Abstract:
A circuit for reducing the noise associated with a clock signal for a flip-flop based circuit has been developed. The circuit includes a charge control portion that stores charge at a pre-determined time of the clock cycle and a dump control portion that releases the stored current also at a pre-determined time of the clock cycle. The charge is released onto the power grid of the system served by the clock signal.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates generally to electronic circuitry. More specifically, the invention relates a method for reducing the noise associated with a clock signal for a flip-flop based circuit. 
     2. Background Art 
     In all microprocessor-based systems, including computers, the clock circuit is a critical component. The clock circuit generates a clock signal that is a steady stream of timing pulses that synchronize and control the timing of every operation of the system. FIG. 1 shows a prior art diagram of an ideal clock signal  10 . An entire clock cycle  12  includes a rising or leading edge  14  and a falling or trailing edge  16 . These edges  14 ,  16  define the transition between the low and high value of the signal. 
     FIG. 2 shows a block diagram of a prior art local clock signal distribution system. The clock signal  30   a  is input to a clock header  32  which serves to buffer the clock signal. From the header  32 , the clock signal  30   b  is input to an edge-triggered flip flop  34  (“flip-flop”) where it serves to trigger the flip-flop. A flip-flop is a memory device that is commonly used in integrated circuits. It is dependent upon a clock signal to initiate its function. Flip-flops generally take input data and distribute output data on the rising edge of a clock signal. However, a flip-flop could be configured to work on the falling edge of a clock signal. 
     FIG. 3 shows a digital logic schematic of the prior art local clock signal distribution system as shown in FIG.  2 . The clock signal  30   a  is input to the clock header  32 . The clock header  32  includes a NAND gate  36  and an inverter  38   a . Once inside the clock header  32 , the clock signal  30   a  is one of the inputs to the NAND gate  36 . The other NAND input  42  is a signal that is HIGH so that the gate  36  simply inverts the value of the clock signal  30   a . The NAND input  42  is switched to LOW to turn off the clock header  32  if needed. Next, the signal  30   a  passes through the inverter  38   a  which inverts the signal back to its original value. The clock signal  30   b  then passes from the clock header  32  to the flip-flop  34 . Once in the flip-flop  34 , the signal  30   b  is split into two paths. The first path passes through one inverter  38   b , and the second path passes through two consecutive inverters  38   c  and  38   d . Each path feeds into the internal circuitry of the flip-flop  40  along with the DATA_IN  44  and DATA_OUT  46  paths of the flip-flop  34 . 
     Clock noise problems on the system power grid are usually caused by the large amount of current that is used in clock signal distribution. This current comes from the switching transistors that control the clock signal. As these transistors switch states, the current noise spikes onto the power grid due to the current demand or “current draw” of the switching transistors. These high current demands cause noise in the system voltage supply due to voltage (IR) drops and inherent system inductance (L di/dt). A clock signal distribution circuit uses a significant amount of current in a short amount of time because the spikes occur twice per clock cycle: once on the current draw of the leading edge and once on the current draw of the falling edge of the signal. This puts the noise at a very high frequency (2× the clock frequency). This noise can cause missed timing if the clock signal voltage is too low or component failure if the clock signal voltage is too high. The noise can even escape “off the chip” and affect the other components of the system. 
     FIG. 4 shows a graph of current draw during a clock cycle period of the prior art embodiment shown in FIG.  3 . The flip-flop of this embodiment is triggered on the rising edge of a clock signal. The value “I”  35  represents the full value of a current draw. The value “¾ I”  37  represents 75% of the full value while the value “½ I”  39  represents 50% of the full value. The first current draw  41  of the graph represents the draw that results from the leading edge of a clock cycle (at clock cycle=0). The second current draw  43  represents the draw that results from the falling edge of the clock cycle (at clock cycle=t/2). As shown, the leading edge draw  41  is the full value of current draw. The trailing edge draw  43  is about half the value of the leading edge draw  41 . In this example, the first draw  41  is larger than the second draw  43  because all of the flip-flop&#39;s change on the rising edge of the clock signal. Since none of the flip-flops change on the falling edge, the second draw  43  is smaller. 
     A common technique to alleviate noise is adding additional power to the grid. This power is added upon sensing a voltage drop due to noise. However, such techniques only respond to noise at a much lower frequency than clock noise and also respond only to a certain threshold of noise. Consequently, a need exists for a technique that generates a response to clock noise at a synchronized frequency with the clock noise itself. 
