Abstract:
Generating a clock signal includes: at a root node of a clock distribution network, receiving a first clock signal; at a first leaf node of the clock distribution network, detecting a reference event and generating a synchronizing signal based on the detection of the reference event; passing the synchronizing signal along a synchronizing signal path from the first leaf node to the root node via one or more clocked storage cells, each storage cell being clocked from a corresponding point within the clock distribution network; at the root node, generating a second clock signal from the first clock signal synchronized to the synchronizing signal received at the root node, and distributing the second clock signal to the leaf nodes of the clock distribution network, the generating of the second clock signal resulting in the second clock signal received at the first leaf node being synchronized to the detected reference event.

Description:
BACKGROUND 
       [0001]    This invention relates to clock signal generation for an integrated circuit. 
         [0002]    A clock on an integrated circuit is often distributed over a network that imposes a significant delay to the clock, but that yields synchronization of the distributed clock at remote nodes of the network. For example, a tree structure may be used for such clock distribution. In some systems, the clock is generated such that it is synchronized with an external reference clock at the leaves of the network. 
         [0003]    It may be desirable to divide the clock to reduce the clocking rate of circuitry, for example, to reduce power consumption. However, such clock frequency division may result in the clock not being suitably synchronized with the external clock reference. 
       SUMMARY 
       [0004]    In a general aspect, an approach to generating a clock signal can provide the ability to divide the frequency of a clock while maintaining suitable synchronization with an external clock reference. 
         [0005]    In one aspect, in general, a method for generating a clock signal includes, at a root node of a clock distribution network, receiving a first clock. At a first leaf node of a plurality of leaf nodes of the clock distribution network, a reference event is detected and a synchronizing signal is generated based on the detection of the reference event. The synchronizing signal is passed along a synchronizing signal path from the first leaf node to the root node via one or more clocked storage cells, each storage cell being clocked from a corresponding point within the clock distribution network. At the root node, a second clock is generated from the first clock synchronized to the synchronizing signal received at the root node. The second clock at the root node is synchronized to the synchronizing signal with a shift in time between corresponding edges that represents a propagation delay through the clock distribution network. The second clock is distributed to the leaf nodes of the clock distribution network. The generating of the second clock results in the second clock received at the first leaf node being synchronized to the detected reference event. The second clock at the first leaf node is synchronized to a signal that caused the reference event without any sift in time between corresponding edges. 
         [0006]    In another aspect, in general, circuitry for generating a clock signal includes a clock divider having an output coupled to a root node of a clock distribution network having clock distribution paths to a plurality of leaf nodes for distributing a clock signal to the leaf nodes. A synchronizing signal generator is coupled to a first leaf node of the plurality of leaf nodes of the clock distribution network. A synchronizing signal path from the first leaf node to the root node is used for passing a synchronizing signal from the first leaf node to the root node. The synchronizing signal path includes one or more clocked storage cells each having a clock input coupled along a clock distribution path of the clock distribution network from the root node to the first leaf node. The synchronizing signal generator has an output coupled to an input of the synchronizing signal path and includes a reference event detection circuit for generating the synchronizing signal. The clock divider includes an input for receiving a generated clock and an input coupled to the synchronizing signal path, and includes a divided clock generator configured to generate a divided clock from the received generated clock synchronized to the synchronizing signal received via the synchronizing signal path. 
         [0007]    In another aspect, in general, a non-transitory computer readable medium storing a data structure which is operated upon by a program executable on a computer system, the program operating on the data structure to perform a portion of a process to fabricate an integrated circuit including circuitry described by the data structure, the circuitry described in the data structure including some or all of the circuitry for generating a clock signal referenced above. 
         [0008]    In another aspect, in general, a method for generating a clock signal includes: at a root node of a clock distribution network, receiving a first clock signal; at a first leaf node of a plurality of leaf nodes of the clock distribution network, detecting a reference event and generating a synchronizing signal based on the detection of the reference event; passing the synchronizing signal along a synchronizing signal path from the first leaf node to the root node via one or more clocked storage cells, each storage cell being clocked from a corresponding point within the clock distribution network; at the root node, generating a second clock signal from the first clock signal synchronized to the synchronizing signal received at the root node, and distributing the second clock signal to the leaf nodes of the clock distribution network, the generating of the second clock signal resulting in the second clock signal received at the first leaf node being synchronized to the detected reference event. 
