Abstract:
A wide-locking range phase locked loop circuit includes a decision unit and a closed loop connection comprising a phase frequency detector, a charge pump, a loop filter, a voltage controlled oscillator, and a multi-modulus divider. The decision unit receives a phase difference signal outputted from phase frequency detector and the control voltage outputted from the loop filter and determines to select a specific divisor form the plurality of divisors provided by the multi-modulus divider if the phase difference signal indicates an unlocked state and the control voltage is not within a standard voltage operation range.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a phase locked loop (PLL) circuit, and more particularly to a wide-locking range phase locked loop circuit using the adaptive post division technique. 
   BACKGROUND OF THE INVENTION 
   Please refer to  FIG. 1 , which illustrates the conventional phase locked loop (PLL) circuit. The PLL circuit  100  includes a phase frequency detector  10 , a charge pump  20 , a loop filter  30 , a voltage controlled oscillator (VCO)  40  and a divider  50 . An input clock signal (CK in ) with a reference frequency (f ref ) is generated by a reference oscillator (not illustrated). Both the input clock signal and a frequency divided signal are inputted into the phase frequency detector  10 . The phase frequency detector  10  detects the difference in phase and frequency between the input clock signal (CK in ) and the frequency divided signal and then outputs a phase difference signal to the charge pump  20 . According to the phase difference signal, the charge pump  20  then outputs the current proportional to the amplitude of the phase difference. The output current charges capacitors C 1  and C 2  of the loop filter  30 , thereby generates a control voltage (Vc) to the VCO  40 . The VCO  40  generates an output clock signal (CK out ) with a voltage controlled frequency (f vco ) in response to the control voltage (Vc). The divider  50  receives the output clock signal (CK out ) and generates a frequency divided signal after dividing the voltage controlled frequency (f vco ) by an integer M (i.e. multiply by 1/M) for being inputted to the phase frequency detector  10 . Therefore, the frequency relation between input clock signal (CK in ) and the output clock signal (CK out ) of the PLL circuit  100  is f voc =M*f ref . 
   As widely known, the frequency operation range of the VCO  40  is restricted in its resonant frequency; further, the control voltage (Vc) is proportional to the voltage controlled frequency (f vco ); hence, the control voltage (Vc) would be restricted within a voltage operation range. That is to say, the conventional frequency locked range of the PLL circuit  100  would be restricted to within the frequency operation range of the VCO  40 . 
   In order to achieve PLL circuit with wide-locking range, as illustrated in  FIG. 2 , a PLL circuit with multi-modulus divider is proposed. The proposed multi-modulus divider  60  of the PLL circuit  150  includes a main divider  62  and a coefficient-selecting unit  64 . The main divider  62  provides a basic numeric M. The coefficient-selecting unit  64  switches using the controlling pins to choose one of the coefficients from many (1, ½, ¼, . . . , ½ N ). For example, if user selects the coefficient ½ from the coefficient-selecting unit  64 , the output voltage controlled clock signal (CK vco ) with a voltage controlled frequency (f vco ) outputted from the VCO  40  is undergoing a first frequency division by the coefficient ½ to generate the output clock signal (CK out ) with an output frequency equal to f vco /2. The output clock signal (CK out ) further undergoes a second frequency division by the main divider  62  according to the basic numeric M, which divides the output frequency (f out ) of the output clock signal (CK out ) by the integer M (multiply by 1/M) to generate the frequency divided signal with frequency equal to f vco /2M. 
   The conventional multi-modulus divider  60  provides a coefficient-selecting unit  64  to the PLL circuit  150 . Through dynamically selecting one value of the coefficient-selecting unit  64  and applying to the PLL circuit  150 , the output frequency (f out ) of output clock signal (CK out ) can achieve the purpose of wide-locking range. However, when designing such kind of PLL circuit in an application specific integrated circuit (‘ASIC’), a set of control pins are needed to be provided additionally in order to control switches (SW 0 ˜SWN) and select one coefficient in the coefficient-selecting unit  64  by user. The additional control pins or terminals would however increase difficulty of operation and the cost and complexity of design and testing. 
