Patent Publication Number: US-6910165-B2

Title: Digital random noise generator

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to digital circuits and, particularly, to a digital random noise generator system and methodology. 
   2. Discussion of the Prior Art 
   It is common in today&#39;s highly integrated semiconductor circuit technologies to identify chip function failures caused by the presence of noise. Most noise problems are found in multi-system chips that include analog, digital, and hybrid mixed-signal circuits. For example, a digital circuit such as a microprocessor chip may produce a significant amount of switching noise, for instance, when a clock frequency of that chip exceeds the GHz range. Such a hostile environment adversely degrades the performance of other (mostly analog) circuit systems. As another example, in a high-density memory chip, significant noise is generated by thousands of sense amplifiers switching simultaneously when a row is accessed. Furthermore, noise generated when hundreds of off-chip drivers that simultaneously switch between ground and Vdd, i.e., bounce, due to sudden change in current flow (dI/dt), become a concern for noise sensitive circuits. As another example, devices built on a structure comprising a thin lightly doped epitaxial layer on top of a heavily doped layer that are commonly used to prevent a latch-up related problem, become susceptible to the coupling noise through the substrate. 
   Noise sensitive circuits are not just limited to the analog circuits. Dynamic logic circuits, those that utilize precharge/predischarge technique, single-ended direct sensing circuits, and area-limiting circuits having floating nodes, etc. are all susceptible to noise disturbance. At the same time, the dynamic circuits themselves also create certain degree of noise due to the simultaneous precharging and discharging action. Achievement of a quiet chip neighborhood is thus an ideal situation, however, is increasingly becoming less realistic in current real chip designs. 
   There currently exist techniques for testing a circuit&#39;s noise immunity. Test macro and analyzing circuits usually small, low density, are mostly operated individually and typically require a noise generator. That is, a semiconductor noise generator is needed for generating noise in order to evaluate newly designed circuits, especially for those circuits which are believed to be noise sensitive. In a testing operation, the noise generator is implemented nearby a test circuit in order to provide the test macro with an artificial switch noise, e.g., for mimicking the noise on the real chip environment. 
   In order to ensure the circuits function as if operating in real life, noise sensitive circuits must survive creation noise disturbance that the circuit may be subject to while operating in real life. Thus it would be highly desirable that the noise generator, its noise pattern and magnitude must be made configurable, so that a full noise analysis at different noise backgrounds may be carried out. 
   Furthermore, it would be desirable that the noise generator be small in size, yet permit the degree of the noise, i.e., its magnitude and frequency, to be made tuneable. This would enable one noise generator to be repeatedly used by many different test circuits to mimic different environments. Furthermore, it would additionally be desirable to provide a random noise pattern so that the test circuits may be tested under a situation very close to the real chip operating condition. 
   U.S. Pat. No. 5,668,507 entitled “Noise generator for evaluating mixed signal integrated circuits” describes a noise generator device comprising a programmable oscillator generating noise signal having a predetermined frequency. It further comprises a programmable load buffer circuit to tune the magnitude of the noise signal. The programmability for this device is accomplished by tapping the capacitor load of a ring oscillator so that the frequency will be altered. The stage of the ring oscillator in this device is also made programmable so that the frequency of the noise can be altered. In addition, the size of the devices may also be changed to cause the frequency of the oscillator and thus the frequency of the noise to change. It provides both coarse and fine tune capabilities to tune the noise frequency. Additionally, in this prior art description, frequency divider circuits are used to divide the frequency. 
   In the device described in U.S. Pat. No. 5,668,507, the resulting noise pattern has a rather regular format, i.e., the noise has a frequency and magnitude which may vary in specific, or predetermined ranges. In the real chips, such a noise pattern is unlikely to occur, especially for asynchronous circuit designs. 
   Therefore, it is highly desirable to provide a random noise generator circuit capable of generating a random, or at least pseudo random, noise pattern for noise study, wherein the random noise is defined as a noise pattern having no detectable frequency or magnitude. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a random noise generator circuit that produces truly random signals, i.e., signals having no discernable frequency or magnitude, for testing noise sensitivity of semiconductor circuits. 
   It is a further object of the invention to provide a random noise generator circuit that produces truly random signals, i.e., signals having no discernable frequency or magnitude, and that are enabled to generate noise mimicking different kinds of noise including, but not limited to noise such as, switching noise, thermal noise, coupling noise, packaging noise, etc. 
   It is another object of the invention to provide a digital random noise generator circuit that can generate variable patterns of random noise having frequency and magnitude that do not follow any particular pattern and, that further provides the capability of tuning both frequency probability and magnitude probability of the noise. 