         [0009]    Aspects can include one or more of the following features. 
         [0010]    Detecting the reference event includes accepting a reference clock signal and detecting a transition of said reference clock signal. 
         [0011]    Generating the second clock signal from the first clock signal includes selecting a repeating pattern of cycles of the first clock signal wherein the repeating pattern is synchronized to the received synchronizing signal, the repeating pattern having a length equal to a length of the number of consecutive cycles of the first clock signal in each cycle of the reference clock signal. 
         [0012]    The second clock signal received at the first leaf node being synchronized to the detected reference event corresponds to the second clock signal having an edge aligned with every rising edge of the reference clock signal or aligned with every falling edge of the reference clock signal. 
         [0013]    Generating the second clock signal from the first clock signal includes selecting a repeating pattern of cycles of the first clock signal, wherein the repeating pattern is synchronized to the received synchronizing signal, the repeating pattern having a length N. 
         [0014]    Generating the second clock signal includes accepting an input representing a division factor M and determining the repeating pattern of cycles according to the division factor such that a rate of the second clock signal is approximately a rate of the first clock signal divided by the division factor. 
         [0015]    The rate of the second clock signal corresponds to a number of rising edges in the repeating pattern of cycles or a number of falling edges in the repeating pattern of cycles divided by the time duration of the repeating pattern of cycles. 
         [0016]    Generating the second clock signal comprises setting a counter to a first value upon receipt of the synchronizing signal, and clocking the counter using the first clock signal in a cyclic sequence of count values, each cycle of length N. 
         [0017]    Generating the second clock signal from the first clock signal includes selecting a repeating pattern of cycles of the first clock signal, wherein the repeating pattern includes fewer than all of the cycles of the first clock signal, and at least every cycle of the first clock signal that is shifted in time by a propagation delay with respect to a rising edge of a reference clock signal that causes the detected reference event or every cycle of the first clock signal that is shifted in time by a propagation delay with respect to a falling edge of the reference clock signal. 
         [0018]    In another aspect, in general, circuitry for generating a clock signal includes: a clock divider having an output coupled to a root node of a clock distribution network having clock distribution paths to a plurality of leaf nodes for distributing a clock signal to the leaf nodes; a synchronizing signal generator coupled to a first leaf node of the plurality of leaf nodes of the clock distribution network; and a synchronizing signal path from the first leaf node to the root node for passing a synchronizing signal from the first leaf node to the root node, the synchronizing signal path including one or more clocked storage cells each having a clock input coupled along a clock distribution path of the clock distribution network from the root node to the first leaf node; wherein the synchronizing signal generator has an output coupled to an input of the synchronizing signal path and includes a reference event detection circuit for generating the synchronizing signal; and wherein the clock divider includes an input for receiving a generated clock signal and an input coupled to the synchronizing signal path, and includes a divided clock generator configured to generate a divided clock signal from the received generated clock signal synchronized to the synchronizing signal received via the synchronizing signal path. 
         [0019]    Aspects can include one or more of the following features. 
         [0020]    The circuitry further includes a reference clock providing a reference clock signal, and said reference event detection circuit detects a transition of said reference clock signal. 
         [0021]    Generating the divided clock signal from the generated clock signal includes selecting a repeating pattern of cycles of the generated clock signal wherein the repeating pattern is synchronized to the synchronizing signal, the repeating pattern having a length equal to a length of the number of consecutive cycles of the generated clock signal in each cycle of the reference clock signal. 
         [0022]    The divided clock signal received at the first leaf node being synchronized to the detected reference event corresponds to the divided clock signal having an edge aligned with every rising edge of the reference clock signal or aligned with every falling edge of the reference clock signal. 
         [0023]    Generating the divided clock signal from the generated clock signal includes selecting a repeating pattern of cycles of the generated clock signal, wherein the repeating pattern is synchronized to the received synchronizing signal, the repeating pattern having a length N. 
         [0024]    Generating the divided clock signal includes accepting an input representing a division factor M and determining the repeating pattern of cycles according to the division factor such that a rate of the divided clock signal is approximately a rate of the generated clock signal divided by the division factor. 
