Abstract:
A delayed signal generation circuit includes a first delay circuit having a plurality of delay elements connected in series and delaying a reference signal applied thereto, a second delay circuit having a plurality of delay elements connected in series each of which sends out an output signal which is delayed with respect to an input signal applied to the second delay circuit, a detector unit, responsive to the reference signal, for detecting a number of delay elements of the first delay circuit which output an output signal that is delayed with respect to the reference signal after a lapse of a predetermined time interval, and a selection unit for selecting one delay element from the second delay circuit according to the number of delay elements of the first delay circuit, and for outputting the output signal from the selected delay element as a delayed signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a delayed signal generation circuit for generating a delayed signal. 
     2. Description of the Prior Art 
     FIG. 8 is a schematic circuit diagram showing the structure of a prior art delayed signal generation circuit. In the figure, reference numeral  1  denotes a delay element for holding an input signal during a predetermined time interval and for outputting an output signal which is delayed by the time interval with respect to the input signal, reference numeral  2  denotes an inverter which constitutes each delay element  1 , and reference numeral  3  denotes a capacitor which constitutes each delay element  1 . 
     In operation, since N delay elements  1  are connected in series in the delayed signal generation circuit of FIG. 8, when the delay time caused by each delay element  1  is T, the output signal from the final-stage delay element  1  is delayed by N×T with respect to the input signal applied to the first-stage delay element  1 . 
     A problem with a prior art delayed signal generation circuit constructed as mentioned above is that when a power supply voltage supplied to each delay element  1  decreases or when the operating temperature rises, the switching time of the inverter  2  which constitutes each delay element  1  increases and hence the delay time caused by each delay element  1  increases, and this results in a change in the delay time by which the output signal is delayed with respect to the input signal. 
     SUMMARY OF THE INVENTION 
     The present invention is proposed to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a delayed signal generation circuit capable of generating an output signal which is delayed by a constant delay time with respect to an input signal even if the power supply voltage and/or the operating temperature changes. 
     In accordance with an aspect of the present invention, there is provided a delayed signal generation circuit comprising: a first delay circuit having a plurality of delay elements connected in series, the first delay circuit delaying a reference signal applied thereto; a second delay circuit having a plurality of delay elements connected in series each of which sends out an output signal which is delayed with respect to an input signal applied to the second delay circuit; a detector unit, responsive to the reference signal applied to the first delay circuit, for detecting a number of delay elements of the first delay circuit which send out an output signal that is delayed with respect to the reference signal after a lapse of a predetermined time interval; and a selection unit for selecting one delay element from among the plurality of delay elements of the second delay circuit according to the number of delay elements of the first delay circuit which is detected by the detector unit, and for sending out the output signal from the selected delay element of the second delay circuit as a delayed signal. Accordingly, the delayed signal generation circuit can generate a delayed signal that is delayed by a constant time interval with respect to the input signal even if the power supply voltage and/or the operating temperature changes. 
     In accordance with another aspect of the present invention, the selection unit stores correspondences between the number of delay elements of the first delay circuit which is detected by the detection unit, and one delay element of the second delay circuit which is to be selected by the selection unit. Accordingly, the delayed signal generation circuit can generate a delayed signal that is delayed by a constant time interval with respect to the input signal without complicating the structure of the delayed signal generation circuit. 
     In accordance with a further aspect of the present invention, the delayed signal generation circuit further comprises a unit for setting a correspondence between the number of delay elements of the first delay circuit which is detected by the detection unit, and one delay element of the second delay circuit which is to be selected by the selection unit in the selection unit and changing the correspondence stored in the selection unit based on the number of delay elements detected by the detector unit. Accordingly, the delayed signal generation circuit can generate a delayed signal that is delayed by a constant time interval with respect to the input signal even if there are variations in the manufacturing processes. 
