PRIORITY CLAIM 
   This application claims priority from Indian patent application No. 836/Del/2002, filed Aug. 12, 2002, which is incorporated herein by reference. 


   TECHNICAL FIELD 
   This invention relates generally to an improved electronic phase locked Loop (PLL) for integrated circuits. 
   BACKGROUND 
   Phase Locked Loops (PLLs) are widely used for synchronizing digital clock signals. Digital integrated circuits make use of PLLs in compensating for internal signal-to-signal delays also known as “skews”. Skew compensations are often critical to the functioning of the circuit, especially when fast clock speeds are involved. The clock signal in high-speed synchronous digital integrated circuits is particularly sensitive to skews, and a great deal of attention is therefore paid to skew compensations in the distribution of this signal. In addition, the internal clock is generally required to be synchronized to an external clock source. Large integrated circuits make of clock tree structures to provide a structured and balanced distribution of the clock across the device. 
   The internal delays of signals in integrated circuits arise from delays contributed by circuit elements as well as delay contributed by the physical path traversed by the signal. Signal path delays are significantly influenced by the physical layout of the device. In signal skew compensation applications it is therefore necessary for the PLL to map the internal signal delays of the two signals to be synchronized. As the signal delays are dependant on the physical layout, the mapping is layout dependant. The design of the PLL block in the integrated circuit is therefore delayed until the rest of the device has been completed. Considering the importance of reducing the development cycle time for integrated circuits it would be desirable to have a mechanism that could enable the PLL design to overlap the design of the remainder of the device. 
   PLL operation requires a continuous operation of the clock sources that are required to be synchronized. In the conventional practice when there is no SLEEP modes in a circuit, the point chosen for the feedback is at the leaf of the clock tree as shown in  FIG. 1  of the accompanying drawings. But for the circuits with sleep mode as shown in  FIG. 2  of the accompanying drawings, during the sleep mode the clock terminates from point  206  onwards, which in turn makes the feedback loop ineffective, as there is no signal activity at point  208 . A detailed description of  FIGS. 1 and 2  has been given below after the description of the accompanying drawings. 
   U.S. Pat. No. 6,023,180 discloses a clock compensation circuit, which provides a PLL circuit without utilizing any analog circuitry (e.g. VCO etc.). In this patent, the digital components available in any Application Specific Integrated Chip (ASIC) vendor&#39;s library are utilized for realization of the equivalent PLL functions. The disclosed PLL includes a clock tree, a reference clock, a phase detector for detecting relative phase information of the clock tree and the reference clock, a controller coupled to the phase detector for determining and controlling the amount of delay necessary to shift the output of the clock tree in phase with the reference clock, and a programmable delay logic coupled to the controller. The programmable delay logic comprises a string of delay elements that selectively determine the amount of delay introduced. This patent also discloses providing the feedback to the PLL from the point at a leaf cell, and hence this PLL is unable to function during SLEEP mode of the circuit. 
   SUMMARY 
   One embodiment of the invention provides an improved PLL circuit that functions effectively even during SLEEP modes of operation. 
   Another embodiment of this invention enables a faster design cycle by enabling a semi custom design approach that provides for basic design of the PLL circuitry in parallel with the design of the remaining circuitry with final adjustments being possible in a short period of time at the end of the design cycle. 
   Yet another embodiment of the invention provides an improved Phase Locked Loop (PLL) for a digital integrated circuit, the PLL comprising:
         a Phase and Frequency Detector (PFD) connected to a Voltage Controlled Oscillator (VCO) providing a clock output,   a routed clock tree connected between said VCO and a leaf cell and a frequency divider in the feedback path to the PFD input, characterized in that,   the VCO output is fedback to the PFD input through a clock tree replica providing a delay equal to the routed clock tree.       

   The PLL may further include selection means connected between VCO and routed clock tree for disabling the PLL output during SLEEP mode. 
   The selection means may be an AND gate. 
   The clock tree replica may be an accurate representation of the on-chip clock tree branch that is required to be driven by the clock signal. 
   The routed clock tree may be implemented by using a semi custom design approach where a netlist replicates the on-chip clock tree with the help of automatic placement, clock tree synthesis, and routing tools to achieve the desired delays. 
   Another embodiment of this invention provides an improved Phase Locked Loop (PLL) for a digital integrated circuit comprising the steps of:
         providing a Phase and Frequency Detector (PFD) for comparing the input clock and the generated clock,   connecting a Voltage Controlled Oscillator (VCO) at the output of the PFD for producing a clock output and a frequency divider in the feedback path to the PFD input, characterized in that the VCO output is fedback to the PFD input through a clock tree replica providing a delay equal to the routed clock tree.       

   The above method may further provide disabling the PLL output during SLEEP mode by coupling a selection means between the VCO output and the PLL output. 
   The routed clock tree may be implemented by using a semi custom design approach in which a netlist replicates the on-chip clock tree with the help of automatic placement, clock tree synthesis, and routing tools to achieve the desired delays. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described with reference to the accompanying drawings. 
       FIG. 1  shows the schematic diagram of a conventional PLL circuit without the SLEEP mode. 
       FIG. 2  shows the schematic diagram of a conventional PLL circuit with the SLEEP mode. 
       FIG. 3  shows the schematic diagram of a PLL circuit with SLEEP mode according to an embodiment of this invention 
       FIG. 4  shows the schematic diagram of a PLL circuit without SLEEP mode according to an embodiment of this invention. 
       FIG. 5  shows one of the possible methods to realize the clock tree replica according to an embodiment of the invention. 
   