         [0025]    The rate of the divided clock signal corresponds to a number of rising edges in the repeating pattern of cycles or a number of falling edges in the repeating pattern of cycles divided by the time duration of the repeating pattern of cycles. 
         [0026]    Generating the divided clock signal comprises setting a counter to a first value upon receipt of the synchronizing signal, and clocking the counter using the generated clock signal in a cyclic sequence of count values, each cycle of length N. 
         [0027]    Generating the divided clock signal from the generated clock signal includes selecting a repeating pattern of cycles of the generated clock signal, wherein the repeating pattern includes fewer than all of the cycles of the generated clock signal, and at least every cycle of the generated clock signal that is shifted in time by a propagation delay with respect to a rising edge of a reference clock signal that causes the reference event or every cycle of the generated clock signal that is shifted in time by a propagation delay with respect to a falling edge of the reference clock signal. 
         [0028]    Aspects can have one or more of the following advantages. 
         [0029]    Frequency division of a clock (GCLK) can be achieved while maintaining the alignment to an external reference clock (REF), which is running at an integer submultiple of the GCLK frequency. 
         [0030]    Other features and advantages of the invention will become apparent from the following description, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0031]      FIG. 1  is a block diagram of a clock generation system. 
           [0032]      FIG. 2  is a timing diagram of clocks in the system of  FIG. 1 . 
           [0033]      FIG. 3  is a schematic of a synchronizing signal path. 
           [0034]      FIG. 4  a timing diagram of signals in the system of  FIGS. 1 and 3 . 
           [0035]      FIG. 5  is a block diagram of a implementation of a clock divider. 
       
    
    
     DESCRIPTION 
       [0036]    Referring to  FIG. 1 , a clock generation system  100  for an integrated circuit generates various clock signals (or simply “clocks”) that exhibit synchronization characteristics that may be useful (or required) for operation of the integrated circuit. The system  100  makes use of a reference clock (REF, also referred to below as φ 0 ), which may be provided externally to the integrated circuit. This reference clock is used by a clock generator  105  to generate a derived clock (GCLK, also referred to below as φ 1 ) with N times the clock rate of the reference clock. A clock divider  110  either passes the derived clock φ 1  without modification or after clock division according to a factor M as its output. The approach to clock division is described in detail below. The divided clock output (DIV, also referred to below as φ 2 ) of the clock divider  110  is passed to a clock distribution network, in this illustration in  FIG. 1  a clock tree  150 , which distributes the divided clock to the leaves of the clock tree. Is should be recognized that a tree is only one possible form of distribution network and other structures of network can be used. In general, the propagation time from the root to the leaves of the clock tree  150  can be more than one clock cycle, and for a balanced clock tree  150  the propagation times from the root to the leaves of the clock tree  150  are substantially equal. The clock generator  105 , for example, uses a Phase-Locked Loop (PLL) approach and compensates for the delay through the clock tree. In some implementations, the delay compensation can be performed using a Delay-Locked Loop (DLL) connected to the output of the PLL. This delay is tuned such that the clock generator  105  causes the rising edge of the reference clock (φ 0 ) at a leaf to be aligned with the rising edge of the clock signal φ 2   (LEAF)  at the leaf when there is no clock division. That is, if M=1, and therefore φ 2   (ROOT) =φ 1   (ROOT)  at the root (as indicated by the superscript “(ROOT)”) of the clock tree  150 , at the leaf each the rising edge of distributed clock φ 2   (LEAF)  is aligned with the rising edge of the φ 0 . While the synchronization procedures are described herein with respect to a particular leaf, it is understood that synchronization may be attained at each of multiple leaves to the extent that the timing of the reference clock φ 0  is substantially the same at each leaf. 
