Abstract:
In a method for testing an embedded phase-locked loop (PLL) circuit, a first clock signal is provided to an embedded phase-locked loop (PLL) circuit to be tested. A PLL clock signal of a first frequency is generated by the embedded PLL in response to the first clock signal. The PLL clock signal of the first frequency is sampled with a second clock signal of a second frequency to generate a first sampled signal, wherein the second frequency is different from the first frequency but has a first correlation with the first frequency so that the first sampled signal toggles at a predetermined frequency when the embedded PLL circuit is in a normal operation condition. The embedded PLL circuit is determined to be in an abnormal operation condition if the first sampled signal does not toggle at said predetermined frequency.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
   The patent application is a continuation application (CA) of a U.S. patent application Ser. No. 10/352,439 filed Jan. 28, 2003, and now U.S. Pat. No. 7,168,020. The content of the related patent application is incorporated herein for reference. 

   FIELD OF THE INVENTION 
   The present invention relates to a circuit for testing an embedded phase-locked loop (PLL) circuit, and more particularly to a testing circuit for diagnosing the clock of an embedded phase-locked loop (PLL) circuit. The present invention also relates to a method for testing an embedded phase-locked (PLL) circuit to diagnose the PLL clock. 
   BACKGROUND OF THE INVENTION 
   Conventionally, test vectors are prevalently employed to screen out the unqualified chips. Since the parameters, such as the frequency, phase and duty cycle, of the clock inputted from a tester into the chip are precisely controlled, a specific test vector inputted into a normal chip is supposed to be outputted as expected. In other words, when the output of the chip in response to the specific test vector shows an unexpected result, the chip is determined to be an unqualified chip and should be ruled out. 
   Presently, a phase-locked loop (PLL) circuit is usually embedded in the chip for providing the clock signals of all required frequencies for the chip, thereby reducing the cost. Since the clock signal generated by the embedded PLL circuit is not a pure digital signal and its phase delay is unpredictable, there have been no specific test vectors, so far, and corresponding outputs for the embedded PLL circuit to perform test. Therefore, the embedded PLL circuit is not particularly tested in the prior art. 
   Therefore, the purpose of the present invention is to develop a circuit and a method for testing an embedded phase-locked loop (PLL) circuit to deal with the above situations encountered in the prior art. 
   SUMMARY OF THE INVENTION 
   The present invention is capable of testing an embedded phase-locked (PLL) circuit to properly diagnose the embedded PLL circuit for reducing errors. 
   In an embodiment, a method for testing an embedded phase-locked loop (PLL) circuit includes steps of: providing a first clock signal to an embedded phase-locked loop (PLL) circuit to be tested; generating a PLL clock signal of a first frequency by the embedded PLL in response to the first clock signal; sampling the PLL clock signal of the first frequency with a second clock signal of a second frequency to generate a first sampled signal, wherein the second frequency is different from the first frequency but has a first correlation with the first frequency so that the first sampled signal toggles at a predetermined frequency when the embedded PLL circuit is in a normal operation condition; and determining the embedded PLL circuit is in an abnormal operation condition if the first sampled signal does not toggle at the predetermined frequency. 
   In an embodiment, a method for testing an embedded phase-locked loop (PLL) circuit includes steps of: providing a first clock signal to an embedded phase-locked loop (PLL) circuit to be tested; generating a PLL clock signal of a first frequency by the embedded PLL in response to the first clock signal; sampling the PLL clock signal of the first frequency with a second clock signal of a second frequency to generate a first sampled signal, wherein the second frequency is different from the first frequency but has a first correlation with the first frequency; sampling the first sampled signal with the second clock signal of the second frequency to generate a second sampled signal; logically operating the first sampled signal and the second sampled signal to obtain a logic operational result; and directly referring to the logic operational result to determine whether the embedded PLL circuit is in a normal operation condition. 
   In an embodiment, a method for testing an embedded phase-locked loop (PLL) circuit includes steps of: providing a first clock signal to an embedded phase-locked loop (PLL) circuit to be tested; generating a PLL clock signal of a first frequency by the embedded PLL in response to the first clock signal; sampling the PLL clock signal of the first frequency with a second clock signal of a second frequency to generate a first sampled signal, wherein the second frequency is different from the first frequency but has a first correlation with the first frequency; sampling the first sampled signal with the second clock signal of the second frequency to generate a second sampled signal; inverting the second clock signal of the second frequency to obtain an inverted second clock signal; sampling the PLL clock signal of the first frequency with the inverted second clock signal to generate a third sampled signal; sampling the third sampled signal with the inverted second clock signal to generate a fourth sampled signal; logically operating the first sampled signal and the second sampled signal to obtain a first logic operational output; logically operating the third sampled signal and the fourth sampled signal to obtain a second logic operational output; and logically operating the first logic operational output and the second logic operational output to obtain an indication signal; and directly referring to the indication signal to determine whether the embedded PLL circuit is in a normal operation condition. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may best be understood through the following description with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic circuit block diagram illustrating a testing circuitry according to the present invention for diagnosing the PLL circuit embedded in a chip; 
       FIG. 2  is a circuit block diagram illustrating a preferred embodiment of the test circuit of  FIG. 1 ; 
       FIG. 3  is a schematic waveform diagram illustrating concerned signals in the IC chip according to the present invention; and 
       FIG. 4  is a circuit block diagram illustrating another preferred embodiment of the test circuit of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed. 
