Abstract:
A circuit and method for selectively and dynamically delaying a signal is presented. A series of delay modules are used to provide progressively finer delays. A multiplexer is used after each delay module to select one of a plurality of signals to pass on to a subsequent delay module. Each multiplexer is controlled by a control signal which can vary in time so that different delays can be selected for different portions of the signal to be delayed. By providing the proper control signals to the multiplexers any delay corresponding to a sum of the available individual delays generated by the individual delay modules is possible. The circuit and method are particularly useful for imposing individual delay times on the pulses in a logic level signal.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to a circuit and method for delaying signals, and in particular, to a circuit and method for selectively delaying electrical signals using a control signal. 
     BACKGROUND OF THE INVENTION 
     Electrical circuits which process logic level signals often require a variable, dynamically controllable delay for each pulse. One example where the capability to selectively delay pulses is desirable is the CD-R/RW writing process. For CD recordable (CD-R) media, the writing process includes generation of burn holes in a thin film. For CD rewritable media (CD-RW), the writing process includes generating marks and spaces by phase-changing spots in the media. 
     This results in better control of the physical dimensions of each mark. Due to thermal heating effects, the length of a mark on the CD is determined not only by the on-time and the optical power of the write laser but also by the characteristics of its neighboring marks and spaces. Thus, it is desirable to individually modify the output power of each laser write pulses according to the content of neighboring marks and spaces. 
     SUMMARY OF THE INVENTION 
     The invention relates to a circuit and method for selectively delaying an electrical signal. A series of delay modules are used to provide progressively finer delays. The path through the delay modules is determined by a series of selectors which select one of a plurality of delayed signals to pass on to a subsequent delay module. Each selector is controlled by a control signal. The control signals are generated according to the desired delay to be imposed on the signal and can provide real-time delay adjustments to the logic signal. 
     In one aspect, the circuit of the invention includes a first delay module, a second delay module, a first selector and a second selector. The first delay module has an input to receive the signal to be delayed, a clock input and a signal output. The first selector has a first and a second input in communication with the first delay module input and first delay module signal output, respectively, a control input and an output. The second delay module has an input in communication with the first selector output, a clock input and an output. The second selector has a first, second and third input in communication with the first delay module input, the first delay module signal output and the second delay module signal output, respectively. The second selector also has an output and a control input. A selected pulse delay is generated in response to a first and a second control signal received at the first and second selector control inputs, respectively. 
     In one embodiment, the second selector control input includes a plurality of control lines and the second control signal includes a plurality of control bits. Each of the plurality of control lines is adapted to receive a respective control bit. In a further embodiment, each control bit is prioritized relative to the other control bits. 
     In one embodiment, the first selector includes a first tri-state buffer and a second tri-state buffer. The first tri-state buffer is in communication with the first delay module signal input and has a buffer enable input and an output. The second tri-state buffer is in communication with the first delay module signal output and has a buffer enable input. The second tri-state buffer also has an output which is in communication with the first buffer output. 
     In another aspect, the method of generating a selectively delayed signal includes the steps of receiving a signal to be delayed, generating a first delayed signal in response to a first clock delay signal having a first frequency, and selecting either the signal to be delayed or the first delayed signal as a first output signal. The method also includes the steps of generating a second delayed signal in response to a second clock delay signal having a second frequency and the first output signal, and selecting one of the signal to be delayed, the first delayed signal and the second delayed signal as the selectively delayed signal. 
     In one embodiment, the method includes the additional step of generating a third delayed signal in response to a third clock delay signal having a third frequency and the selected one of the signal to be delayed, the first delayed signal and the second delayed signal. In this embodiment, the step of selecting the signal to be the selectively delayed signal includes selecting one of the signals to be delayed, the first delayed signal, the second delayed signal and the third delayed signal as the selectively delayed signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will become apparent from the following more particular description of various embodiments of the invention, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed on illustrating the principles of the present invention. 