     SUMMARY OF INVENTION 
     In some aspects, the invention relates to an apparatus for reducing noise of a clock signal for a flip-flop based circuit, comprising: a charge control circuit that initiates storing a charge upon receipt of a first pre-determined signal; and a dump control circuit that initiates dumping the charge onto a system power grid upon receipt of a second predetermined signal. 
     In another aspect, the invention relates to an apparatus for reducing the noise of a clock signal for a flip-flop based circuit, comprising: means for initiating a charge control circuit that stores a charge upon receipt of a first predetermined signal; and means for initiating a dump control circuit that dumps the charge onto a system power grid upon receipt of a second pre-determined signal. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a graph of an ideal clock signal. 
     FIG. 2 shows a block diagram of a prior art embodiment of a clocking circuit for a flip-flop. 
     FIG. 3 shows a digital logic schematic of a prior art embodiment of a clocking circuit for a flip-flop. 
     FIG. 4 shows a graph of current draw during a clock cycle period of the prior art embodiment shown in FIG.  3 . 
     FIG. 5 shows a block diagram of a block diagram of one embodiment of the present invention. 
     FIG. 6 shows a digital logic schematic of one embodiment of the present invention. 
     FIG. 7 a  shows an equivalent circuit of a portion of the digital logic schematic shown in FIG. 6 during a charge phase. 
     FIG. 7 b  shows an equivalent circuit of a portion of the digital logic schematic shown in FIG. 6 during a discharge phase. 
     FIG. 8 shows a digital logic schematic of an alternative embodiment of the present invention. 
     FIG. 9 shows a digital logic schematic of an alternative embodiment of the present invention. 
     FIG. 10 shows a graph of current draw during a clock cycle period of the embodiments of the present invention shown in FIGS.  6 - 9 . 
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the invention will be described with reference to the accompanying drawings. Like items in the drawings are shown with the same reference numbers. 
     FIG. 5 shows a block diagram of a local clock signal distribution system (similar to that shown in FIG. 2) with a clock noise reduction circuit  48  added in accordance with one embodiment of the present invention. The clock signal  30   a  is input to a clock header  32  which serves to buffer the clock signal. From the header  32 , the clock signal  30   b  is input to a flip-flop  34  where it serves to trigger the device. In this embodiment of the present invention, the initial clock signal  30   a  is split before the signal  30   a  is input into the header  32 . The parallel split of the signal  30   a  is input into a clock noise reduction circuit  48 . Upon sensing the rising edge of the clock signal  30   a , the clock noise reduction circuit  48  will dump a voltage charge  50  onto the power grid of the system. The dumped charge  50  will alleviate the current noise spike associated with the clock cycle. 
     FIG. 6 shows a logic schematic of a clock noise reduction circuit  48  in accordance with one embodiment of the present invention. Once inside the noise reduction circuit  48 , the clock signal  30   a  is input into to a first inverter  50   a . This inverter  50   a  simply inverts the signal value. Next the signal is input to a second inverter  50   b  which inverts the signal back to its original value. The signal  52  (hereafter referred to as “charge signal”) is then split off into two branches. One branch of the charge signal  52  is input into a third inverter  50   c  which once again inverts the signal. The output of the third inverter  50   c  (hereafter referred to as “dump signal”) is then input, along with the charge signal  52 , into three circuit control transistors: a charge control transistor  56 ; a dump control transistor  58 ; and a connecting transistor  60 . It is important to note that the charge signal  52  and the dump signal  54  will have opposite values because the charge signal passes through the third inverter  50   c.    
     The charge control transistor  56  connects the system power supply (Vdd) with the system ground (Vss) through an charge capacitor  62   a . The charge capacitor  62   a  is located between the charge control transistor  56  and Vss. The transistor  56  is controlled (i.e. switched on and off) with the charge signal  52 . The transistor  56  is a “P-type” transistor which means that the transistor is “on” (allows current to pass) when the charge signal  52  is low. Conversely, the transistor  56  is “off” (does not allow current to pass) when the charge signal  52  is high. 
     The dump control transistor  58  also connects the system power supply (Vdd) with the system ground (Vss) through a dump capacitor  62   b . The dump capacitor  62   b  is located between the dump control transistor  58  and Vdd. The transistor  58  is controlled (i.e. switched on and off) with the dump signal  54 . The transistor  58  is an “N-type” transistor which means that the transistor is “on” (allows current to pass) when the dump signal  54  is high. Conversely, the transistor  58  is “off” (does not allow current to pass) when the dump signal  54  is low. 