   The present invention provides a method for testing an embedded phase-locked loop (PLL) circuit. First of all, a clock signal of a relatively low frequency is inputted to an embedded PLL circuit to be tested. After the embedded PLL circuit reaches a stable condition, the embedded PLL stably outputs a PLL clock signal in response to the clock signal of the low frequency. Subsequently, the PLL clock signal is sampled with a clock signal of a relatively high frequency to generate a sampled signal. According to the sampled signal, it is determined whether the embedded PLL circuit operates normally. If the embedded PLL circuit is in the normal operation, the clock signal outputted thereby toggles at a predetermined frequency with substantially 50% duty cycle. Hence, if the high frequency is 2 or 2/(2n+1) times the low frequency, where n is zero or positive integer, then one of two adjacent states of the sampled signal should be of a high level, i.e. logic 1, and the other is of a low level, i.e. logic 0. Accordingly, by comparing the logic levels of two successive states of the sampled signal, the toggling situation of the embedded PLL can be determined. On the other hand, if the duty cycle of the clock signal outputted from the embedded PLL circuit is much larger or smaller than 50%, for example larger than 75% or smaller than 25%, then the embedded PLL circuit is still considered “abnormal” even though the frequency of the outputted clock signal is correct. In the case that the duty cycle is far away from 50%, the two successive sampled signals are likely to be of the same logic level, i.e. both logic 1 or both logic 0. Accordingly, the duty cycle can be determined on the basis of two successive sampled states of the clock signal. Therefore, it is easy to determine whether the embedded PLL circuit is in the normal operation condition by comparing the logic levels of two successive sampled signal states. 
   Please refer to  FIG. 1  which is a schematic circuit block diagram including a test circuitry according to the invention. A test circuit  120  is integrated into the integrated circuit (IC) chip  12  and electrically connected to the embedded PLL circuit  121  to be tested. An external clock signal (CLK) from a tester  11  is transmitted to the PLL circuit  121 , thereby generating the PLL clock signal (PLL_CLK). The tester  11  further provides another external clock signal (EXT_CLK) for the test circuit  120  for the test of the PLL clock signal (PLL_CLK). In this embodiment, both the PLL circuit  121  and the test circuit  120  are embedded in the chip  12 . Alternatively, the test circuit  120  can be disposed outside the chip  12 . 
   When performing a test on the chip  12 , the tester  11  outputs a clock signal CLK into the embedded PLL circuit  121 , and another clock signal EXT_CLK to the test circuit  120 . After a predetermined time period, for example 2.05 ms, the embedded PLL circuit  121  oscillates stably. If the embedded PLL  121  operates normally, a stable clock signal PLL_CLK will be outputted in response to the clock signal CLK. For the purpose of test, the clock signal PLL_CLK is transmitted to the test circuit  120 . The duty cycle of the clock signal EXT_CLK is 50% and the frequency thereof is, for example, twice that of the clock signal PLL_CLK. After receiving the clock signal PLL_CLK from the embedded PLL  121  and the clock signal EXT_CLK from the tester  11 , the test circuit  120  outputs an indication signal FAIL to the tester  11 . If the indication signal FAIL is at a high level, i.e. logic “1”, the embedded PLL circuit  121  is determined to operate abnormally by the tester  11 . On the contrary, if the indication signal FAIL is at a low level, i.e. logic “0”, the embedded PLL circuit  121  is determined to be in normal operation. In practice, the frequency of the clock signal EXT_CLK can be 2/(2n+1) times that of the clock signal PLL_CLK where n is zero or positive integer. 
   Please refer to  FIG. 2  which is a circuit block diagram exemplifying the test circuit  120  in  FIG. 1 . The test circuit includes two flip-flops  201  and  202  and an XNOR gate  203 . An input end of the flip-flop  201  is connected to the output end of the embedded PLL circuit (not shown) for receiving therefrom the clock signal PLL_CLK. Another input end of the flip-flop  201  is connected to the output end of the tester  11  (not shown) for receiving therefrom the clock signal EXT_CLK, and so is an input end of the flip-flop  202 . The output end of the flip-flop  201  is connected to an input end of the flip-flop  202 . The output ends of the flip-flops  201  and  202  are connected to the two input ends of the XNOR gate  203 , respectively, and the output end of the XNOR gate  203  is connected to the testing end of the tester  11 . 
