Abstract:
The present invention implements an apparatus for calibrating a phase locked loop (PLL) circuit. The apparatus includes a detector for detecting frequencies of a reference signal and a controlled oscillator contained in the PLL circuit. The detector outputs the frequency difference to a control circuit. The control circuit is programmed to adjust one or more control signals to the controlled oscillator based upon the frequency difference in an orderly fashion to complete the calibration process.

Description:
FIELD OF THE INVENTION 
   The invention pertains to the calibration of high speed circuitry. More specifically, the invention pertains to calibrating a voltage controllable oscillator used in a phase locked loop circuit with a reference signal. 
   BACKGROUND OF THE INVENTION 
   As semiconductor line widths decrease, designers and programmers are configuring the devices to run at faster speeds. In addition to the demand for higher speed devices, demands also exist for device performance with higher levels of signal accuracy. One example of electronic devices where signal accuracy is of great importance is devices that communicate wirelessly. 
   One tool used by designers to improve the accuracy of a signal is a phase locked loop (PLL). A PLL is an electronic control system that generates a signal that is locked to the phase (and frequency) of an input or “reference” signal. PLLs are widely used in radio, telecommunications and computers to generate stable frequency signals, or to recover a signal from a noisy communication channel. A PLL includes specific circuit components to adjust the control voltage of a voltage controllable oscillator until its output phase is identical to that of the reference signal. 
     FIG. 1  illustrates a sample PLL  100 . Here, the reference signal  102  is input to the reference divider  104 . The reference divider  104  reduces the clock rate of the reference signal to a rate that the other components can process. After dividing the frequency of the reference signal (if needed), the reference divider  104 , passes the signal to the phase detector  106 . 
   The phase detector  106  is the control element of the PLL  100 . The phase detector  106  compares the phases of its two input clocks (in this example, the reference signal supplied by the reference divider  104  and the output of the voltage controllable oscillator feedback divider  112 ) and provides a corrective signal that is filtered which controls the oscillator to force the phase between the two compared clock signals to zero. In this example, the phase detector  106  provides a control signal to the loop filter  108 . 
   The optional loop filter  108  receives the control signal from the phase detector  106  and creates a low pass filtered version of the phase detector output (VLF). For a voltage controlled oscillator (VCO) with a positive transfer function slope, the loop filter output voltage increases if the oscillator should produce a higher frequency signal, and conversely, is lowered if the oscillator should produce a lower frequency signal to match the two clock phases. The voltage controlled oscillator output is provided to an optional feedback divider  112 . In  FIG. 100 , the feedback divider splits the output of the controlled oscillator  110  into two signals; one output provides the feedback input to the phase detector  106  for further comparison; the second output signal is an output signal  114 . 
   The PLL described in  FIG. 1  is a typical approach to producing phase locked signals. When implemented in an integrated circuit, PLL circuit performance can vary greatly over different silicon processes, voltage and temperature. A slight temperature change can change the value of the circuit components (e.g., the loop filter) or functionality of other PLL elements. Under some combinations of silicon process, supply voltage and temperature, there are occasions when the loop filter cannot produce a voltage signal that will result in the controlled oscillator producing a signal with the same phase as the reference signal. To avoid this undesirable condition, a PLL trimming method that calibrates components of the PLL, in particular the controlled oscillator, is often employed. 
   SUMMARY OF THE INVENTION 
   The present invention is a programmable linear trimming apparatus and method for quick calibration of a phase locked loop (PLL) circuit. The apparatus and method search through an operating range of a controlled oscillator and centers the output frequency of the controlled oscillator to an optimal frequency. 
   A first embodiment of the present invention is an apparatus for calibrating a PLL circuit. The apparatus includes a detector to compare the frequency difference between the reference signal and the output of the voltage controlled oscillator contained in the PLL circuit. A control circuit is programmed to adjust control signals to the controlled oscillator based upon the comparison of the frequencies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a conventional Phase Locked Loop (PLL) circuit for aligning the phase of an output signal to the phase of a reference input signal. 
       FIG. 2  is a modified PLL according to principles of the present invention. 
       FIG. 3  is a flow diagram illustrating one exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows a trimming circuit  200  according to one preferred embodiment of the present invention. The trimming circuit  200  includes the PLL  100  from  FIG. 1 , along with several additional components. A first set of additional components are digital control circuits. In trimming circuit  200 , the digital control circuits include frequency detector  202 , the center frequency trimming state machine  204  and the power on reset (POR) startup state machine  206 . A second set of additional components are analog control circuits and include the current regulator  208  and the voltage regulator  210 . 