   DETAILED DESCRIPTION 
     FIG. 1  shows a conventional PLL circuit for an electronic device without the SLEEP mode. An external clock  100  is fed to the phase and frequency detector PFD  101 , which connects to the Voltage Controlled Oscillator VCO  102 . The output A  103  of the VCO  102  connects to the leaf  106  of the clock tree Leaf Cell  106  through the routed clock tree  104 . The routed clock tree  104  within the design is selected to provide a feedback loop to correct phase difference between the external system clock and the internal clock. Point B  105  where the routed clock tree net terminates is fed back to the PFD  101  through the frequency divider  108 . The main drawback of this circuit lies in the selection of the clock-tree termination point on which the entire feed back loop is dependent. 
     FIG. 2  shows the schematic diagram of a conventional PLL circuit with SLEEP mode  204 . In this circuit, a deselect means  203  is introduced for deselecting the routed clock tree. In the present example, an AND gate  203  is introduced between the routed clock tree  207  and the Voltage Controlled Oscillator VCO  202 , such that one of the inputs of AND gate  203  is used to select the SLEEP mode, while the second input  205  of the AND gate is the output of VCO  202 . In this case, when the circuit is in SLEEP mode, the clock is terminated from point  206  onwards making the feedback loop ineffective, as there is no signal activity at point  208 . 
   An embodiment of the present invention is explained in FIG.  3 . In this figure, an external clock  300  is fed to the phase and frequency detector PFD  301 , which connects to the Voltage Controlled Oscillator VCO  302 . The output C  305  of the VCO  302  connects to the selection means (here, AND gate  303 ) and the output of the selection means is connected to the leaf cell  308  through the routed clock tree  307 . The output  305  of the VCO  302  is selected to provide a feedback loop to correct the phase difference between the external system clock and the internal clock through the clock-tree replica  309  joining the optional frequency divider  310  to the phase and frequency detector  301 , the clock tree replica providing a delay equal to the routed clock tree. 
   The clock-tree replica  309  is inserted in the feedback loop along with frequency divider so that when the circuit is in sleep mode, i.e., the clock signal is terminated from point A onwards, the PLL circuit can still function to keep the clock in phase. The input and output pins of the clock-tree replica  309  are strategically placed in the layout so as to minimize routing delay. 
     FIG. 4  shows another possible embodiment of the PLL circuit without SLEEP mode. The output of the Voltage Controlled Oscillator VCO  402  is fed back to the phase and frequency detector  401  through the clock-tree replica  406  and the optional frequency divider  407 . 
     FIG. 5  shows one of the possible methods for realizing the clock-tree replica  309 . The delay module viz. clock tree-replica  309  is implemented by using a semi-custom design approach where a netlist replicates the on-chip clock tree with the help of automatic placement, clock-tree synthesis, and routing tools. The method uses a set of registers whose clock pins CLK are connected to top-level pin CLK_IN and the input pin (not shown) is tied to logic level high. The output pin is purposely left unconnected, as there is no data flow through the circuit. After clock-tree synthesis, routing of the CLK_IN node is carefully selected to minimize routing delay, and the CLK_IN node is connected to the CLK_OUT of the module. The flip-flops (registers) and clock-tree buffers used are the same type as used in the actual on chip clock tree, making the clock-tree replica an accurate replica of the on-chip clock tree branch, thereby providing an effective control over insertion delay of this branch. Any shift in the clock-tree delay due to process voltage, temperature (PVT) conditions would affect this replica branch in the same way as it affects the on-chip clock tree delay. The output  305  of the VCO  302  (as shown in  FIG. 3 ) and the output  403  of the VCO  402  (as shown in  FIG. 4 ) are selected to provide a feedback loop to correct the phase difference between the external system clock and the internal clock. 
   The PLLs of  FIGS. 3 and 4  may be incorporated into an integrated circuit (IC) such as a processor, and the IC may be incorporated into a system such as a computer system. 
   It will be apparent to those with ordinary skill in the art that the foregoing is merely illustrative, and is not intended to be exhaustive or limiting, having been presented by way of example only and that various modifications can be made within the scope of the above invention. For instance, the number of series-connected buffers and parallel-connected registers can be varied. Similarly, the intermediate voltage levels can be different from what has been described. Accordingly, this invention is not to be considered limited to the specific examples chosen for purposes of disclosure, but rather to cover all changes and modifications. The invention is therefore not limited by the description contained herein or by the drawings.
An improved Phase Locked Loop (PLL) for digital integrated circuits. A characteristic of this PLL is that the Voltage Controlled Oscillator (VCO) output is fed to the phase and frequency detector (PFD) input through a clock-tree replica providing a delay equal to the routed clock tree. "This enables the PLL to maintain the proper phase even during a sleep mode of operation."