     Finally, the connecting transistor  60  connects both sides of the circuit. Specifically, the connecting transistor  60  connects the sides between the control transistors  56 ,  58  and the respective capacitors  62   a ,  62   b . The connecting transistor  60  is a “P-type” transistor which means that the transistor is “on” (allows current to pass) when the dump signal  54  is low. Conversely, the transistor  60  is “off” (does not allow current to pass) when the dump signal  54  is high. 
     In normal operation, the control circuit has two phases of operation: a charge phase and a dump phase. In each phase, the circuit is activated by an “active low” signal. This means that the respective control signal (charge  52  or dump  54 ) initiates its respective phase when it is low rather than high. Specifically, during the charge phase, the charge signal  52  will be low and the dump signal  54  will be high. As a result, the charge control transistor  56  and the dump control transistor  58  are both “on” while the connecting transistor  60  is “off”. This allows both capacitors  62   a ,  62   b  to charge in preparation for the dump phase. During the dump phase, the charge signal  52  will be high and the dump signal  54  will be low. As a result, the charge control transistor  56  and the dump control transistor  58  are both “off” while the connecting transistor  60  is “on”. This allows both capacitors  62   a ,  62   b  to dump their charge on the power grid and consequently reduce the peak current draw. 
     FIGS. 7 a  and  7   b  show the equivalent circuits of a portion of the digital logic schematic shown in FIG. 6 during a charge phase and discharge phase respectively. In each figure, the “off” transistors have been deleted while the “on” transistors have been replaced by a standard circuit connection. Specifically, FIG. 7 a  shows an equivalent circuit during the charge phase. It shows the two capacitors  62   a  and  62   b  connected in parallel between Vdd and Vss. FIG. 7 b  shows an equivalent circuit during the dump phase. It shows the two capacitors  62   a  and  62   b  connected in series between Vdd and Vss. 
     When the capacitors  62   a  and  62   b  are in parallel during the charge phase, the each store a charge value “Q”, where Q=(Capacitance Value “C”)×Vdd. Consequently, the total charge stored by the circuit is 2Q. When the capacitors  62   a  and  62   b  are in series during the dump phase, each capacitor  62   a  and  62   b  will have a voltage equal to Vdd/2 across it. Consequently, each capacitor will store only Q/2 for a total stored charge of Q by the circuit. The excess charge equal to Q will be dumped onto the power grid. 
     In comparing FIG. 6 with FIG. 3, it is important to note that the clock header  32  and flip-flop  34  are synchronized with the clock noise reduction circuit  48 . The header  32  and flip-flop  34  have a three separate layers of inverters  38   a ,  38   b ,  38   c ,  38   d  along with the NAND gate  36 , while the clock noise reduction circuit  48  has only three inverters  50   a ,  50   b ,  50   c . In order to synchronize the signals, the components of both paths  38   a-d ,  36 ,  50   a-c  are sized such that the delays of both paths are identical. 
     The circuit  48  shown in FIG. 6 triggers the dump phase on the falling edge of the clock signal  30   a  because the dump phase begins when the dump signal  54  is “low” or on the falling edge. However, the circuit could easily be arranged to trigger the dump phase on the falling edge of the clock signal  30   a.    
     FIG. 8 shows a logic schematic of a clock noise reduction circuit  63  in accordance with one embodiment of a falling edge triggered circuit. The noise reduction circuit  63  is similar to the rising edge triggered circuit  48  (shown in FIG. 6) in that is has the same configuration of three sequential inverters  50   a ,  50   b ,  50   c  that generate the charge signal  52  and the dump signal  54  in the same manner. Additionally, the falling edge circuit  63  has a charge control transistor  64 , a dump control transistor  68 , and a connecting transistor  66 . Each is arranged in a similar configuration with respect to Vdd, Vss, and capacitors  62   a ,  62   b , as the rising edge circuit  48 . 
     However, in the falling edge circuit  63 , each of the transistors  64 ,  66 ,  68  are the opposite type of transistor with respect to the transistors  56 ,  58 ,  60  of the rising edge circuit  48 . Specifically, the charge control transistor  64  and the connecting transistor  66  are both “N-type” transistors while the dump control transistor  68  is a “P-type” transistor. This means that the charge control transistor  64  is “on” (allows current to pass) when the charge signal  52  is high. Conversely, the transistor  64  is “off” (does not allow current to pass) when the charge signal  52  is low. Additionally, the dump control transistor  68  is “on” (allows current to pass) when the dump signal  54  is low. Conversely, the transistor  68  is “off” (does not allow current to pass) when the dump signal  54  is high. Finally, the connecting transistor  66  is “on” (allows current to pass) when the dump signal  54  is high. Conversely, the connecting transistor  66  is “off” (does not allow current to pass) when the dump signal  54  is low. 