     In accordance with another aspect of the present invention, the delayed signal generation circuit further comprises a unit for reducing a frequency of a clock supplied to the plurality of delay elements of the second delay circuit when the number of delay elements detected by the detector unit is less than a reference number. Accordingly, the delayed signal generation circuit can secure a low voltage margin. 
     In accordance with a further aspect of the present invention, the delayed signal generation circuit further comprises a unit for outputting an interruption signal that informs a decrease in a power supply voltage when the number of delay elements detected by the detector unit is less than a reference number. Accordingly, the delayed signal generation circuit can give an alarm indicating a decrease in the power supply voltage and can provide an instruction for saving of the contents of a RAM. 
     In accordance with another aspect of the present invention, the delayed signal generation circuit further comprises a unit for raising a power supply voltage when the number of delay elements detected by the detector unit is less than a reference number. Accordingly, the delayed signal generation circuit can keep the power supply voltage constant. 
     In accordance with a further aspect of the present invention, the delayed signal generation circuit further comprises a unit for controlling a power supply voltage according to the number of delay elements detected by the detection unit. Accordingly, the delayed signal generation circuit can keep the power supply voltage constant. 
     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the structure of a delayed signal generation circuit according to a first embodiment of the present invention; 
     FIG. 2 is a diagram showing the logical values of various signals in the delayed signal generation circuit according to the first embodiment; 
     FIG. 3 is a block diagram showing the structure of a delayed signal generation circuit according to a second embodiment of the present invention; 
     FIG. 4 is a block diagram showing the structure of a delayed signal generation circuit according to a third embodiment of the present invention; 
     FIG. 5 is a block diagram showing the structure of a delayed signal generation circuit according to a fourth embodiment of the present invention; 
     FIG. 6 is a block diagram showing the structure of a delayed signal generation circuit according to a fifth embodiment of the present invention; 
     FIG. 7 is a block diagram showing the structure of a delayed signal generation circuit according to a sixth embodiment of the present invention; and 
     FIG. 8 is a schematic circuit diagram showing the structure of a prior art delayed signal generation circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1. 
     FIG. 1 is a block diagram showing the structure of a delayed signal generation circuit according to a first embodiment of the present invention. In the figure, reference numeral  11  denotes a delay element for outputting a reference clock (e.g., a P1 clock from a microcomputer into which the delayed signal generation circuit is incorporated) which is a reference signal after holding it during a predetermined time interval. A first delay circuit is comprised of N delay elements  11 . Reference numeral  12  denotes a delay element for holding a signal A which is an input signal during a predetermined time interval, and for sending out an output signal which is delayed by the predetermined time interval with respect to the input signal. A second delay circuit is comprised of S delay elements  12 . 
     Reference numeral  13  denotes a latch circuit (i.e., detection means) for latching the contents of each delay element  11  every time the reference clock falls, and for clearing the contents thereof every time a reset signal (e.g., a P2 clock from the microcomputer in which the P1 clock and the P2 clock are provided as two-phase clocks which do not overlap with each other) falls, and reference numeral  14  denotes a determination circuit for, every time the reset signal rises, determining the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state from the contents of the latch circuit  13 , and for outputting a control signal having a value corresponding to the determined number of delay elements  11 . Reference numeral  15  denotes a selector (i.e. selection means is  15  an  14 ) for selecting a delay element  12  according to the control signal output from the determination circuit  14 , and for sending out an output signal, which is delayed by a predetermined time interval with respect to the input signal A, from the selected delay element  12 , as a delayed signal B. 
     For example, each delay element  11  included in the first delay circuit has the same structure as each delay element as shown in FIG. 8, and each delay element  12  included in the second delay circuit also has the same structure as each delay element as shown in FIG.  8 . Therefore, when a power supply voltage supplied to each delay element  11  and each delay element  12  or the operating temperature changes, the operation characteristics of each delay element  11  change in the same manner that the operation characteristics of each delay element  12  change. Assume that in the first embodiment a time delay provided for an input signal by each delay element  12  is two times as large as that provided for an input signal by each delay element  11 . 