   SUMMARY OF THE INVENTION 
   One of the objects of the present invention is to provide a wide-locking range phase locked loop circuit with built-in auto-adjust mechanism. 
   The present invention provides a phase locked loop circuit including: a phase frequency detector receiving a frequency divided signal and an input clock signal with a reference frequency and detecting the difference in phase and frequency between the input clock signal and the frequency divided signal and then outputting a phase difference signal; a charge pump outputting an output current in response to the phase difference signal; a loop filter generating a control voltage in response to the phase difference signal; a voltage controlled oscillator generating a voltage controlled clock signal with a voltage controlled frequency in response to the control voltage; a multi-modulus divider receiving the voltage controlled clock signal and then generating the frequency divided signal and an output clock signal with an output frequency, wherein a first divisor can be selected from a plurality of divisors provided by the multi-modulus divider to achieve a relation that the voltage controlled frequency divided by the first divisor equals the output frequency; and a decision unit receiving the phase difference signal and the control voltage and determining to select a second divisor form the plurality of divisors provided by the multi-modulus divider if the phase difference signal indicates an unlocked state and the control voltage is not within a standard voltage operation range. 
   The present invention further provides a method of controlling a phase locked loop circuit, wherein the phase locked loop circuit divides a voltage controlled clock signal by a divisor for generating a frequency divided signal and generates a control voltage in response to a difference between the frequency divided signal and an input clock signal, the method including steps of: setting the divisor to an initial value; and changing the divisor if the control voltage is not within a standard voltage operation range over a period of time. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above contents of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
       FIG. 1  illustrates the conventional phase locked loop (PLL) circuit. 
       FIG. 2  illustrates a PLL circuit with multi-modulus divider. 
       FIG. 3  illustrates the PLL circuit of the present invention. 
       FIG. 4  illustrates the frequency operation range of the PLL circuit of the present invention. 
       FIG. 5  illustrates the state diagram of the decision unit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Please refer to  FIG. 3 , which illustrates the PLL circuit of the present invention. The PLL circuit  200  comprises a phase frequency detector  210 , a charge pump  220 , a loop filter  230 , a VCO  240 , a multi-modulus divider  250  and a decision unit  260 . The multi-modulus divider  250  comprises a main divider  252  and a coefficient-selecting unit  254 . The main divider  252  provides a basic numeric value M, while the coefficient-selecting unit  254  controls switches (SW 0 ˜SWN) through the decision unit  260 , which is used to choose one coefficient from several coefficients (1, ½, ¼, . . . , ½ N ). That is to say, after the decision unit  260  selects one coefficient from the coefficient-selecting unit  254 , the voltage controlled clock signal (CK vco ) with the voltage controlled frequency (f vco ) outputted by VCO  240  undergoes a first frequency division by the coefficient-selecting unit  254  and then becomes an output clock signal (CK out ) with an output frequency (f out ). Further, the output clock signal (CK out ) further undergoes a second frequency division by the main divider  252  according to the basic numeric M which divides the output frequency (f out ) of output clock signal (CK out ) by the integer M (multiplied by 1/M) to generate the frequency divided signal. 
   According to the embodiment of the present invention, the decision unit  260  and the PLL circuit  200  with multi-modulus divider are designed and integrated into an ASIC. In this way, the embodiment enables the PLL circuit  200  to have the characteristic of wide-locking range without using control pins to select one coefficient in the coefficient-selecting unit  254  by user. 