     The charge phases and dump phases of the falling edge circuit  63  will function in the same manner as the rising edge circuit  48 . However, these phases will be triggered by an “active high” control signal (charge  52  or dump  54 ). During the charge phase the charge signal  52  will be high and the dump signal  54  will be low. As a result, the charge control transistor  64  and the dump control transistor  68  are both “on” while the connecting transistor  66  is “off”. This allows both capacitors  62   a ,  62   b  to charge in preparation for the dump phase. During the dump phase, the charge signal  52  will be low and the dump signal  54  will be high. As a result, the charge control transistor  64  and the dump control transistor  68  are both “off” while the connecting transistor  66  is “on”. This allows both capacitors  62   a ,  62   b  to dump their charge on the power grid and consequently reduce the peak current draw. Thus, this circuit  63  will initiate the dump phase on the falling edge of the clock signal  30   a  because the dump phase begins when the dump signal  54  is “high” or on the rising edge. 
     FIG. 9 shows a logic schematic of a clock noise reduction circuit  69  in accordance with another embodiment of a falling edge triggered circuit. The noise reduction circuit  69  is similar to the rising edge triggered circuit  48  (shown in FIG. 6) in that is has the same configuration of three sequential inverters  50   a ,  50   b ,  50   c  that generate the charge signal  52  and the dump signal  54  in the same manner. Additionally, the falling edge circuit  69  has a charge control transistor  64 , a dump control transistor  68 , and a connecting transistor  66 . Each is arranged in a similar configuration with respect to Vdd, Vss, and capacitors  62   a ,  62   b , as the rising edge circuit  48 . 
     However, in this embodiment of a falling edge circuit  69 , the dump signal  54  and the charge signal  52  are switched as inputs to the control transistors  56  and  58 . Specifically, the charge signal  52  is input into the “N-type” control transistor  58  (the dump control transistor of the falling edge circuit  48  shown in FIG. 6) while the dump signal  54  is input into the “P-type” control transistor  56  (the charge control transistor of the falling edge circuit  48  shown in FIG.  6 ). 
     The charge phases and dump phases of the falling edge circuit  69  will function in the same manner as the rising edge circuit  48 . However, these phases will be triggered by an “active high” control signal (charge  52  or dump  54 ). During the charge phase the charge signal  52  will be high and the dump signal  54  will be low. As a result, the both control transistors  56  and  58  are “on” while the connecting transistor  60  is “off”. This allows both capacitors  62   a ,  62   b  to charge in preparation for the dump phase. During the dump phase, the charge signal  52  will be low and the dump signal  54  will be high. As a result, both control transistors  56  and  58  are “off” while the connecting transistor  60  is “on”. This allows both capacitors  62   a ,  62   b  to dump their charge on the power grid and consequently reduce the peak current draw. Thus, this circuit  69  will initiate the dump phase on the falling edge of the clock signal  30   a  because the dump phase begins when the dump signal  54  is “high” or on the rising edge. 
     FIG. 10 shows a graph of current draw during a clock cycle period of the rising edge or falling edge noise reduction circuits as shown in FIGS. 6-9. In both circuits, the results in reducing the current draw during the clock signal switching are similar. Specifically, the graph of FIG. 10 is set up on the same scale as the graph of the prior art performance shown in FIG.  4 . The value “I”  35  represents the full value of a current draw. The value “¾ I”  37  represents 75% of the fall value while the value “½ I”  39  represents 50% of the full value. The first current draw of the graph  70  represents the draw that results from the leading edge of a clock cycle (at clock cycle=0). The second current draw  72  represents the draw that results from the falling edge of the clock cycle (at clock cycle=t/2). As shown, the leading edge draw  70  and the trailing edge draw  72  are both at about 75% (¾I)  37  of the full current draw. This represents a substantial improvement in noise reduction by reducing the peak current draw while only slightly increasing the companion current draw. These results are consistent for either a falling edge or a rising edge noise reduction circuit. Consequently, such a reduction in the current draw during the switching for a clock signal will reduce the noise generated by the clock signal. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.