   Further refer to  FIG. 3  showing a schematic waveform diagram illustrating concerned signals according to the present invention. Assume that the embedded PLL circuit  121  generates the clock signal PLL_CLK with a predetermined frequency stably. Since the frequency of the clock signal EXT_CLK is twice that of the clock signal PLL_CLK, the flip-flop  201  is triggered twice every cycle of the clock signal PLL_CLK. Thus, the sampled signal SAMP outputted from the flip-flop  201  toggles, and has a period twice that of the clock signal EXT_CLK from the tester  11 . That is, the SAMP toggles with the same frequency as the clock signal PLL_CLK. Similarly, since the frequency of the clock signal EXT_CLK is twice that of the sampled signal SAMP outputted by the flip-flop  201 , the flip-flop  202  is triggered twice every cycle of the sampled signal SAMP. Hence, the sampled signal SAMP_D 1  outputted from the flip-flop  202  toggles, and has a frequency twice that of the EXT_CLK from the tester  11 . 
   In addition, since both the sampled signals SAMP from the flip-flop  201  and SAMP_D 1  from the flip-flop  202  are triggered in response to the clock signal EXT_CLK from the tester  11 , they have the same frequency. Since the starting operation of the flip-flop  202  lags behind the flip-flop  201  by one cycle of the clock signal EXT_CLK. the sampled signal SAMP_D 1  is delayed by one cycle of the clock signal EXT_CLK. Thus, the waveforms of the sampled signals SAMP and SAMP_D 1  are in inverse phase as shown in  FIG. 3 . 
   The sampled signals SAMP and SAMP_D 1  are transmitted to the XNOR gate  203 , as shown in  FIG. 2 , and the XNOR gate  203  performs an XNOR operation on them and then outputs an indication signal FAIL. Due to the inverse phase of the two sampled signals, the indication signal FAIL is logic “0”. The low level of the indication signal FAIL indicates the embedded PLL circuit  121  operates normally. On the other hand, if the embedded PLL circuit  121  abnormally operates, the output signal FAIL will be logic “1”. 
   For example, if the output signal of the embedded PLL circuit is kept at logic “0”, i.e. not toggling at all, the sampled signals SAMP and SAMP_D 1  are both be logic “0”. Through the operation of the XNOR  203 , the output of logic “1” is obtained, and the high level of the indication signal FAIL indicates the abnormal operation, i.e. untoggling in this case. Therefore, the invention can precisely determine whether the embedded PLL circuit operates normally or not according to the logic state of the output signal from the XNOR gate  203 . 
   In another example, when the frequency of the clock signal PLL_CLK is not as stable as expected, the embedded PLL circuit  121  is determined abnormal according to the following criterion. If the frequency of the clock signal PLL_CLK is not stable, the same logic status may happen for the sampled signals SAMP and SAMP_D 1 , i.e. both logic “1” or logic “0” at the same sampling point. Under this circumstance, the indication signal FAIL from the XNOR gate  203  becomes logic “1”, and the abnormal condition of the PLL circuit  121  is indicated. For example, after one million cycles of the clock signal EXT_CLK, the logic “1” status of the indication signal FAIL may happen. Of course, it can be determined that the embedded PLL circuit  121  normally operates if the indication signal from the XNOR gate  203  always keeps at logic “0” for a predetermined time period. 
   Moreover, when the duty cycle of the clock signal PLL_CLK is much larger or much smaller than 50%, e.g. larger than 75% or smaller than 25%, the sampled signals SAMP and SAMP_D 1  are possibly at logic “1” or logic “0” simultaneously because the adjacent half-cycles of the jittering signal PLL_CLK are sampled with the ideal double frequency signal EXT_CLK. Under this circumstance, the indication signal FAIL outputted from XNOR gate  203  is logic “1”, and the abnormal condition of the PLL circuit  121  is indicated. In other words, when the duty cycle of the clock signal PLL_CLK is far away from 50%, the embedded PLL circuit  121  is considered abnormal even though the frequency of the PLL_CLK is as stable as expected. Of course, it can be determined that the embedded PLL  121  normally operates in response to the continuous low-level state of the indication signal FAIL from the XNOR gate  203 . 