   The decision unit  260  comprises a lock detector  262 , a comparator  264 , an accumulator  266  and a switch controller  268 . The comparator  264  receives and monitors the control voltage (Vc). When the control voltage (Vc) is smaller or larger than the standard voltage operation range, the comparator  264  will output pulses from either a low Vc terminal or a high Vc terminal to the accumulator  266 . The accumulator  266  will count the number of pulses from low Vc terminal or the high Vc terminal. As the number accumulated by the accumulator  266  reach a predetermined value (X times), the accumulator  266  will generate an adjust-up signal (UP) or an adjust-down signal (DN) to the switch controller  268 . The switch controller  268  can select a coefficient in the coefficient-selecting unit  254  according to the adjust-up signal (UP) or the adjust-down signal (DN). Further, the lock detector  262  receives the phase difference signal from the phase frequency detector  210  and determines whether the PLL circuit  200  is in a locked state or an unlocked state. When the lock detector  262  determines that the PLL circuit  200  is in the locked state, the lock detector  262  outputs a clear signal to the accumulator  266  to clear the number counted in the accumulator  266 . 
   Please refer to  FIG. 4 , which illustrates the frequency operation range of the PLL circuit of the present invention. The horizontal axis and vertical axis represent respectively the control voltage (Vc) and the output frequency (f out ) of output clock signal (CK out ). As illustrated, standard voltage operation range of the VCO  240  is in between Vx and Vy. When a divisor of the multi-modulus divider  250  is M, (multiply by 1/M), the frequency operation range of the PLL circuit  200  falls in between B MHz and A MHz; when the divisor of multi-modulus divider  250  is 2M, (multiply by ½M), the frequency operation range of the PLL circuit  200  falls in between B/2 MHz and A/2 MHz; when the divisor of the multi-modulus divider  250  is 4M, (multiply by ¼M), the frequency operation range of the PLL circuit  200  falls in between B/4 MHz and A/4 MHz; the same applies for the divisor of 2 N  M. Therefore, the PLL circuit  200  of the present invention can be operated between B/4 MHz and A MHz. Similarly, the more coefficients in the coefficient-selecting unit  254  for selection, the wider the frequency operation range of the PLL circuit  200 . 
   Please refer to  FIG. 5 , which illustrates the state diagram of the decision unit. When the PLL circuit  200  begins to operate, the decision unit  260  is in state A, which is the initial state. As input clock signal (CK in ) with reference frequency (f ref ) is input into PLL circuit  200 , the control voltage (Vc) starts to change; and the decision unit  260  is in state B, which is the state of detecting control voltage (Vc). In state B, the comparator  264  of the decision unit  260  monitors whether the control voltage (Vc) is operated within the standard voltage operation range (Vx˜Vy). When the control voltage (Vc) is operated within the standard voltage operation range (Vx˜Vy) and the lock detector  262  confirms that the PLL circuit  200  has been locked, the decision unit  260  enters into state G, which is the locked state. In state G, when the lock detector  262  detects the PLL circuit  200  is unlocked, the decision unit  260  enters into state B. 
   Further, in state B, when the reference frequency (f ref ) of the input clock signal (CK in ) changes and makes the control voltage (Vc) smaller than Vx, the decision unit  260  enters into state C, which is a counting state in which Vb&lt;Vc&lt;Vx. In state C, the accumulator starts to count the number of pulses output from the low Vc terminal; from here, (1) when the control voltage (Vc) is larger than Vx and the number of pulses does not reach the predetermined value (X times), then the decision unit  260  enters into state B; (2) when the control voltage (Vc) is smaller than Vx and the number of pulses does not reach the predetermined value (X times) and the lock detector  262  confirms that PLL circuit  200  has been locked, then the decision unit  260  enters into state G; (3) when the control voltage (Vc) is even smaller than Vb and the number of pulses does not reach the predetermined value (X times), then the decision unit  260  enters into state D, which is a counting state in which Vc&lt;Vb; and (4), when the control voltage (Vc) is smaller than Vx and the number of pulses reach the predetermined value (X times), then the decision unit  260  enters into state F, which is the state of increasing divisor and reset. 