     FIG. 1 is a block diagram of a system for receiving encoded EFM signals and generating post-processed EFM signals for CD laser control according to the present invention; 
     FIGS. 2A and 2B are illustrations of examples of post-processor module output waveforms for CD-R and CD-RW embodiments, respectively, of FIG. 1; 
     FIG. 3 is a block diagram of one of the delay units shown in FIG. 1; 
     FIGS. 4A and 4B are timing diagrams depicting the clock signals and waveforms of the delay unit of FIG. 3; 
     FIG. 5 is a block diagram of a priority multiplexer shown in FIG. 3; 
     FIG. 6 is a schematic diagram of a tri-state buffer priority multiplexer; 
     FIG. 7 is a block diagram of a translation unit for generation of enable signals used to control the priority multiplexer of FIG. 6; 
     FIG. 8 is a truth table illustrating the relationship between the select signals used to control the priority multiplexer of FIG.  5  and the enable signals used to control the priority multiplexer of FIG. 6; and 
     FIG. 9 is a schematic diagram of a translation unit as shown in FIG.  7 . 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a CD-Recordable/CD-Rewritable delay generation circuit  10  includes five delay blocks  12 ( a-e ) although only  12   a  and  12   b  are shown for clarity. Each delay block  12 ( a-e ) receives a respective clock signal CLK 1  to CLK 5  generated by a clock generation unit  16 . Pre-processor module  18  receives an Eight to Fourteen Modulation code (EFM) signal and generates intermediate signals EFMA to EFME. Each delay block  12 ( a-e ) receives a respective one of the intermediate signals EFMA to EFME and generates a delayed intermediate signal DEFMA to DEFME, respectively. A post-processor module  20  receives the delayed intermediate signals DEFMA to DEFME and generates multiple post-processed EFM signals EFM 1  to EFM 3  to control the CD laser. 
     Referring to FIG. 2A, post-processed EFM signals EFM 1  to EFM 3  are combined (i.e., summed) to generate a laser power signal P R  which controls the output power of the laser for a CD-R system. EFM 1  controls the basic optical power during the on-time T(on) of the laser. EFM 2  controls the additional optical power used during the peak period T(peak) of the optical pulse. EFM 3  controls application of optical power during a pre-heat period T(preheat). The post-processed EFM signals EFM 1  to EFM 3  are the logic signals determined according to the data content on the nearby region of the CD and which dynamically control corresponding current sources. The sum of the currents is used to drive the laser, resulting in the laser output power P R . This results in better control of the physical dimensions of each mark on the CD. Referring to FIG. 2B, post-processed EFM signals EFM 1  to EFM 3  are logic signal similarly used to control individual current. The sum of the current is used to drive the laser, resulting in the laser output power P RW  for a CD-RW system. 
     FIG. 3 illustrates a delay block  12   a  for dynamically delaying by variable times the pulses in an EFM signal received at the delay block input  24   a . The delay block  12   a  includes five D flip-flops  30 ( a-e ) which are used as delay modules to provide delay to the signals received at the delay block input  24   a . Each flip-flop  30 ( a-e ) is followed by a selector or multiplexer  32 (a-e, respectively) which receives the EFM signal and the delayed outputs of the prior flip-flops  30 , and provides a delayed output signal to the subsequent flip-flop  30 . For example, the third multiplexer  32   c  receives the EFM signal and the delayed output signals from the first, second and third flip-flops  30   a ,  30   b  and  30   c , respectively. The third multiplexer  32   c  provides a delayed output signal to the fourth flip-flop  30   d . The output of the fifth multiplexer  32   e  is the desired selectively delayed intermediate signal DEFMA to DEFME and is provided at the delay block output  26   a.    