     The latch circuit  13  and the determination circuit  14  detect the number of delay elements  11  that have made a “Low” to “High” transition during the interval that the reference clock is at a “High” state in cooperation with each other, so that the selector  15  selects a delay element  12  from among the plurality of delay elements included in the second delay circuit according to the detected number of delay elements  11 . At that time, the period of the reference clock is maintained constant and the pulse duration of the reference clock is also maintained constant regardless of changes in the power supply voltage and/or the operating temperature. In other words, the interval during which the latch circuit  13  and the determination circuit  14  detect the number of delay elements  11  that make a “Low” to “High” transition in their contents remains constant regardless of changes in the power supply voltage and/or the operating temperature. As a result, the delayed signal generation circuit can generate a delayed signal B that is delayed by a predetermined time interval with respect to the input signal A, the predetermined time interval being constant regardless of changes in the power supply voltage and/or the operating temperature, by establishing correspondences between the number of delay elements  11  detected by the latch circuit  13  and the determination circuit  14 , and one delay element  12  included in the second delay circuit. 
     For simplicity, assume that 1 cycle of the reference clock is 100 nsec and the delayed signal B that is delayed by 20 nsec with respect to the input signal A needs to be output from the selector  15 , as shown in FIG.  2 . Furthermore, assume that when the power supply voltage is 3 Volts, the first to tenth delay elements  11  included in the first delay circuit make a “Low” to “High” transition in their contents and all other delay elements  11  remain at a “Low” state during the interval that the reference clock is at a “High” state, whereas when the power supply voltage is 2 Volts, the first to eighth delay elements  11  make a “Low” to “High” transition in their contents and all other delay elements  11  remain at a “Low” state during the interval that the reference clock is at a “High” state. 
     When the delayed signal generation circuit operates on the power supply of 3 Volts, the plurality of delay elements  11  make a “Low” to “High” transition in their contents in rotation in the order in which they are running from the input side to the output side every time the reference clock rises. The latch circuit  13  then latches the contents of each of the plurality of delay elements  11  when the reference clock falls. In this case, since the power supply voltage is 3 Volts, the contents of each of the first to tenth delay elements  11  are “High” and the contents of each of the 11th to Nth delay elements  11  are “Low”. 
     Every time the reset signal rises, the determination circuit  14  determines the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state from the contents of the latch circuit  13 , and then outputs a control signal having a value corresponding to the determined number of delay elements  11  to the selector  15 . The contents of the latch circuit  13  are cleared when the reset signal falls. That is, the determination circuit  14  stores a correspondence between the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state and one delay element  12  included in the second delay circuit to be selected. For example, if the number of delay elements  11  make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state is 10, the one delay element  12  to be selected is the fifth delay element, and if the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state is 8, the one delay element  12  to be selected is the fourth delay element. Therefore, in the example of FIG. 2, the determination circuit  14  outputs a control signal that directs the selector  15  to select the fifth delay element  12  to the selector  15 . 
     When the selector  15  receives the control signal from the determination circuit  14 , the selector  15  selects one delay element  12  according to the control signal and furnishes an output signal from the selected delay element  12  as the delayed signal B. In this example, since the selector  15  receives the control signal that directs the selector to select the fifth delay element  12 , it outputs an output signal output from the fifth delay element  12  as the delayed signal B. 
     Next, the description is directed a case where the power supply voltage decreases to 2 Volts because of battery drain or the like. Even when the delayed signal generation circuit operates on the power supply of 2 Volts, the plurality of delay elements  11  make a “Low” to “High” transition in their contents in rotation in the order in which they are running from the input side to the output side every time the reference clock rises. The latch circuit  13  then latches the contents of each of the plurality of delay elements  11  every time the reference clock falls. In this case, since the power supply voltage is 2 Volts, the contents of each of the first to eighth delay elements  11  are “High” and the contents of each of the ninth to Nth delay elements  11  are “Low”. 