   In State D, as control voltage (Vc) is already too low, the PLL circuit  200  is impossible to enter into state G (locked state). Therefore, unless reference frequency (f ref ) of input clock signal (CK in ) changes to enable the control voltage (Vc) larger than Vx again which causes the decision unit  260  to enter into state B, when the number of pulse reaches the predetermined value (X times), the decision unit  260  will enter into state F. 
   In state F, the switch controller  268  can select another coefficient from the coefficient-selecting unit  254  to increase the divisor of multi-modulus divider  252 ; for instance, increasing the divisor from M to 2M, or from divisor 2M to divisor 4M. After such, the decision unit  260  enters into state A and continues operation. 
   Further, in state B, when the reference frequency (f ref ) of the input clock signal (CK in ) changes and makes control voltage Vc larger than Vy, the decision unit  260  enters into state E, which is the counting state in which Vy&lt;Vc&lt;Vt. In state E, the accumulator starts to count number of pulses output from high Vc terminal. Following such, (1) when the control voltage (Vc) is smaller than Vy and the number of pulses is short of the predetermined value (X times), the decision unit  260  enters into state B; (2) when the control voltage (Vc) is larger than Vy, the number of pulses is short of the predetermined value (X times) and the lock detector  262  confirms that PLL circuit  200  has been locked, the decision unit  260  enters into state G; (3) when the control voltage (Vc) is further larger than Vt and the number of pulses does not reach the predetermined value (X times), the decision unit  260  enters into state I, which is the counting state in which Vc&gt;Vt; and (4) when the control voltage (Vc) is larger than Vt and the number of pulses reach the predetermined value (X times), the decision unit  260  enters into state H, which is a state of decreasing divisor and reset. 
   In State I, as the control voltage (Vc) is already too high, the PLL circuit  200  is impossible to enter into state G (locked state). Thus, unless the reference frequency (f ref ) of input clock signal (CK in ) changes making the control voltage (Vc) smaller than Vy again to make the decision unit  260  enter into state B, when the number of pulses reaches the predetermined value (X times), the decision unit  260  enters into state H. 
   In state H, the switch controller  268  can select another coefficient from the coefficient-selecting unit  254  to decrease divisor of the multi-modulus divider  252 ; e.g. decreasing from divisor 2M to divisor M or from divisor 4M to divisor 2M. After such, the decision unit  260  then enters into state A and continues operation. 
   According to the embodiment of the present invention, the predetermined value (X times) is 24, and the frequency of pulses generated from low Vc terminal or high Vc terminal is f ref /256. That is to say, when the control voltage (Vc) is not operated in the standard voltage operation range (Vx˜Vy), the decision unit  260  can change the divisor of the multi-modulus divisor  250  after a period of 24*(256/f ref ). 
   For instance, when the reference frequency (f ref ) of the input clock signal (CK in ) is very low, the output current from the charge pump  220  suppresses the control voltage (Vc), making the decision unit  260  enter into state C or D. After a period in which the PLL circuit  200  remains unlocked, the decision unit  260  will control the multi-modulus divider  250  to increase the divisor; after resetting, the control voltage (Vc) is returned to within the standard voltage operation range (Vx˜Vy) and then the decision unit  260  enters into state G. 
   By the same logic, when the reference frequency (f ref ) of the input clock signal (CK in ) is very high, the output current from the charge pump  220  drive up the control voltage (Vc), and cause the decision unit  260  to enter into state E or I. After a period in which the PLL circuit  200  remains unlocked, the decision unit  260  will control the multi-modulus divider  250  to decrease divisor; the control voltage (Vc) is enabled to return to the standard voltage operation range (Vx˜Vy) and then the decision unit  260  enters into state G. 
   Therefore, the present invention provides a wide-locking range phase locked loop circuit, which enables application of PLL circuit to the ASIC without an additional control pin that increases user&#39;s loading. The present invention of the decision unit  260  is also achieved using only digital circuit; hence it has a higher immunity against the disturbance from manufacturing process. 
   While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.