Abstract:
A circuit and method for synchronized clocking of components such as registers. Registers are clocked by individual component clock signals having the same frequency but potentially different phases due to differing propagation delays. Separate component clock signals are received by registers are brought into phase by evaluating the phases of the component clock signals at the registers, and synchronizing the component clock signal of each register to that of the previous register in a sequence.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to clock signal synchronization in integrated circuits.  
       BACKGROUND  
       [0002]     Integrated circuits (IC), including application specific integrated circuits (ASIC), are increasing in processing capability and are shrinking in physical size. Smaller ICs contain added components such as digital receiving and processing devices. Decreasing the size of ICs has led to an increase in IC processing speed since communication paths are decreased between IC components.  
         [0003]     As IC size decreases, however, resistance-capacitance (RC) time delay of metal interconnects between IC components begins to limit IC performance. Interconnect RC time delay is associated with metal resistance of interconnections and capacitance associated with dielectric media. Because metal resistance and dielectric media are inherently part of the materials used in construction of an IC, only a change in materials will affect (improve) RC time delay. A change in materials may be technically impossible or cost prohibitive.  
         [0004]     Differences in propagation delay, when compounded across all interconnections, such as clock nets or paths, in a complex IC may lead to unacceptable degradations in overall system-timing. This problem is often referred to as “clock skew.” 
         [0005]      FIG. 1  illustrates a clock tree that distributes clock signals in a controlled manner. An IC may contain numerous clocked components requiring clock signals. A clock tree or similar clock architecture provides the necessary clock signals to the components. Components within an IC, specifically registers of the components, may require that the clock signals be synchronized. To be considered “synchronized,” clock signals have the same phase at different receivers, despite propagation delays.  
         [0006]     In this particular example, clock receiving components  10 ,  15 ,  20 , and  25  reside on a single IC. Components  10 ,  15 ,  20 , and  25  may be at varying distances from one another. In other words component  10  may be an unequal distance from component  15 , as component  15  is to component  20 . Oftentimes, due to IC design constraints or physical architecture restrictions on an IC, components must be placed at varying locations at varying distances from one another. In this example, components  10 ,  15 ,  20 , and  25  are components that must be synchronized with one another (i.e. have the same phase clock signals). Further, since components are placed at varying distances from one another, components may also be located at varying distances from a clock source such as clock driver  30 . Since clock signals travel over varying distances from the clock source to the components, assuring that each clock signal is in phase with the other clock signals becomes a complicated task.  
         [0007]     In typical clock architectures such as the clock tree of  FIG. 1 , a controller such as controller  35  initiates a clock signal. Controller  35  can be located on an IC (on-chip) or external to an IC (off-chip). Controller  35  instructs clock driver  30  to generate a clock signal. Clock driver  30  may be implemented for example as a clock oscillator or clock generator or similar component. Alternatively, clock driver  30  may be a clock buffer. A clock signal transmitted by clock driver  35  is passed on to fan-out clock drivers  40 ,  45 ,  50 ,  55 ,  60 , and  65 . All clock signals derived from clock driver  30  have the same frequency, although clock signals arriving at various components or registers may have different phase values.  
         [0008]     To assure that the clock signals arriving at components  10 ,  15 ,  20 , and  25  are properly synchronized and have the same phase, paths  70 ,  75 ,  80 , and  85  must have approximately the same length and propagation delay characteristics. If components  10 ,  15 ,  20 , and  25  are not located equidistant from their respective clock drivers  50 ,  55 ,  60 , and  65 , certain paths may have to be wrapped around to assure equal lengths and propagation characteristics of all paths. When IC space is at a premium, this approach may not be feasible. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a clock tree diagram in accordance of the prior art.  
         [0010]      FIG. 2  is a schematic illustrating component synchronization for multiple registers in an IC.  
         [0011]      FIG. 3  is a schematic illustrating a phase feedback element that makes use of a phase comparator and clock skew register.  
         [0012]      FIG. 4  is a schematic illustrating a phase feedback element that makes use of matched current sources. 
     
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 2  illustrates a circuit having registers that are to be synchronously clocked. Specifically, this circuit has a plurality of components  200 ,  205 , and  210 . In this example, each component comprises one or more multi-bit or byte-word registers. Each byte-word register might have eight individual bit registers, as shown, or some other number of bit registers, typically from 8 to 16 bit registers. In certain other embodiments, all bit registers might be treated as separate registers.  