         [0037]    In some situations, it is desirable to divide the distributed clock by a factor M, for example, to reduce power consumption of clocked circuitry. If N is a multiple of M, and the φ 2   (ROOT)  were generated from φ 1   (ROOT)  according to a division by M that is achieved by passing every M th  clock pulse, then it is possible that each rising edge of φ 2   (LEAF)  would be aligned with the rising edge of the φ 0  at the leaves of the clock tree, but it is also possible that the rising edges of φ 0  would never be aligned with a rising edge of φ 2   (LEAF) . Furthermore, if N is not a multiple of M, then division by M may result in some rising edges of φ 0  being aligned with a rising edge of φ 2   (LEAF) , and some not. Clock divider  110  implements an approach that guarantees that for any M≠1 the derived clock φ 2   (LEAF)  has a clock pulse aligned with each cycle of the reference clock, and more particularly in this embodiment, that φ 2   (LEAF)  has a rising edge aligned with each rising edge of the reference clock φ 0 . 
         [0038]    Referring to  FIG. 2 , a situation in which N=9 is illustrated such that there are N=9 clock periods of φ 1  for every period of φ 0 . Note that in practice, the value of N may be much larger, for example, N=50, while N=9 is chosen for here for illustration. In the case that there is no clock division (M=1), the clock at the leaf, φ 2   (LEAF) (DIV M=1) is simply a delayed version of the generated clock at the root, φ 1   (ROOT) . In this illustration, the delay through the clock tree  150  is approximately 1.2 clock periods T 1  of the generated clock φ 1 . As illustrated by the dotted lines in  FIG. 2 , each rising edge of the reference clock φ 0  (occurring at the times at which they would occur at the synchronized leaf of the clock tree  150 ) is aligned with the rising edge of a clock pulse of φ 2   (LEAF)  (DIV M=1). 
         [0039]    For M≠1, operation of the clock divider  110  can be understood by the illustration of a division by M=4. The system  100  is configured, as described in more detail below, to enable the clock divider  110  to generate a periodic output signal that has most rising edges occurring at a divided frequency, and retains additional pulses whose rising edges are synchronized with the reference clock φ 0 , which does not sacrifice a desired power savings of operation at the lower divided frequency. In this case, φ 2   (LEAF)  (DIV M=4) has a clock pulse aligned with each rising edge of φ 0  (these pulses are indicated by dashed circles), and other pulses separated by no more than M−1=3 inhibited pulses. In this example, if the cycles of the an undivided clock (GCLK) are numbered 0, 1, 2, . . . , 7, 8, 0, 1, 2, such that cycle φ 0  of the undivided but delayed clock at the leaf φ 2   (LEAF)  (DIV M=1) is aligned with the reference clock φ 0  rising edge, clock pulses 0, 4, 8, 0, 4, . . . of φ 1   (ROOT)  are selected (i.e., retained) and the others are inhibited to form φ 2   (ROOT)  at the root. The clock divider  110  generates the clock φ 2   (ROOT)  both to have such a pattern, and also to ensure that the corresponding pattern at the leaf φ 2   (LEAF)  (DIV M=4) is aligned with (i.e., has coincident rising edges with) the reference clock φ 0  at the leaf. For example, a misaligned sequence that retained pulses 2, 6, 1, 2, . . . would not be suitable. 
         [0040]    The clock divider  110  does not simply make use of φ 0  at the root to properly align the divided clock pattern. Rather, a sync generator  120  coupled to at least one leaf detects a rising edge of φ 0  at the leaf and generates a synchronization signal at the detected time at the leaf. This synchronizing signal is propagated to the clock divider  110  at the root of the clock tree along a synchronizing signal path. The synchronizing signal path for propagating the synchronization signal from the leaf to the root of the tree may include a series of clocked storage cells (e.g., flip flops), such that the propagation of the synchronization signal takes a fixed number cycles of the clock being distributed over the clock tree. Generally, the clock divider  110  makes use of knowledge of this fixed number of cycles, and the factors M and N, to select pulses of the generated clock φ 1   (ROOT)  to form φ 2   (ROOT) , such that after propagation φ 2   (LEAF)  is aligned as desired with the reference clock φ 0 . Alternatively, in some implementations, if the propagation delay over the path from the root to the leaf of the clock distribution network is less a full period T 1  of the generated clock φ 1 , then it may be possible to provide a synchronizing signal path without any clocked storage cells. 
