Abstract:
The invention is related to a method and an apparatus for generating an output clock. The method comprises: measuring a reference clock according to a free-run clock to produce a counter signal in a normal mode; suspending the reference clock; and generating the output clock according to the counter signal and the free-run clock in a power-saving mode.

Description:
This application claims the benefit of Taiwan application serial no. 93103420, filed on Feb. 13, 2004 and Taiwan application serial no. 93101101, filed on Jan. 16, 2004, the subject matter of which is incorporated herein by reference. 
   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   This invention relates to an apparatus and a method for generating an output clock, particularly relates to an apparatus and a method for generating an output clock signal using a free-run clock generator. 
   2. Description of the Prior Art 
   The reference clock generator is a very popular device for providing a reference clock. Conventionally, a reference clock generator can be an oscillator or a combination of a crystal and an oscillation circuit. The power consumptions of these kinds of reference clock generators are high. 
   The electronic device can operate in a power-saving mode or a sleep mode to reduce the power consumption of the electronic device. When the electronic device operates in the power-saving mode or the sleep mode, the electronic device periodically check whether the electronic device receives the link signal or not and determines whether the electronic device need to operate in the normal mode. 
   The conventional method is utilized an external component, such as a resistor or a capacitor, and an internal component, such as a resistor or a capacitor, to generate a clock signal having a long period according to a RC constant. However, this long period clock signal is not stable that the period may change with the changes of temperature, voltage or/and the manufacture process of semiconductor. 
   In additions, an integrated circuit (IC) includes a phase-lucked loop (PLL) and needs at least one pin, which receives an external clock. The PLL produces the other reference clock according to the external clock. 
   SUMMARY OF INVENTION 
   It is therefore one of the objectives of the claimed invention to provide an apparatus and method for generating an output clock. The apparatus and method can reduce the power consumption of generating the output clock. 
   It is therefore one of the objectives of the claimed invention to provide an apparatus and method for generating an output clock. The frequency of the output clock will be robust to the changes of the voltage, temperature and/or the manufacture process. 
   It is therefore one of the objectives of the claimed invention to provide an apparatus and method for generating an output clock. The apparatus and method have a self-calibration or/and real-time calibration functions. 
   According to the present invention, a method for generating an output clock comprises: generating a free-run clock; generating a first clock in a first mode; measuring the first clock according to the free-run clock to generate a counter signal; generating the output clock according the free-run clock and the counter signal in a second mode; and suspending the first clock in the second mode. 
   Preferably, the period of the output clock in a second mode can be adjusted when the counter signal is adjusted. For example, the counter signal can be divided by or multiplied by the adjusting value. 
   According to the present invention, an apparatus for generating an output clock comprises: a clock generator for generating a reference clock in a first mode and suspending the reference clock in a second mode; a free-run clock generator for generating a free-run clock; a comparator for measuring the reference clock according to the free-run clock in the first mode to generate a counter signal; and an output circuit for outputting the output clock according to the free-run clock and the counter signal in the second mode. 
   Preferably, the clock generator of the invention further includes an adjusting unit. This adjusting unit receives the counter signal from the comparator, adjusts the value of the counter signal and outputs the adjusted counter signal to the output circuit. 
   These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The details of the present invention will be more readily understood from a detailed description of the preferred embodiments taken in conjunction with the following figures. 
       FIG. 1  shows a block diagram of a first embodiment of the clock generator according to the present invention; 
       FIG. 2  shows a block diagram of a second embodiment of the clock generator according to the present invention; and 
       FIG. 3  shows a flowchart of an embodiment of the method for generating an output clock according to the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 1 .  FIG. 1  shows an embodiment of the low power consumption clock generator of this invention. In  FIG. 1 , the low power consumption clock generator  300  comprises a clock generator  310 , a measuring unit  320 , an output unit  330 , and a free-run clock generator  340 . The power consumption of the free-run clock generator  340  is lower than that of the clock generator  310 . 
   Generally speaking, the clock generator  310  is a high power consumption clock generator and generates an accuracy reference clock  319 . In an embodiment, the clock generator  310  further includes a clock generator  313  and a first frequency-adjusting unit  315 . The clock generator  313  generates a clock signal  317 . The first frequency-adjusting unit  315  can be omitted. The first frequency-adjusting unit  315  can be a divider or a counter. 
   In the first mode, the measuring unit  320  uses the free-run clock  343  generated by the free-run clock generator  340  to measure (count) the accuracy reference signal  319  and obtain the measured value  325 . The power consumption of the free-run clock generator is lower than that of the clock generator  310 . In an embodiment, the measured value  325  generated by the measuring unit  320  can be adjusted by the adjusting unit  316  and then stored in the storage unit (not show in  FIG. 1 ). The storage unit can be a register or an on-chip memory or RAM or others. The adjusting unit  316  can be omitted. 