   According to the principles of the invention, there is provided a system and method for generating random noise for use in testing electronic devices comprises a first random pattern generator circuit for generating first sets of random bit pattern signals; one or more delay devices each receiving a trigger input signal and a random bit pattern signal set for generating in response a respective delay output signal, each delay output signal being delayed in time with respect to a respective trigger signal, a delay time being determined by the bit pattern set received; and, an oscillator circuit device associated with a respective one or more delay devices for receiving a respective delay output signal therefrom and generating a respective oscillating signal, each oscillator signal being used to generate artificial random noise for emulating a real noise environment in an electronic device. 
   Advantageously, a second random pattern generator circuit may be provided for generating second sets of random bit pattern signals for receipt by each of the oscillator circuit devices associated with a respective one or more delay devices in order to frequency adjust in a random manner, each of the oscillator signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features, aspects and advantages of the apparatus and methods of the present invention will become better understood with regard to the following description, appended claims, and the accompanying drawings in which: 
       FIG. 1  illustrates a schematic diagram of the random noise generator circuit  10  according to a first embodiment of the invention. 
     FIG.  2 ( a ) illustrates the various waveform diagrams of the set signal  22 , propagating control signals  11 ′- 14 ′ for delay elements D 1 -D 4  and oscillator signals X 1 -X 4  according to a first embodiment of the invention. 
     FIG.  2 ( b ) illustrates resultant noise signal  31  generated in accordance with the circuit of FIG.  1 . 
     FIG.  2 ( c ) illustrates resultant noise signal  31  generated in accordance with the random noise generator circuit of the third embodiment of the invention depicted in FIG.  4 . 
       FIG. 3  illustrates a schematic diagram of the random noise generator circuit  9 ′ according to a second embodiment of the invention. 
       FIG. 4  illustrates a schematic diagram of the random noise generator circuit  9 ″ according to a third embodiment of the invention. 
       FIG. 5  illustrates a circuit schematic diagram of an example random generator circuit  10 . 
     FIG.  6 ( a ) illustrates a circuit schematic diagram  120  of a shift register unit implemented in the random noise generator circuit of the invention. 
     FIG.  6 ( b ) illustrates a circuit schematic diagram of seed circuit  100  for generating data signals that set the initial conditions of each shift register unit. 
       FIG. 7  illustrates an example HSPICE simulation of the random pattern generator circuit  10  showing the output signals RN 1  to RN 5  of respective shift registers of FIG.  5 . 
   

   DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1  illustrates a schematic diagram of the random noise generator circuit  9  providing an adjustment scheme according to a first embodiment of the invention. As shown in  FIG. 1 , the random noise generator circuit  9  includes a random pattern generator component  10  for generating a random pattern of binary bits  26 , e.g., 5-digit random binary bits; at least one delay element component; at least one ring-oscillator component; and, at least one output driver.  FIG. 1  illustrates an example system comprising four components, namely, a parallel configuration of four digital delay elements numbered D 1 -D 4  for simultaneously receiving the generated random binary bit pattern  26 ; a parallel configuration of four ring-oscillator components C 1 -C 4  corresponding to each digital delay element D 1 -D 4  for receiving an output from its respective delay element output; and, a parallel configuration of four corresponding output driver circuits  19  for driving respective generated noise signals X 1 , . . . , X 4  to a device under test, e.g., power supply or ground  20 . 
   In operation, a set signal  22  and clock signal (not shown) are simultaneously input to the random pattern generator  10  for initiating generation of a random pattern  26 . The set signal  22  is additionally input to an XOR gate  25 . At every clock cycle or number of clock cycles, the random pattern generator  10  generates 5-digit random binary bits  26 , e.g., 11001, 10110, 00011, 10110, etc. for input to each of the digital delay components D 1  to D 4 . In response to an input trigger signal, each delay element will generate a corresponding output signal  11 ′, . . . , 14 ′ that functions to control the turn-on time of each respective oscillator C 1  to C 4  in accordance with a received random binary bit pattern input. Further details regarding the operation of the digital delay elements D 1 -D 4  may be found in commonly-owned, issued U.S. Pat. No. 6,348,827 entitled PROGRAMMING DELAY ELEMENT AND SYNCHRONOUS DRAM USING SAME, the whole contents and disclosure of which is incorporated by reference as if fully set forth herein. 