   The trimming circuit  200  begins operation when the POR state machine  206  signals the PLL to be brought up from a powered-down state. The POR state machine  206  will wait a pre-determined period of time during which any values stored in any registers or gates of the circuit reset to zero. Additionally, this time period allows any voltage or current regulators to initialize. During this initialization, the POR state machine sends a signal to PLL  100  to begin its standard closed loop functionality. After signaling PLL  100 , the POR  206  sends a signal to the center frequency trimming state machine  204  indicating the PLL is being powered-up. Once the center frequency trimming state machine  204  receives this indication, it sends an enable signal to the frequency detector  202  to begin functioning. 
   Once the feedback divider provides an output, however, the phase locked loop feedback connection is opened by disconnecting the output of the loop filter  108  from the control voltage input of controlled oscillator  110  and replacing the loop filter output with a reference voltage or current. The reference voltage or current is chosen to set the VCO close to the center of its frequency range. In this open loop configuration, the frequency detector  202  directly compares the divided VCO frequency with the divided reference clock frequency. 
   The output of the frequency detector  202  is determined by the frequency difference of the two input signals. The design described herein uses a higher output from the frequency detector  202  to indicate the voltage controllable oscillator  110  is producing a signal with too high a frequency, however, the design can easily be modified to accommodate opposite control directions. 
   The trimming state machine  204  provides signals that serve as inputs to components of the phase locked loop. These inputs are typically encoded as digital signals and allow adjustment of the frequency of the voltage controllable oscillator when its control voltage input is set to the reference voltage or current. The trimming state machine  204  contains two registers, one for storing the initial value of the trimming state machine output (TRIMIV) and one for storing the end value of the trimming control (TRIMEV). An additional value, TRIMCV represents the current value of the trimming state machine output. Upon initialization of the frequency trimming state machine  204 , TRIMCV is set equal to TRIMIV. 
   The center frequency trimming state machine  204  acts as controller for the PLL  100  as well as the current regulator  208  and voltage regulator  210  once the PLL  100  has entered its open-loop state. First, the center frequency trimming state machine  204  sends a signal to the loop filter  108  to replace its output with a constant reference voltage or current to the control voltage input of the controlled oscillator  110 . This will stop the PLL  100  from making any changes to the operation of the controlled oscillator  110 . Changes to the controlled oscillator  110  will now be determined by center frequency trimming state machine  204  and signaled to the controlled oscillator  110  by signals from either current regulator  208  or voltage regulator  210 . By controlling the controlled oscillator  110  with center frequency trimming state machine  204 , the control is moved to a digital component, and time is saved as the operation of phase detector  106  and loop filter  108  is eliminated. 
   Once center frequency trimming state machine  204  has received both TRIMIV and TRIMEV signals, the control of controlled oscillator  110  fully begins. Again, as before, TRIMCV is set equal to TRIMIV. If the TRIMCV value is smaller than that of TRIMEV, the center frequency trimming state machine  204  starts calibrating the controlled oscillator at its lowest programmed output frequency by sending a signal to either current regulator  208  or voltage regulator  210  to set the operating frequency of the controlled oscillator  110 . After a controlled settling time (during which the output of controlled oscillator  110  filters through the various components), the center frequency trimming state machine  204  makes another comparison. If the signal from the controlled oscillator is slower than that of the reference, the center frequency trimming state machine  204  increments the control signal sent to the current regulator  208  or the voltage regulator  210  by one and repeats the settling time and makes another comparison. This process is repeated until the frequency of the voltage controlled oscillator  110  output is equal or greater than the reference signal frequency. If the output frequency of the voltage controlled oscillator  110  is higher than the frequency of the reference signal, a step-back function is initiated. If the output frequency of the voltage controlled oscillator is unacceptably higher than that of the reference signal, the control signals are decremented by one and the trimming algorithm is complete. 
   Conversely, if TRIMIV value is larger than that of TRIMEV, the center frequency trimming state machine  204  starts calibrating the controlled oscillator at its highest programmed output frequency by sending a signal to either current regulator  208  or voltage regulator  210  to set the operating frequency of the controlled oscillator  110  to its maximum frequency. After a controlled settling time (during which the output of controlled oscillator  110  filters through the various components), the center frequency trimming state machine  204  makes another comparison. If the frequency of the controlled oscillator is higher than that of the reference frequency, the center frequency trimming state machine  204  decrements the control signal sent to the current regulator  208  or the voltage regulator  210  by one and repeats the settling time and makes another frequency comparison. This process is repeated until the frequency of the voltage controlled oscillator  110  output is equal to or less than the frequency of the reference signal. If the frequency of the voltage controlled oscillator  110  is a lower frequency than that of the controlled oscillator  110 , a step-back function is initiated. If the frequency of the voltage controlled oscillator is unacceptably less than that of the reference signal, the control signals are incremented by one and the trimming algorithm is complete. 