         [0041]    Referring back to  FIG. 1 , the synchronizing signal path of a synchronizing signal SYNC (LEAF)  at a leaf of the clock tree  150  to yield a synchronizing signal SYNC(ROOT) passes through a number of flip-flops  162 , and over clock distribution lines  164  linking the flip-flops. In  FIG. 1 , there are K=4 flip-flops  162  on the path from the leaf to the root. In this example, these flip-flops are clocked on the negative edge of the clock signals at respective points (A, B, C, D) along the distribution lines  164 . Note that the flip-flops are arranged along a clock path from the root to the leaf of the clock tree  150  (or at points in the distribution network with equivalent delay from the root node), and therefore due to the propagation delay of the clock signal, are clocked in succession. Of course, any of a variety of implementations could be used in which various digital circuitry, such as the flip-flops  162 , are clocked on the positive edges instead of the negative edges, or on the negative edges instead of the positive edges, as long as the entire system  100  is designed consistently to use the appropriate clock edges at the appropriate locations. 
         [0042]    Referring to  FIG. 3 , the K=4 flip-flops  162 , are illustrated in schematic form with the clock distribution lines  164  separating the flip-flops  162 . In this example, the propagation delay along each of the lines  164  is about 40% of a period of φ 1 , such that the total delay from the root to the leaf of the clock distribution network is about 1.2 times the period of φ 1 . A more precise constraint on the relationship between the period of φ 1  and the propagation times along lines  164  between the flip-flops  162  is discussed below. 
         [0043]    Referring to  FIG. 4 , a transition from an undivided clock (M=1) to a divided clock (M=4) is illustrated. During the undivided phase, φ 2   (ROOT) =φ 1   (ROOT)  is generated such that after propagation through the clock distribution network, φ 2   (LEAF)  is aligned with φ 0 , as indicated by the left-most dotted vertical line. When the clock divider  110  receives an input to make a transition to M=4 clock division, it continues to pass each clock pulse of φ 1   (ROOT)  as the divided clock φ 2   (ROOT)  until it receives a synchronizing signal from the leaf. The clock divider  110  ignores subsequent synchronizing signals it receives until the division factor M is again changed. Alternatively, there may be an explicit signal that enables and disables the division even without the division factor M changing. When division is disabled, the undivided clock would again be provided. 
         [0044]    At the leaf, the sync generator  120  generates a synchronizing signal SYNC (LEAF)  at the first clock pulse of the received clock φ 2   (LEAF)  after it detects the rising edge of the reference clock φ 0 . This synchronizing signal is passed through the series of flip-flops  162  that are illustrated in  FIG. 3  and in  FIG. 1 . Continuing to refer to  FIG. 4 , with the flip-flops  162  clocked on the falling edges of the clock signal in the clock distribution network, the synchronizing signal propagates back to the root. Note that at the root, the synchronizing signal SYNC (ROOT)  is asserted K=4 clock cycles after the clock pulse of φ 0   (ROOT)  that after propagation to the leaf was aligned with φ 0 . Therefore, after the arrival of the synchronizing signal at the root, there remain N−K=9−4=5 cycles before the next clock pulse of φ 1   (ROOT)  that would propagate through the clock distribution network to be synchronized with φ 0 . Generally, the clock divider  110  uses the arrival of the synchronizing signal at the root of the clock tree to initialize a cyclic counter (i.e., counting in a cycle of length N), with count n. The count sequence is illustrated in  FIG. 4 . In this embodiment, the counter is initialized at n=K, and then increments the counter K, K+1, . . . , N−1, 0, 1, . . . on the negative edges of φ 1   (ROOT)  Clock pulses of φ 2   (ROOT)  generated at n=0 propagate to the leaf of the tree to be aligned with the rising edge of φ 0 . Therefore, the clock divider  110  decodes the counter value n to select pulses of the generated clock φ 1   (ROOT)  at n=0 as well as at intervals n=M, 2M, . . . etc for n&lt;N. For the case of N=9 and M=4, the clock divider  110  selects the clock pulses at n=0, 4, and 8. Reversion to an undivided clock (M=1) is performed by the clock divider  110  by again passing all the pulses of φ 1   (ROOT)  until it yet again receives a signal to divide the clock. 