   As further shown in  FIG. 1 , a respective output signal  11 ′, . . . ,  13 ′ of each delay element functions as a respective trigger input to a next successive digital delay element for initiating the delay operation of each delay element. For example, digital delay element output  11 ′ is input to the delay element D 2  to trigger operation of D 2 , delay element output  12 ′ is input to the D 3  element, and delay element output  13 ′ is input to the D 4  element. The final delay element output  14 ′ is provided as a feedback input to the XOR gate  25  along with the set input functioning to either terminate or commence the delay operation of delay element D 1 . 
   The noise circuit  10  particularly operates as follows: The Set signal  22 , and a clock signal (not shown) start the random pattern generator circuit  10  which yields the first 5 binary digits for input to D 1  to determine the turn on time of the C 1 . The Set signal  22  and the feedback signal  14 ′ are preferably initiated at zero to activate the XOR gate  25 , and also kick on the D 1  delay circuit. As known, the delay element D 1  implements the incoming 5 bits to set the delay time. It is understood that the clock period must be longer than the delay time. Once the delay element D 1  is on it generates output  11 ′ to enable C 1  to oscillate and result in a first noise set. 
   FIGS.  2 ( a )- 2 ( c ) illustrate the various waveform diagrams of the set signal  22 , and example oscillator output signals X 1  to X 4  and the example generated noise (Vdd or ground) signals. As shown in FIGS.  1  and  2 ( a ), the random noise generating process may be initiated by a rising edge of the set signal  22 . After a first delay time t 1  relative to set time (determined by the initial program (5 bits) as input to D 1  from random circuit  10 , D 1  signal  35  and the corresponding oscillator C 1  is activated as exemplified in FIG.  2 ( a ). Once the C 1  is activated as indicated by the rising edge of signal  11 ′, C 2  will be activated after a delay time t 2  which is determined by the newly issued random digits (5 bits) from random circuit  10  as input to D 2 . Activation of C 2 , as indicated by the rising edge of signal  12 ′, will cause the oscillator C 2  to oscillate as exemplified by signal X 2  of FIG.  2 ( a ). At this moment in time, the resulting noise will be the sum of the signals X 1  and X 2  from C 1  and C 2  respectively, since both are on. Once the D 2  is activated, C 3  will be activated after a delay time t 3  which is determined by the newly issued random digits (5 bits) from random circuit  10  as input to D 3 . Activation of C 3 , as indicated by the rising edge of signal  13 ′, will cause the oscillator C 3  to oscillate as exemplified by signal X 3  of FIG.  2 ( a ). Likewise, once the D 3  is activated, C 4  will be activated after a delay time t 4  which is determined by the newly issued random digits (5 bits) from random circuit  10  as input to D 4 . Activation of C 4 , as indicated by the rising edge of signal  14 ′ will cause the oscillator C 4  to oscillate as exemplified by signal X 4  of FIG.  2 ( a ). At this moment, all the oscillators C 1  to C 4  are oscillating with a constant frequency to provide outputs X 1 -X 4  as shown in FIG.  2 ( a ). 
   As in the case of propagating active signals for turning on each oscillator (C 1 -C 4 ), similarly, oscillator turnoff control signals are propagated sequentially to shut off each of the oscillators. In this instance, the turning off of each oscillator is initiated when oscillator C 4  is turned on. That is, the feedback  14 ′ signal (output of D 4 ) is switched from low to high, causing the XOR gate  25  to shut off the D 1 . The delay time between the turning on of oscillator C 4  (D 4 ) on and turning off of oscillator C 1  (D 1 ) is indicated as a time t 5 , as set by the newly issued random digits (5 bits) from random circuit  10  as input to D 1 . The turning off of oscillator C 1  due to falling edge of signal  11 ′ initiates turning off of oscillator C 2  after a delay time t 6  as set by the newly issued random digits (5 bits) from random circuit  10  as input to D 2 . Likewise, the turning off of oscillator C 2  due to falling edge of signal  12 ′ initiates turning off of oscillator C 3  after a delay time t 7  as set by the newly issued random digits (5 bits) from random circuit  10  as input to D 3 . Finally, the turning off of oscillator C 3  due to falling edge of signal  13 ′ initiates turning off of oscillator C 4  after a delay time t 8  as set by the newly issued random digits (5 bits) from random circuit  10  as input to D 4 . 
   It should be understood that the process repeats itself such that, at the time output signal  14 ′ of delay circuit D 4  is turned off, e.g., falling edge, the signal is input to the XOR gate  25  to initiate again the turning on of oscillator C 1  after a delay time t 9 , which propagates signal  12 ′ to turn on oscillator C 2  after delay time t 10 , and so on. 