     Every time the reset signal rises, the determination circuit  14  determines the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state from the contents of the latch circuit  13 , and then outputs a control signal having a value corresponding to the determined number of delay elements  11 . The contents of the latch circuit  13  are cleared every time the reset signal falls. Therefore, in the example of FIG. 2, since the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state is 8, the determination circuit  14  outputs a control signal that directs the selector  15  to select the fourth delay element  12  to the selector  15 . 
     When the selector  15  receives the control signal from the determination circuit  14 , the selector  15  selects a delay element  12  according to the control signal and outputs an output signal from the selected delay element  12  as the delayed signal B. In this example, since the selector  15  receives the control signal that directs the selector to select the fourth delay element  12 , it outputs an output signal from the fourth delay element  12  as the delayed signal B. 
     As previously mentioned, in accordance with the first embodiment of the present invention, the delayed signal generation circuit is so constructed as to determine the number of delay elements  11  that make a “Low” to “High” transition state in their contents during the interval that the reference clock is at a “High” state from the contents of the latch circuit  13 , and to output a control signal having a value corresponding to the determined number of delay elements  11  to the selector  15 . Accordingly, the delayed signal generation circuit can generate a delayed signal B that is delayed by a constant time interval with respect to an input signal A even if the power supply voltage supplied to each delay element  11  and each delay element  12  changes. In the first embodiment shown, the delayed signal generation circuit can also generate a delayed signal B that is delayed by a constant time interval with respect to an input signal A in the same manner even if the operating temperature changes. 
     Embodiment 2. 
     FIG. 3 is a block diagram showing the structure of a delayed signal generation circuit according to a second embodiment of the present invention. In the figure, since the same reference numerals as shown in FIG. 1 denote the same components as those of the first embodiment or like components, the explanation of those components will be omitted hereafter. Reference numeral  21  denotes a CPU for, when a power supply voltage of 3 Volts is supplied to a microcomputer, into which the delayed signal generation circuit is incorporated, from an external tester  23  disposed outside the microcomputer at the time of production test, for example, furnishing the contents of a latch circuit  13  to the tester  23  by writing the contents of the latch circuit  13  in a flash memory  22 , and for, when the external tester  23  writes a reset vector (i.e. a correspondence between the number of delay elements  11  that make a “Low” to “High” transition state in their contents during the interval that a reference clock applied to a first delay circuit is at a “High” state and a delay element  12  to be selected by a selector  15 , which is to be stored in a determination circuit  14 ) which indicates a reference number specific to the microcomputer in the flash memory  22 , writing the reset vector in the determination circuit  14 . The reference number is the number of delay elements that make a “Low” to “High” transition state in their contents during the interval that the reference clock is at a “High” state when the power supply voltage from the tester  23  is 3 Volts. The flash memory  22  stores the contents of the latch circuit  13  and the reset vector. The tester  23  calculates the reset vector from the contents of the latch circuit  13 . 
     The above-mentioned first embodiment does not refer to the setting of a correspondence and changing of a correspondence stored in the determination circuit  14 . However, since the switching characteristics of delay elements  11  and  12  installed in the microcomputer are not necessarily invariant due to variations in the manufacturing processes, it is necessary to set a proper correspondence to the determination circuit  14  as required. So, in accordance with the second embodiment, to generate a delayed signal B that is delayed by a constant time interval with respect to an input signal A even if there are variations in the manufacturing processes, the CPU  21  sets a proper correspondence between the number of delay elements  11  of the first delay circuit which is determined by the determination circuit  14 , and one delay element of the second delay circuit which is to be selected by the selector  15  in the determination circuit  14  or changes the correspondence stored in the determination circuit  14  based on the contents of the latch circuit  13  at the time of a production test or a reset of the microcomputer. 
     Concretely, at the time of a production test or a reset of the microcomputer, the external tester  23  supplies a power supply voltage (for example, a voltage of 3 Volts), on which the microcomputer operates under normal operating conditions, to the microcomputer. After that, when the reference clock falls, the CPU  21  reads the contents of the latch circuit  13  and furnishes the contents of the latch circuit to the external tester  23  by writing the contents of the latch circuit in the flash memory  22 . 
