  is determined correspondingly to the clock frequency of the reference clock generator circuit  12  and the control voltage Vs is constant. This is also applied to each of the inverters  2   b  of the delay element circuit  11 , which operates under the same control voltage Vs, since the inverters  2   b  of the delay element circuit  11  are integrated in the IC together with the inverters  2   a.    
     The delay time of the delay element circuit  11  is determined without substantial influence of the external condition such as variation of temperature and power source voltage of the device, etc. Assuming that the number of the series-connected inverters  2   b  is P and a delay time of each inverter  2   b  is τ, the delay time of the delay element circuit  11  with respect to the input signal Din of the input terminal  9  is determined by τ×P. A delay signal having a finely determined delay amount is outputted from the output terminal  15   a  of the delay element circuit  11 , which is connected to the output of the last stage inverter  2   b.    
     Particularly, since the frequency of the clock signal CLK generated by the reference clock generator circuit  12 , which is constructed with a crystal oscillator, etc., is hardly influenced by fabrication process variation of each delay circuit or each semiconductor circuit including a delay circuit, the variation of delay time between the delay circuits can be substantially absorbed. 
     Incidentally, in this embodiment, the delay time τ each inverter is programmable. The delay time can be changed by changing the frequency dividing ratio n of the 1/n frequency divider  3  and the frequency dividing ratio m of the 1/m frequency divider  8  by the controller  13 . The 1/n frequency divider  3  and the 1/m frequency divider  8  are constructed with a n-adic counter and an m-adic counter, respectively, such that the maximum count values n and m thereof can be changed by data setting from the controller  13 . 
     FIG. 2 is a circuit diagram of the delay circuit according to another embodiment of the present invention, which comprises a PLL circuit la and a delay element circuit  11   a . The delay element circuit  11   a  comprises, in addition to series-connected inverters  2   b  similar to those of the delay element circuit  11  shown in FIG. 1, a plurality of 3-state buffer amplifiers  2   c  connected to outputs of the inverters  2   b  in the odd numbered stages as loads thereof, respectively, and a plurality of 3-state buffer amplifiers  2   d  connected to an input of the initial stage inverter  2   b  and to outputs of the inverters  2   b  of the even numbered stages, respectively, so that the delayed input signals can be derived from the inverters  2   b  of the even numbered stages through the buffers  2   d  connected thereto, respectively. The 3-state buffer amplifiers  2   c  and  2   d  alternately connected to the outputs of the inverters  2   b  as the loads thereof operate with a control voltage Vs from a voltage follower  7  as the power source, as in the first embodiment shown in FIG.  1 . The 3-state buffer amplifiers  2   c  function as dummy loads of the respective inverters  2   b  to obtain equal delay times in the respective stages. The 3-state buffer amplifiers  2   d  functioning as the outputs of the respective inverters of the even numbered stages are grounded by a selection signal SEL from the controller  13  and supply output signals in a predetermined level H or L to a level shifter  14 . The level shifter  14  receives the output signals from the inverters  2   d  in the even numbered stages, matches output levels “H” and “L” of the data thereof and sends them to an output terminal  15   b.    
     The output of the inverter  2   b  in the last stage is sent to an output terminal  15   a  as in the first embodiment shown in FIG.  1 . 
     In a PLL circuit  1   a  shown in FIG. 2, a VCO  20  is provided correspondingly to the described construction of the delay element circuit  11   a . The VCO  20  is a ring oscillator, which includes, in addition to the inverters  2   a  of the VCO  2  shown in FIG. 1, 3-state buffer amplifiers  2   e , which are dummy circuits connected to outputs of the respective inverters  2   a  as loads thereof. The 3-state buffer amplifiers  2   e  connected to the outputs of the respective inverters  2   a  are dummy load circuits corresponding to the 3-state buffer amplifiers  2   c  and  2   d , which operate with the control voltage Vs and have GND terminals grounded. 
     An output of the VCO  20  is supplied to a 1/n frequency divider  3  through a level shifter  14   a  equivalent to the level shifter  14 . Thus, the inverters  2   a  of the VCO  20  are provided as a circuit substantially equivalent to the inverters  2   b  of the delay element circuit  11   a . As a result, the delay times of the inverters  2   b  of the delay element circuit  11   a  become substantially equal to those of the inverters  2   a  of the VCO  20 . 
     Since other portions of the delay circuit shown in FIG. 2 than those mentioned above are the same as those of the delay circuit shown in FIG.  1  and depicted by the same reference numerals, respectively, details thereof are omitted for avoidance of duplication. 
     FIG. 3 a  shows a practical layout of the inverters  2   a  and  2   e  of the VCO  20  as well as the inverters  2   b ,  2   c  and  2   d  of the delay element circuit  11   a , in which one of the inverters  2   a  (or  2   b ) and one of the 3-state buffer amplifiers  2   e  (or  2   c  and  2   d ) are paired as a cell  16   i  (i is a to n) and the cells  16   b  to  16   n  are formed as a 2-stage arrangement. In the VCO  20 , the output of the inverter  2   a  of the cell  16   n  of the last stage is fed back to the input of the inverter  2   a  of the cell  16   a  of the initial stage through a wiring  17   a  and the output of the inverter  2   a  of the cell of the upper last stage is connected to the input of the inverter  2   a  of the cell immediately below the upper last stage through a wiring  17   b , resulting in the ring oscillator circuit. 
     Lengths of the wiring  17   a  and the wiring  17   b  are selected as close to the length of the connection wiring between adjacent inverters  2   a  or between adjacent inverters  2   b  as possible. 
     The delay element circuit  11   a  is the same as that of the VCO  20  expect that the wiring  17   a  is removed. 
     By employing the described layouts of the VCO  20  and the delay element circuit  11   a , it is possible to minimize an error of the delay time of each stage, which is set by the input signal. 
     As to the wiring of the power source line or the ground line GND, it is preferable that the lengths of the wiring between the power source line or the ground line and all of the cells  16  are made equal by using a tournament path wiring  18  shown in FIG. 3 b , in which lengths of paired symmetrical wiring depicted by a symbol ⊚, X, // or \ are substantially the same. Such tournament path wiring  18  may be employed in deriving the outputs of the 3-stage buffer amplifiers  2   d  of the respective stages. In such case, it becomes possible to minimize the error of the delay time of each stage, which is set by the input signal. 
     FIG. 3 c  shows a wiring layout of the power source line for supplying the control voltage Vs of the voltage follower  7  to the delay element circuit  11  or  11   a  and the VCO  2  or  20 . In the shown layout, the control voltage Vs is supplied to the VCO  2  or  20  and the delay element circuit  11  or  11   a  through wiring  19   a  and  19   b , which have substantially the same lengths, and the VCO  2  or  20  and the delay element circuit  11  or  11   a  are grounded through wiring  19   c  and  19   d , which have substantially the same lengths. 
     Although the inverters are used as the delay elements in the described embodiments, other elements may be used as the delay elements, alternatively. For example, any circuit element such as a flip-flop circuit, which operates with a predetermined delay with respect to an input signal., may be used therefor.