     Operation of each delay block  12  can be understood by sequential description of its circuit components. Referring to delay block  12   a , the first flip-flop  30   a  receives the intermediate signal EFMA and a 34 MHz clock signal CLK 1 . Clock signal CLK 1  is synchronous with the intermediate signal EFMA received at the delay block input  24   a . The value of the intermediate signal EFMA immediately before each positive voltage transition of the clock signal CLK 1  is provided as a first delayed signal DS 1  at the output  36   a  of the first flip-flop  30   a . The first multiplexer  32   a  receives the intermediate signal EFMA, the first delayed signal DS 1 , and a control signal CS 1 . A single bit control signal CS 1  determines which of the two signals EFMA and DS 1  received at the first multiplexer  32   a  to pass to the second flip-flop  30   b . A 34 MHz clock signal CLK 2 , shifted in phase by 180° with respect to the first clock signal CLK 1 , is provided to the second flip-flop  30   b . In this embodiment, the second clock signal CLK 2  is generated from the first clock signal CLK 1  by a logical inverter  38   a . The signal M 1 OUT selected by the first multiplexer  32   a  is provided to the second flip-flop  30   b . The value ofthe signal M 1 OUT immediately before the positive voltage transition of the second clock signal CLK 2  is provided at the second flip-flop output  36   b . The second multiplexer  32   b  receives the intermediate signal EFMA, the first delayed signal DS 1 , the second delayed signal DS 2  and a two bit control signal CS 2 . The values of the control signal CS 2  determines which of the three input signals EFMA, DS 1  and DS 2  is selected for input to the third flip-flop  30   c.    
     The remainder of the delay unit  12   a  functions in a similar manner. The input signals to the third, fourth and fifth flip-flops  30   c ,  30   d  and  30   e  include the delayed signal from the previous multiplexer  32   b ,  32   c  and  32   d , respectively, and a clock signal CLK 3 , CLK 4  and CLK 5 , respectively, which is twice the frequency of the clock signal used by the previous flip-flop  30   b ,  30   c  and  30   d , respectively. The input signals to the third, fourth and fifth multiplexers  32   c ,  32   d  and  32   e  include the intermediate signal EFMA and the delayed signals from all of the prior flip-flops  30 . The input signal selected by the third, fourth and fifth multiplexers  32   c ,  32   d  and  32   e  is determined by control signal CS 3 , CS 4  and CS 5 , respectively. The number of bits in each control signal CS is equal to the number of input signals for the corresponding multiplexer  32 ( a-e ). 
     Referring to FIG. 4A, each flip-flop  30 ( a-e ) delays its input signal according to the period of its respective clock signal CLK 1  to CLK 5 . For example, the first flip-flop  30   a  delays its input signal by T where T is approximately 28.9 ns. Because each flip-flop  30 ( a-e ) is triggered on the first positive transition of its received clock signal CLK, the second flip-flop  30   b will trigger at a time T/2 corresponding to the positive transition of logically inverted CLK 1  or, equivalently, 180° phase shifted CLK 1 . Similarly, the delays generated by the third, fourth and fifth flip-flops  30   c ,  30   d  and  30   e , respectively, are T/4, T/8and T/16, respectively. 
     The total delay imposed on the EFM signal by the delay block  12   a  (FIG. 3) is approximately equivalent to the sum of the individual delays generated along the signal path selected through the delay block  12   a  as determined by the five multiplexers  32 ( a-e ). Thus, by providing the proper control signals CS 1  to CS 5  to the five multiplexers  32 ( a-e ), the EFM signal can be selectively delayed by a time value in a range from 0 to 31 T/16with a resolution of T/16. For example and with reference back to FIG. 2, to delay an EFM signal pulse by approximately 38 ns (i.e., 21T/16) the delays generated by the first, third and fifth flip-flops  30   a ,  30   c  and  30   e , respectively, are selected. Thus, the second multiplexer  32   b  selects the first delayed signal DS 1  as the input signal for the third flip-flop  30   c , the fourth multiplexer  32   d  selects the third delayed signal DS 3  as the input to the fifth flip-flop  30   e , and the fifth multiplexer  32   e  selects the fifth delayed signal DS 5  as the signal presented at the delay block output terminal  26 . 