         [0045]    Referring to  FIG. 5 , one of many alternative implementations of the clock divider  110  is shown. Load/enable logic is responsive to the SYNC (ROOT)  signal from the leaf to assert a load input of a mod N counter  530  thereby initializing the counter to K as discussed above and enable the counter after the receiving the synchronizing signal. Select logic  540  uses the count output of the counter  530  to generate a select signal that is passed to an AND gate  550 . The AND gate selects the pulses of the clock φ 1   (ROOT)  to generate the divided clock φ 2   (ROOT)  Note that alternative implementations of the clock divider do not necessarily use a counter while maintaining the same function. For example, rather than a counter, a shift register, multiple counters (e.g., one with a cycle of length N and one with a cycle of length M) or an arrangement of flip-flops can be used to generate a select signal that passes the selected pulses of φ 1   (ROOT) . 
         [0046]    Note that the approach above does not depend on the specific period of φ 1 . For example, if the period of φ 1  is doubled, then instead of the delay through the example clock tree  150  of  FIG. 1  being 1.2 periods, the delay would be 0.6 periods. However, the propagation of the synchronizing signal would nevertheless take K=4 periods to propagate from the leaf to the root. Therefore, correct operation is maintained. The clock period of φ 1  can be reduced subject to a timing constraint that can be understood with reference to  FIG. 3 . Consider for example a falling edge of φ 2  passing point “A” at time 0. The flip-flop at point “B” will be clocked at the propagation delay from point “A” to point “B” along path  164 . There is then a clock-to-Q delay for the flip-flop  162 , and a propagation delay from the output (Q) of the flip-flop  162  to the input (D) of the next flip-flop  162 . The input needs to be setup for at least the minimum setup time for the flip-flop. This must all happen before the next falling edge of the φ 2  clock at point “A”, which occurs at time T 2 , the period of φ 2 . Generally, there are the following constraints. 
         [0000]    Setup constraint: 
         [0047]    T 2 &gt; (clock propagation time between flip-flops along path  164 
       + clock-to-Q delay of flip-flop  162     + signal propagation time between flip-flops   + setup time for D input of flip-flop  162 )
 
Hold constraint (typically independent of clock frequency):
       
 
         [0051]    (clock propagation time between flip-flops along path  164 
       + clock-to-Q delay of flip-flop  162     + signal propagation time between flip-flops)   &gt; hold time for D input of flip-flop  162 
 
In general, N will be greater than K. However, this is not required as instead if initializing the counter n at K as described above, it can be initialized at K mod N to achieve the required alignment. Also, the approach is not dependent on a particular delay through the clock distribution network remaining stable. As long as the constraint on the period T 2  is maintained, then the approach continues to function correctly even if the clock delay changes, for example, due to power of environmental (e.g., temperature) changes.
       
 
         [0055]    It should be understood that although discussed in the context of generating a divided clock φ 2  to be aligned with a reference clock φ 0 , the approach can be applied to a variety of situations in which a clock φ 2   (ROOT)  must be generated so that after propagation over a delay path is synchronized to an reference event that is detected at the end of the delay path. The rising or falling edge of a reference clock is but one example of a reference event to which the clock is to be synchronized. Furthermore, clock division is but one example of the type of clock that can be synchronized with the reference event using the approaches described above. 
         [0056]    In some implementations, a computer accessible storage medium includes a database representative of the system  100 . Generally speaking, a computer accessible storage medium may include any non-transitory storage media accessible by a computer during use to provide instructions and/or data to the computer. For example, a computer accessible storage medium may include storage media such as magnetic or optical disks and semiconductor memories. Generally, the database representative of the system may be a database or other data structure which can be read by a program and used, directly or indirectly, to fabricate the hardware comprising the system. For example, the database may be a behavioral-level description or register-transfer level (RTL) description of the hardware functionality in a high level design language (HDL) such as Verilog or VHDL. The description may be read by a synthesis tool which may synthesize the description to produce a netlist comprising a list of gates from a synthesis library. The netlist comprises a set of gates which also represent the functionality of the hardware comprising the system  100 . The netlist may then be placed and routed to produce a data set describing geometric shapes to be applied to masks. The masks may then be used in various semiconductor fabrication steps to produce a semiconductor circuit or circuits corresponding to the system  100 . In other examples, the database may itself be the netlist (with or without the synthesis library) or the data set. 
         [0057]    Other embodiments are within the scope of the following claims.