         [0014]     In  FIG. 2 , component  200  is made up of bit registers  200 A-H. Component  205  is made up of bit registers  205 A-H. Component  210  is made up of bit registers  210 A-H. In this example, bit registers  200 A-H,  205 A-H, and  210 A-H make up a continuous string of bit registers. In other words, bit registers  200 A-H,  205 A-H, and  210 A-H are physically laid out contiguous to one another (side by side). Bit registers  200 A-H,  205 A-H, and  210 A-H may be arranged in a particular sequence. For example, for a pair of components, the last bit register of a first component may be located adjacent the first bit register of a second component. Therefore, bit register  200 H is placed directly adjacent to bit register  205 A, and bit register  205 H is placed directly adjacent to bit register  210 A. Logically, however, bit registers  200 A-H,  205 A-H, and  210 A-H are configured to comprise separate components (i.e., they are logically part of components  200 ,  205 , and  210 ). As separate components, bit registers  200 A-H,  205 A-H, and  210 A-H receive separate component clock signals.  
         [0015]     Components  200 ,  205 , and  210  and their respective bit registers are intended to be synchronized with one another. In other words, these components are intended to be synchronously clocked. To achieve this, component clock signals to each byte-word register are adjusted to have matching phases at the byte-word registers, after accounting for any differing propagation delays of the component clock signals. A factor determining propagation delay difference is the difference between the lengths of the paths. In a preferred embodiment, the difference between propagation delays is less than 15%.  
         [0016]     The described embodiment includes a clock driver corresponding to each set of components, which in this case equates to a separate clock driver for each respective byte-word register. Thus, a clock driver  215  provides a component clock signal  218  to bit registers  200 A-H of component  200 . Component clock signal  218  travels along a path  219  from clock driver  215 . Path  219  branches out to sub-paths  219 A-H which lead to individual bit registers  200 A-H, respectively. Clock driver  220  provides a component clock signal  222  to bit registers  205 A-H of component  205 . Component clock signal  222  travels along a path  224  from clock driver  220 . Path  224  branches out to sub-paths  224 A-H which lead to individual bit registers  205 A-H, respectively. Clock driver  225  provides a component clock signal  227  to bit registers  210 A-H of component  210 . Component clock signal  227  travels along a path  229  from clock driver  225 . Path  229  branches out to sub-paths  229 A-H which lead to individual bit registers  210 A-H, respectively. Therefore, the clock drivers  215 ,  220 , and  225  provide separate clock signals to each of the bit registers  200 A-H,  205 A-H, and  210 A-H by way of separate paths.  
         [0017]     Clock driver  215 ,  220 , and  225  may receive input clock signals from a common source such as a clock tree. Such a clock tree architecture may be part of the same IC in which components  200 ,  205 , and  210  reside or may be part of another IC.  
         [0018]     In this example, a master clock driver  230  produces a common clock signal  232  that branches out to clock drivers  215 ,  220 , and  225 . Since clock drivers  215 ,  220 , and  225  derive respective component clock signals  218 ,  222 , and  227  from common clock signal  232 , each of the component clock signals is a variably-delayed version of common clock signal  232 .  
         [0019]     Since component clock signals  218 ,  222 , and  227  originate from a common clock signal source, they have the same frequency. However, as component clock signals  218 ,  222 , and  227  travel across respective paths  219 ,  224 ,  229 , and the sub-paths leading to individual bit registers, component clock signals  218 ,  222 , and  227  traverse potentially different distances. Different distances result in differing propagation delays, which result in component clock signals that are potentially out of phase with each other as they are received at the respective components  200 ,  205 , and  210 . Clock drivers  215 ,  220 , and  225  are capable of varying the phase of component clock signals  218 ,  222 , and  227  so that the phases of the component clock signals  218 ,  222 , and  227  are synchronized upon arrival at the bit registers of components  200 ,  205 , and  210 .  
         [0020]     A reference clock signal  240  is used to correct the phases of component clock signals  218 ,  222 , and  227 , so that they are in phase with each other at the physical locations of the byte-word registers  210 ,  215 , and  220 . Reference clock signal  240  has the same frequency as clock signals  218 ,  222 , and  227 . Reference clock signal  240  may be generated by an arbitrary clock source; however, it is contemplated that reference clock signal  240  may be provided by or derived from the same clock tree or clock architecture from which component clock signals  218 ,  222 , and  227  are derived. In certain cases, one of clock signals  218 ,  222 , and  227  may be branched and used as reference clock signal  240 . It is not necessary for reference clock signal  240  to have any particular phase relationship with the component clock signals  218 ,  222 , and  227 , although its phase preferably remains constant over time as compared to the component clock signals.  