   In the second mode, the high power consumption clock generator  310  can be suspended to reduce the power consumption of the clock generator of the invention. The output unit  330  outputs a clock signal  335 , which is corresponding to the reference signal  319 , according to the measured value  325  and the free-run clock  343 . The output unit  330  can also adjust the frequency of the clock signal  335  according to a control signal  345 . In one embodiment, the free-run clock generator  340  comprises a RC oscillator circuit. The measuring unit  320  can be a first counter and the measured value is a counter value. The output unit  330  can be a second counter. The measured value can be a non-integer. 
   In a preferred embodiment, the first mode can be a calibration mode or a normal mode. In another preferred embodiment, the second mode can be a power-saving mode or a sleep mode. 
   Please refer to  FIG. 2 .  FIG. 2  shows another embodiment of the clock generator of this invention. In the  FIG. 2 , the low power consumption clock generator  400  comprises a free-run clock generator  410 , a measuring unit  420 , an output unit  430 , a reference clock generator  440  and the adjusting unit  450 . The power consumption of the free-run clock generator  410  is lower than that of the precise clock generator  440 . 
   In the embodiment, the free-run clock generator  410  is different from the high power consumption clock generator  310  in  FIG. 1 . The free-run clock generator  410  generates a free-run clock  419  to the output unit  430 . In an embodiment, the free-run clock generator  410  further comprises a clock generator  411  and a first frequency-adjusting unit  413 . The clock generator  411  generates a clock signal  415  which is then transmitted to and adjusted by the first frequency-adjusting unit  413 . The first frequency-adjusting unit  413  can be omitted. 
   In a first mode, the measuring unit  420  uses the clock signal  445 , which generated by the clock generator  440 , to measure (count) the free-run clock  419  from the free-run clock generator  410 , and obtains a measuring value  425 . In an embodiment, the measured value  425 , which is outputted from the measuring unit  420 , is modified by the adjusting unit  450  and outputted to an output circuit  430 . The output circuit  430  can record (store) the measured value  425 . In this embodiment, the adjusting unit  450  can be omitted. 
   In a second mode, the clock generator  440  stops operating. The output unit  430  uses the stored measured value  425  and the free-run clock  419 , and outputs the output clock  460 . 
   In one embodiment, the free-run clock generator  410  comprises a RC oscillator circuit. 
   As illustrated in  FIG. 1  and  FIG. 2 , whether through using the free-run clock signal to count (measure) the reference signal or through using the reference signal to count (measure) the free-run clock signal, the relationship between (ratio of the period of) the free-run clock signal and the reference signal can be obtained. This ratio is the measured value. Although the frequency of the free-run clock signal is unknown, the output clock which is substantially the same as the reference signal generated by the clock generator  310  or  440  is generated according to the measured value and the free-run clock. In additions, when the operating temperature changes, the clock generator of this invention can be self-calibration to optimize the measured value, which is the ratio of the period of the free-run clock signal and the reference signal. Therefore, the errors caused by the changes of operation environment, such as temperature, voltage, can be avoided. 
   Please refer to  FIG. 3 .  FIG. 3  shows the procedures of an embodiment of the low power consumption clock generator in this invention. Please also refer to  FIG. 1  and  FIG. 2 . 
   In step  201 : in the first mode, the free-run clock  343  ( 419 ) is used to measure (count) the reference signal  319  ( 445 ) generated by the clock generator. A measured value  325  ( 425 ) is then obtained. The measured value  325  ( 425 ) can be an integer or a non-integer value. 
   In step  202 : the output clock  330  ( 430 ) is outputted according to the free-run clock  343  ( 419 ) and the measured value  325  ( 425 ) obtained in the step  201 . Because the measured value  325  ( 425 ) is corresponding to the reference signal  319  ( 445 ) outputted from the clock generator  310  ( 440 ). After obtaining the measured value  325  ( 425 ), the low power consumption clock generator of the present invention enters the second mode in which the clock signal generator  310  ( 440 ) stop operating to reduce the power consumption of the clock generator of the present invention. In an embodiment, the measured value  325  ( 425 ) can be adjusted by the adjusting unit  316  ( 450 ). 
   In step  203 : the measured value  325  ( 425 ) can be adjusted according to an adjusting signal  345  to control the frequency of the output clock  460 . 
   As illustrated previously, through a free-run clock  419 , the relationship between (ratio of the period of) the free-run clock and the reference signal can be obtained, whether using the free-run clock signal to count the reference signal or using the reference signal to count the free-run clock signal. This ratio is the measured value. The output clock can be generated according to the measured value. If the reference signal is inputted externally, the output clock will not vary with the changes of the voltage, temperature or the manufacturing process and the frequency of the output clock can be fixed. 
   In other words, the invention can utilize the free-run clock generator to output an output clock which has the same frequency of the reference clock generated by the clock generator. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, that above disclosure should be construed as limited only by the metes and bounds of the appended claims.