     Referring to FIG. 4B, a set of signal waveforms corresponding to the delay unit of FIG. 3 is shown for fixed control signals CS 1  to CS 5 . In this example CS 5  has a hexadecimal value of 15. The five bits of CS 5  are determined by the binary equivalent value for CS 5 , namely 10101. Control signals CS 1  to CS 4  correspond to the most significant bit, two most significant bits, three most significant bits and four most significant bits, respectively, of CS 5 . Clock signals CLK 1  to CLK 5  are synchronous with the intermediate signal EFMA received at the delay block input  24   a . M 1 OUT to M 4 OUT are the output signal waveforms of multiplexers M 1  to M 4 , respectively, and DEFMA is the delayed intermediate signal waveform provided at the output  26  of the delay block  12   a.    
     Referring to FIG. 5, multiplexer operation can be explained by examination of an individual multiplexer  32   c  representative of any of the multiplexers  32 ( a-e ). The third multiplexer  32   c  provides as an output signal M 3 OUT one of its four input signals A, B, C and D according to the three bits CS[ 1 ], CS[ 2 ] and CS[ 3 ] of its control signal CS 3 . The multiplexer output signal M 3 OUT is typically delayed with respect to the selected input signal A, B, C or D. The delay can be different for each signal A, B, C and D because typically the corresponding internal paths through the multiplexer  32   c  are unequal. 
     FIG. 6 shows a multiplexer  44  based on tri-state buffers  46 ( a-d ) which can be used to achieve the same functionality of the multiplexer  32   c  of FIG. 5 while providing a substantially uniform multiplexer path delay for input signals A, B, C and D. The multiplexer  44  includes a tri-state buffer  46 ( a-d ) for each input signal A, B C and D, respectively. Each buffer  46 ( a-d ) is enabled with a single enable signal E 1 , E 2 , E 3  and E 4 , respectively. The outputs of the buffers  46 ( a-d ) are coupled to the multiplexer output  48 . A translation unit  50  as shown in FIG. 7 is used to convert the three-bit multiplexer control signal CS 3  used to control the third multiplexer  32   c  into four enable signals E 1  to E 4  used to control the tri-state multiplexer  44 . 
     In order to avoid an indeterminate state at the multiplexer output  48 , only one enable signal E 1  to E 4  is asserted at one time. Thus, there must be a priority (i.e., a relative significance) assigned to the control bits s 1  to s 3 . FIG. 8 is a logic table indicating the relationship between the input signals A, B, C and D, control bits CS[ 1 ] to CS[ 3 ], and enable signals E 1  to E 4 . A “ 1 ” indicates an asserted bit or signal and a “ 0 ” is used to indicate an unasserted bit or signal. An “X” is used to indicate that the state of the bit or signal is not relevant to determination of the multiplexer output signal M 3 OUT. Control bit CS[ 3 ] has the highest priority. Thus, when control bit CS[ 3 ] is asserted, the output signal M 3 OUT is signal D regardless of the values of bits CS[ 1 ] and CS[ 2 ]. Control bit CS[ 2 ] has intermediate priority and therefore determines the output M 3 OUT only when control bit CS[ 3 ] is unasserted. Control bit CS[ 1 ] has the lowest priority. Thus, even if control bit CS[ 1 ] is asserted, it cannot determine the output M 3 OUT unless both CS[ 2 ] and CS[ 3 ] are unasserted. Enable signals E 1  to E 4  are generated such that only one can be asserted at one time, therefore, there is no similar priority requirement. 
     FIG. 9 illustrates a translation unit  50  using inverters  52 ( a-c ) and AND gates  54 ( a-c ) that can be used to generate the enable signals E 1  to E 4  from the control bits CS[ 1 ] to CS[ 3 ]. Control bit CS[ 3 ] is provided as enable signal E 1  and as an input to an inverter  52   a . AND gate  54   a  receives control bit CS[ 2 ] and inverted control bit CS[ 3 ] as inputs and generates enable signal E 3 . AND gate  54   b  receives inverted control bits CS[ 2 ] and CS[ 3 ] from inverters  52   b  and  52   a , respectively, and generates enable signal E 2 . AND gate  54   c  receives inverted control bits CS[ 1 ], CS[ 2 ] and CS[ 3 ] from inverters  52   c ,  52   b  and  52   a , respectively, and generates enable signal E 4 . 
     Equivalents 
     While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.