         [0021]     The circuit of  FIG. 2  has a reference feedback element  250  that receives component clock signal  218  from path  252 . Path  252  is a continuation of one of the sub-paths  219 A-H and originates from near register  200 . In this example, path  252  is connected to path  219 D. Reference clock signal  240  travels along path  254  to reference phase feedback element  250 . Reference phase feedback element  250  compares the phases of component clock signal  218  and reference clock signal  240 , and provides an adjustment signal  251  to clock driver  215 . Adjustment signal  257  represents an advance or delay value that allows component clock signal  218  to become in phase with reference clock signal  240 . An adjusted component clock signal  218  may then be used as a reference clock signal for other component clock signals. In other words, when the components are considered in sequence, the component clock signal to any particular component is matched in phase to the component clock signal of the immediately preceding component in the sequence.  
         [0022]     Note that in certain embodiments, reference clock signal  240  may not be used. In this case, the component clock signals of the components are simply synchronized to that of the first component in the sequence.  
         [0023]     In addition to reference phase feedback element  250 , the circuit includes phase feedback elements  255  and  260  corresponding to adjacent pairs of components. The phase feedback element corresponding to a particular pair of components receives the component clock signal from a register of each of the components of the particular pair. The component clock signal in each case is routed from a point physically near its corresponding register (of the corresponding component). The phase feedback element is responsive to the received component clock signals to adjust the phase of one of the component clock signals to match the phase of the other component clock signal. More particularly, each phase feedback element receives a first component clock signal from a particular register and a second component clock signal from an immediately subsequent register in sequence, and adjusts the second component clock signal to match the phase of the first component clock signal.  
         [0024]     With specific reference to  FIG. 2 , phase feedback element  255  receives component clock signal  218  from path  256 , and component clock signal  222  from path  257 . Path  256  is a continuation of one of the sub-paths  219 A-H, and path  257  is a continuation of one of the sub-paths  224 A-H. In this example, path  256  continues sub-path  219 H and path  257  continues sub-path  224 A. Sub-paths  219 H and  224 A may and are expected to differ in length. To assure that component clock signals  218  and  222  have the same phase at the respective registers, paths  256  and  257  should be equal in length and have the same or similar propagation delay characteristics. A factor determining propagation delay difference is the difference between the lengths of the paths. In a preferred embodiment, the difference between propagation delay of length of paths  256  and  257  is less than 15%.  
         [0025]     Paths  256  and  257  couple their respective components to phase feedback element  255 . Phase feedback element  255  determines the phase difference between component clock signals  218  and  222 , and generates an adjustment signal  267 , which is provided to clock driver  220  in either analog or digital form (analog skew or digital skew values). Adjustment signal  267  is a measure of an advance or delay that allows component clock signal  222  to become in phase with component clock signal  218 . An adjusted clock signal  222  may then be used as a “reference clock” signal for other component clock signals.  
         [0026]     In a similar manner, phase feedback element  260  receives component clock signal  222  from path  261 , and component clock signal  227  from path  262 . Path  261  is a continuation of one of the sub-paths  224 A-H, and path  262  is a continuation of one of the sub-paths  229 A-H. In this example, path  261  continues sub-path  224 H and path  262  continues sub-path  229 A. Sub-paths  224 H and  229 A may and are expected to differ in length. To assure that component clock signals  222  and  227  have the same phase at the respective registers, paths  261  and  262  should be equal in length and have the same propagation delay characteristics.  
         [0027]     Paths  261  and  262  couple their respective components to phase feedback element  260 . In this example, phase feedback element  260  corresponds to the adjacent pair of components  200  and  205 . Phase feedback element  260  determines the phase difference between component clock signals  218  and  222 , and generates an adjustment signal  277  which is provided to clock driver  225  in either analog or digital form (analog skew or digital skew values). Adjustment signal  277  is a measure of an advance or delay that allows component clock signal  227  to become in phase with component clock signal  222 . Since component clock signal  222  has been adjusted to match the phase of component clock signal  218 , it follows that component clock signal  227  is adjusted to match the phase of component clock signal  218 .  