   At this time, the reference voltage or current signal at the control voltage input to the voltage controllable oscillator is removed and its input is provided by the loop filter. This closes the feedback loop of the phase locked loop in preparation for closed loop operation. 
     FIG. 3  illustrates a flowchart detailing the algorithm. The process begins at step  302  after the power-on reset operation has been completed and the PLL  100  is functioning in open-loop operation. At step  302 , TRIMCV is set equal to the value of the TRIMIV, and Trim done is set equal to zero. Trim done is a value representing the end of the trim function. Once the TRIMCV value and Trim done signal have been set, the process proceeds to step  304 . Here, an initial comparison of TRIMCV and TRIMEV is done. Three possible outcomes can occur during this comparison: (1) TRIMCV is smaller than TRIMEV; (2) TRIMCV is larger than TRIMEV; and (3) TRIMCV and TRIMEV are equal. For case (1), the process proceeds to the left side of the flow chart, to step  306   a.  Here, a delay of one frequency detection period to allow any changes to the controlled oscillator to occur and the inputs to frequency detector  202  to update. Then the process proceeds to step  308   a . Here, one frequency detection period elapses during which the frequency detector records the frequency difference between the reference signal and the voltage controlled oscillator  110 . The process proceeds to step  310   a . At step  310   a , the TRIMCV value is compared to TRIMEV. If the TRIMCV value does not equal TRIMEV, the process proceeds to step  312   a . Here the center frequency trimming state machine  204  determines if the frequency of the voltage controlled oscillator is less than the frequency of the reference signal. If it is, the process proceeds to step  314   a  where the TRIMCV value is incremented and steps  306   a - 310   a  are repeated. 
   If at step  310   a  the TRIMCV value does equal TRIMEV, or at step  312   a  the center frequency trimming state machine  204  determines that the frequency of the voltage controlled oscillator is higher than the frequency of the reference signal, the process moves to step  316   a . Here, the step back criteria is examined. Essentially, the center frequency trimming state machine  204  determines if the frequency difference between the voltage controlled oscillator and the frequency of the reference signal is outside an acceptable range. If it is, the process proceeds to step  318   a  where the control signal is decremented. The process then proceeds to step  320  where Trim done is set to one and the process ends. 
   For case (2), the process is basically the inverse of case (1). The process proceeds to the right side of the flow chart, to step  306   b . Hero, there is a wait time of one frequency detection period to allow for settling of the frequency of the controlled oscillator and frequency detector  202 . Then the process proceeds to step  308   b  where one frequency detection period elapses during which the frequency detector records the frequency difference between the reference signal and the voltage controlled oscillator  110 . Once the frequency detector  202  has recorded the difference frequency, the process proceeds to step  310   b  where the TRIMCV value is compared to TRIMEV. If the TRIMCV value does not equal TRIMEV, the process proceeds to step  312   b . At step  312   b,  the center frequency trimming state machine  204  determines if the frequency of the voltage controlled oscillator is greater than the frequency of the reference signal. If it is, the process proceeds to step  314   b  where the TRIMCV value is decremented and steps  306   b - 310   b  are repeated. 
   If at step  310   b  the TRIMCV value does equal TRIMEV, or at step  312   b  the center frequency trimming state machine  204  determines that the frequency of the voltage controlled oscillator is less or equal to the frequency of the reference signal, the process moves to step  316   b  where the step back criteria is examined. Essentially, the center frequency trimming state machine  204  determines if the frequency difference between the voltage controlled oscillator and the reference clock is outside an acceptable range. If it is, the process proceeds to step  318   b  where the control signal is incremented by one. The process then proceeds to step  320  where Trim done is set to one and the process ends. 
   For case (3), where TRIMCV and TRIMEV are initially equal, the process proceeds directly to step  320  where Trim done is set to one and the process ends. 
   The circuit described in  FIG. 2  is merely shown by way of example and is in no way meant to limit the scope of the present invention. It would be obvious to one of ordinary skill in the art to make various modifications to the process and still achieve the same result, for example, the digital components (i.e., frequency detector  202 , center frequency trimming state machine  204  and power on reset state machine  206 ) could be included in a software application. Another possible modification would be to include additional registers for storing additional trimming level values. 
   Similarly, the process described in  FIG. 3  is merely shown by way of example and is in no way meant to limit the scope of the present invention. It would be obvious to one of ordinary skill in the art to make various modifications to the process and still achieve the same result, for example, by using different step back criteria. 
   Additional alterations, modifications, and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.