     The external tester  23  reads the contents of the latch circuit  13  from the flash memory  22  and measures a time interval by which a delayed signal output from each delay element  12  is delayed. The external tester  23  then calculates a reset vector (a correspondence to be stored in determination circuit  14 ) from measurement results and the contents of the latch circuit, and stores the reset vector in the flash memory  22 . The CPU  21  then reads the reset vector from the flash memory  22 , and performs a process of writing the reset vector in the determination circuit  14 . Similarly, the external tester  23  can supply a power supply voltage of 2 Volts to the microcomputer, for example, and then calculate a reset vector associated with the power supply voltage that thus decreases to 2 Volts, so that the CPU  21  can perform a process of writing the reset vector in the determination circuit  14 . 
     Embodiment 3. 
     In the above-mentioned second embodiment, the external tester  23  calculates a-reset vector and the CPU  21  writes the reset vector in the determination circuit  14 . In accordance with a third embodiment of the present invention, a sequencer  25  having the same functions as the CPU  21  and the tester  23  of the second embodiment calculates a reset vector and writes the reset vector in the determination circuit  14  in response to a trigger signal applied thereto by way of an external terminal  24 , as shown in FIG.  4 . As a result, even if no flash memory is built in the microcomputer, the reset vector can be written in the determination circuit  14 . 
     Embodiment 4. 
     FIG. 5 is a block diagram showing the structure of a delayed signal generation circuit according to a fourth embodiment of the present invention. In the figure, since the same reference numerals as shown in FIG. 1 denote the same components as those of the first embodiment or like components, the explanation of those components will be omitted hereafter. Reference numeral  31  denotes a determination circuit that has the same function as the determination circuit  14  mentioned above, and that, when the number of delay elements  11  included in a first delay circuit that make a “Low” to “High” transition in their contents during the interval that a reference clock is at a “High” state is less than a reference number of delay elements  11 , outputs a control signal indicating the fact, reference numeral  32  denotes a sequencer (i.e., frequency control means) for outputting a write signal that directs reduction of the frequency of a clock supplied to each of a plurality of delay elements (not shown) included in a second delay circuit (not shown) in response to the control signal from the determination circuit  31 , and for outputting an interruption signal that informs an interruption control block  34  of a decrease in the power supply voltage, and reference numeral  33  denotes an operation clock control register for reducing the frequency of the clock in response to the write signal from the sequencer  32 . The interruption control block  34  gives an alarm indicating a decrease in the power supply voltage and provides an instruction for saving of the contents of a RAM in response to the interruption signal from the sequencer  32 . A selector  15  is not shown in FIG.  5 . 
     In operation, when the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state is less than the reference number, the delayed signal generation circuit according to the fourth embodiment can reduce the frequency of the clock supplied to each of the plurality of delay elements (not shown) included in the second delay circuit (not shown). Concretely, since when the power supply voltage becomes less than an acceptable voltage because of battery drain or the like, the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state becomes less than the reference number and therefore the correct functioning of each of the plurality of delay elements (not shown) included in the second delay circuit (not shown) is not ensured, the determination circuit  31  outputs the control signal indicating that the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state is less than the reference number to the sequencer  32 . 
     In response to the control signal from the determination circuit  31 , the sequencer  32  reduces the frequency of the clock so as to secure a low voltage margin by outputting the write signal that directs the reduction of the frequency of the clock supplied to each of the plurality of delay elements (not shown) included in the second delay circuit (not shown) to the operation clock control register  33 . In response to the control signal from the determination circuit  31 , the sequencer  32  outputs an interruption signal that informs the interruption control block  34  of a decrease in the power supply voltage to the interruption control block  34 , so that the interruption control block  34  can give an alarm indicating a decrease in the power supply voltage and can provide an instruction for saving of the contents of a RAM. 