   Please refer to  FIG. 4  which is a circuit block diagram illustrating another preferred embodiment of the test circuit  121  of  FIG. 1 . The test circuit includes two sub-circuits  1201  and  1202 , a inverter  407 , an AND gate  408  and an output flip-flop  409 . The sub-circuit  1201  includes two flip-flops  401  and  402  and an XNOR gate  403 , and the sub-circuit  1202  includes two flip-flops  404  and  405  and an XNOR gate  406 . The duplication of the sub-circuits  1201  and  1202  double checks the testing procedure, and assures of the accuracy of the test system. 
   Two respective input ends of the flip-flops  401  and  404  are both connected to the output end of the embedded PLL (not shown in  FIG. 4 ) for receiving therefrom the clock signal PLL_CLK. The other input ends of the flip-flops  401  and  404  are both connected to the output end of the tester (not shown in  FIG. 4 ) for receiving therefrom the clock signal EXT_CLK. The output ends of the flip-flops  401  and  404  are connected to two input ends of the flip-flops  402  and  405 , respectively. The other input ends of the flip-flops  404  and  405  are connected to the output end of the inverter  407 . The input end of the inverter  407  is connected to the tester (not shown in  FIG. 4 ) for receiving therefrom the clock signal EXT_CLK. The output ends of the flip-flops  401  and  402  are connected to the two input ends of the XNOR gate  403 , respectively. The output ends of the flip-flops  404  and  405  are connected to the two input ends of the XNOR gate  406 , respectively. The output ends of the XNOR gates  403  and  406  are connected to the two input ends of the AND gate  408 , respectively. The output end of the AND gate  408  is connected to the input end of the output flip-flop  409  which has another input end connected to the output end of the inverter  407 , as shown in  FIG. 4 . The output end of the inverter  407  is further connected to the tester  11  for outputting therefrom the indication signal FAIL. 
   The structure and function of each of the sub-circuits  1201  and  1202  are similar to those of the test circuit  120  of  FIG. 1 . Since the phase of the clock signal EXT_CLK is shifted by 180 degrees by the inverter  407 , the time points for sampling the clock signals PLL_CLK are different for the test circuits  1201  and  1202 , and the time difference therebetween is a half cycle of the clock signal EXT_CLK. The inverter  407  functions for staggering the sampling time points of the sub-circuits  1201  and  1202 , for example, by a half cycle of the clock signal EXT_CLK. By this way, while one of the sub-circuits  1201  and  1202  samples at a transition state of the clock signal PLL_CLK, which possibly causes an error, the other samples within the stable range of the clock signal PLL_CLK, which will obtain a correct result. 
   When the embedded PLL circuit is under the normal operation condition to generate a stable and predetermined frequency, both outputs FAIL 1  and FAIL 2  of the sub-circuits  1201  and  1202  are logic “0”. Since both the input ends of the AND gate  408  are logic “0”, the input end of the output flip-flop  409  connected, to the AND gate  408  is logic “0”. Accordingly, the flip-flop  409  outputs logic “0” to the test end of the tester  11  in response to the reverse clock signal EXT_CLK. 
   Even when one of the sub-circuits  1201  and  1202  samples at a time point around the transition state, which possibly causes an error, the other samples at the stable range of the clock signal PLL_CLK, which will obtain a correct result. In other words, no matter what logic state is rendered for the sub-circuit  1201  or  1202 , either “0” or “1”, the other sub-circuit always outputs logic “0”. Under this circumstance, through the AND gate  408 , a logic “0” status will be guaranteed at the input end of the output flip-flop  409 , resulting in the output of a logic “0” from the output flip-flop  409  to the test end of the tester  11  in response to the clock signal EXT_CLK. 
   On the contrary, when both the sub-circuits  1201  and  1202  output logic “1” to the two input ends of the AND gate  408 , the embedded PLL circuit is determined to be under an abnormal operation condition. Since both input ends of the AND gate  408  are at logic “1”, a logic “1” status is rendered at the input end of the output flip-flop  409 , resulting in that the output flip-flop  409  outputs logic “1” to the test end of the tester  11  in response to the reverse clock signal EXT-CLK. In practice, the abnormal operation condition may include signal drift, signal delay and/or signal jitter. 
   According to this embodiment using two sub-circuits  1201  and  1202  and the AND gate  408  to prevent over-killed, an output of logic “1” from the AND gate  408  to the tester  11  will be rendered only when both input ends of the AND gate  408  are logic “1”. Moreover, in a case that glitches occasionally occur at the circuit output end, the erroneous indication can be prevented by sampling the output flip-flop  409  only at the triggered moment. It should be understood that the probability of simultaneous occurrence of the glitch and the sampling operation is extremely low, so the output flip-flop  409  may be used for eliminating the glitch to reduce the erroneous indication of the tester  11 . 
   While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need 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.