         [0028]     Although this example describes synchronization of component clock signals from a left to right sequence beginning with a left most component, it is contemplated that synchronization may start with any component clock signal, including a component clock signal received at a middle component (byte-word register) or middle bit register.  
         [0029]      FIG. 3  illustrates an exemplary embodiment of phase feedback element  255 . Phase feedback element  260  is similarly implemented. This implementation of feedback element  255  is particularly appropriate in circuits where components or registers have 10 or fewer bits.  
         [0030]     Phase feedback element  250  includes a phase comparator  305 . Phase comparator  305  receives component clock signals from a pair of components; allowing the phase feedback element to adjust the phase of one of the component clock signals to match that of the other component clock signal. In particular examples, the phase comparator  305  receives a clock signal from a first bit register of a plurality of bit registers in a component and a clock signal from a last bit register of a plurality of bit registers in a second component. The clock signals from these bit registers may be routed through paths that have matched lengths. In this example, phase comparator  305  receives component clock signals  218  and  222 , and determines the phase difference between component clock signals  218  and  222 . Phase comparator  305  may include a phase converter that converts the phase difference to a phase offset value or digital skew time value  310 . Digital phase offset value or digital skew time value  310  may be stored in a clock register  315 . Based on digital phase offset value or digital skew time value  310 , clock register  315  instructs clock driver  220  to advance or delay transmission of component clock signal  222 . Digital phase offset value or digital skew time value  310  is used as adjustment value  267  of  FIG. 2 .  
         [0031]     Referring now to  FIG. 4 , illustrated is an exemplary embodiment of a phase feedback element  255  using current sources. Phase feedback element  260  is similarly implemented. This implementation is particularly appropriate in circuits having more than 10 bits in each component or register.  
         [0032]     In this embodiment, phase feedback element  255  has an integrator or capacitance  405  and current sources  415  and  420 . Current sources  415  and  420  are controlled by switches  425  and  430 , respectively. Current source  415  corresponds to component  200  and current source  420  corresponds to component  205 , where in this example components  200  and  205  are treated as a pair. Switches  425  and  430  are preferably implemented as transistors. Current sources  415  are connected through the respective switches  425  and  430  to charge and discharge capacitance  405 . Specifically, current source  415  is connected through and enabled by switch  425  to charge capacitance  405  when switch  425  is closed. Current source  420  is connected through and enabled by switch  430  to discharge capacitance  405  when switch  430  is closed. Current sources  415  and  420  preferably source equal currents, albeit in opposite directions. In other words, current sources  415  and  420  are matched current sources.  
         [0033]     Switches  425  and  430  are selectively enabled or controlled by the component clock signals of the two adjacent components corresponding to phase feedback element  255 , in this case by component clock signals  218  and  222 . Switch  425  is closed when component clock signal  218  is logically true or high, and is open when component clock signal  218  is logically false or low. Switch  430  is closed when component clock signal  222  is logically true or high and is open when component clock signal  222  is logically false or low. If the component clock signals  218  and  222  are in phase, switches  425  and  430  close and open at the same times, and the net effect of the opposite current sources  415  and  420  is null—the capacitance  405  is neither charged nor discharged.  
         [0034]     If, on the other hand, component clock signals  218  and  222  are out of phase, switches  425  and  430  do not close and open at the same times, and there is a net charging or discharging effect on capacitance  405 . Assuming a relatively large capacitance  405 , the voltage at the capacitance will increase or decrease, relative to ground, in accordance with any phase difference between the two component clock signals  218  and  222 . Thus, the voltage at capacitive node  435  represents a phase difference between component clock signals  218  and  222 . In the illustrated embodiment, phase feedback element  255  has an analog to digital (A/D) converter  440  that converts the analog skew or voltage value at capacitive node  435  to a digital skew time value  445 . Clock driver  220  advances or delays component clock signal  222  based on digital skew time value  445 . Clock driver  220  therefore is responsive to the voltage value at capacitive node  435 . In other embodiments, analog skew value  435  is passed directly on to clock driver  220 .  
         [0035]     Although details of specific implementations and embodiments are described above, such details are intended to satisfy statutory disclosure obligations rather than to limit the scope of the following claims. Thus, the invention as defined by the claims is not limited to the specific features described above. Rather, the invention is claimed in any of its forms or modifications that fall within the proper scope of the appended claims, appropriately interpreted in accordance with the doctrine of equivalents.