     Embodiment 5. 
     FIG. 6 is a block diagram showing the structure of a delayed signal generation circuit according to a fifth embodiment of the present invention. In the figure, since the same reference numerals as shown in FIG. 5 denote the same components as those of the fourth embodiment or like components, the explanation of those components will be omitted hereafter. Reference numeral  35  denotes a ring oscillator for generating a clock OSC to cause a voltage booster  36  to perform a pumping operation in response to a control signal from a determination circuit  31 . The voltage booster  36  performs a pumping operation so as to raise a power supply voltage in response to the clock OSC from the ring oscillator  35 . Delay elements  12  and a selector  15  are not shown in FIG.  6 . 
     In operation, when the number of delay elements  11  included in a first delay circuit that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state becomes less than a reference number, the delayed signal generation circuit of the fifth embodiment can raise the power supply voltage. Concretely, when the power supply voltage decreases to 3 Volts or less and therefore the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state becomes less than the reference number, the determination circuit  31  outputs a control signal indicating the fact to the ring oscillator  35 . 
     In response to the control signal from the determination circuit  31 , the ring oscillator  35  generates the clock OSC to cause the voltage booster  36  to perform a pumping operation. The voltage booster  36  then performs a pumping operation so as to raise the power supply voltage in response to the clock OSC from the ring oscillator  35 . When the supply of the clock OSC is stopped, the voltage booster  36  stops the pumping operation. 
     When the power supply voltage is thus raised, the boosted power supply is consumed by a circuit that uses the boosted power supply and the power supply voltage then decreases gradually. When the power supply voltage becomes 3 Volts or less again, the ring oscillator  35  generates the clock OSC again in the same way. 
     Embodiment 6. 
     FIG. 7 is a block diagram showing the structure of a delayed signal generation circuit according to a sixth embodiment of the present invention. In the figure, since the same reference numerals as shown in FIG. 5 denote the same components as those of the fourth embodiment or like components, the explanation of those components will be omitted hereafter. Reference numeral  37  denotes a reference generation circuit for generating a plurality of different reference voltages, reference numeral  38  denotes a selector for selecting a reference voltage from among the plurality of different reference voltages according to a control signal applied thereto for indicating the number of delay elements  11  included in a first delay circuit that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state from a determination circuit  31 , and reference numeral  39  denotes a VDC for comparing the reference voltage selected by the selector  38  with a power supply voltage, and for controlling the power supply voltage according to a comparison result. Delay elements  12  and a selector  15  are not shown in FIG.  7 . 
     In operation, when the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state becomes less than a reference number, the delayed signal generation circuit according to the sixth embodiment selects a reference voltage from among the plurality of different reference voltages according to the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state and controls the power supply voltage based on the reference voltage, instead of raising the power supply voltage. Concretely, the reference generation circuit  37  generates a reference voltage of 2 Volts, a reference voltage of 1.5 Volts, and a reference voltage of 1 Volt, for instance. For example, when the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state is 8, the selector  38  selects the first reference voltage of 2 Volts, when the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state is 10, the selector  38  selects the second reference voltage of 1.5 Volts, and when the number of delay elements  11  that make a “Low” to “High” transition in their contents during the interval that the reference clock is at a “High” state is 12, the selector  38  selects the third reference voltage of 1 Volt. 
     Then, when the selector  38  selects the first reference voltage of 2 Volts, the VDC  39  controls the power supply voltage so that the power supply voltage becomes 3.5 Volts. When the selector  38  selects the second reference voltage of 1.5 Volts, the VDC  39  controls the power supply voltage so that the power supply voltage becomes 3 Volts. When the selector  38  selects the third reference voltage of 1 Volt, the VDC  39  controls the power supply voltage so that the power supply voltage becomes 2.5 Volts. As a result, even if the operating temperature changes, the delayed signal generation circuit can keep the power supply voltage constant. 
     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.