   Returning to  FIG. 1 , connected to each respective oscillator is a pair of inverter devices  19  which are properly sized to provide a load in order to meet the desired noise magnitude. The device under test  20  for which noise is to be coupled, preferably comprises a power supply, e.g., a Vdd (such as fabricated in a n-well) or a ground (e.g., a p-substrate). FIG.  2 ( b ) illustrates the corresponding noise signals  31  generated as a result of coupling oscillator outputs X 1 -X 4  to the power supply circuit in accordance with the first embodiment. It should be noted from FIG.  2 ( b ) that the magnitude of the noise spikes are identical. 
   In short, in the first embodiment, the random digits are used to cause variable turn-on and turnoff time of the oscillator components, so that a first order of random noise may be generated. 
     FIG. 3  illustrates a schematic diagram of the random noise generator circuit  9 ′ providing an adjustment scheme according to a second embodiment of the invention. The embodiment of the random noise generator circuit  9 ′ depicted in  FIG. 3  is exactly the same as in  FIG. 1 , however, a second random digit pattern generator random pattern  10 ′ is provided for generating a random pattern of binary bits  32  used for adjusting the frequency of each oscillator circuit C 1 -C 4 . Preferably, the frequency adjustment only occurs when the respective oscillator is turned on. As it is already established that the frequency of the oscillator may be altered if the transistor size or R/C loading of each oscillator stage is changed, then the random pattern of binary bits  32  may be used to adjust the size of the pull-up, pull-down device, or size of the R/C loading components included in the respective ring oscillator circuit. Thus, in the second embodiment, for example, shunt transistors (not shown) are implemented to by-pass particular R/C components in an oscillator stage in order to adjust the oscillator loading. In other words, whenever an oscillator C 1 -C 4  is activated in the propagated manner described herein and turned on, the random digits generated by the random digit generator  10 ′ and present at the respective oscillator circuit C 1 -C 4  will initiate the turning on/off of these by-pass transistors to selectively bypass the “R” or “C” component in order to adjust the frequency of the respective oscillator. This frequency control adjustment in conjunction with the random delay adjustment will result in different respective output signals X 1 -X 4  for generating noise in power supply and ground circuits that are different from the noise generated by the circuit  9  of the first embodiment. 
     FIG. 4  illustrates a schematic diagram of the random noise generator circuit  9 ″ providing an adjustment scheme according to a third embodiment of the invention. The embodiment of the random noise generator circuit  9 ″ depicted in  FIG. 4  is exactly the same as the second embodiment (FIG.  3 ), however, in the third embodiment, an additional random digit pattern generator random pattern  10 ″ is provided for generating a pattern of binary bits  36  used for randomly adjusting the magnitude of the noise. That is, by activating one or more shunt transistors (not shown) under control of a respective bit of the random pattern of generated bits  36  present at the inverter buffers  19  corresponding to the current operating oscillators, the sizes of the pull-up and pull-down devices present in the inverter buffers  19  may be adjusted, i.e., increased, or decreased, for example. Adjusting the size of the inverter buffers, in effect, alters the magnitude of the resultant noise X 1 -X 4  generated for the corresponding oscillator C 1 -C 4 . For example, the larger the buffer driver size, the greater in magnitude of the resultant noise spike generated, and vice versa, the smaller the buffer driver size, the smaller in magnitude the resultant noise spike generated. FIG.  2 ( c ) illustrates the resulting noise waveforms  35  having mixed magnitude generated in accordance with the circuit of the third embodiment of FIG.  4 . 
   In accordance with another aspect of the invention, the random pattern generator circuit  10 ,  10 ′ and  10 ″ described in accordance with the first through third embodiments, is now described with respect to FIG.  5 . Although a description is provided for the random pattern generator circuit  10  of  FIG. 1 , it is understood that the principles of operation described are applicable for each generator circuit  10 ′ and  10 ″ provided for the other two embodiments. In the example random generator circuit  10  of  FIG. 5 , an amount of stage shift registers corresponding to the number of bits in the respective pattern is provided. In the example circuit  10  described, five (5) stage shift registers SR- 1  to SR- 5  are implemented since five (5) digits are generated. FIG.  6 ( a ) illustrates a circuit schematic diagram  120  of a shift register unit implemented in the random noise generator circuit of the invention. In operation, a clock signal “CLK”  146  having a certain frequency is chosen to simultaneously trigger each of the shift registers. FIG.  6 ( b ) illustrates a circuit schematic diagram of seed circuit  100  for generating data signals  111 - 115  that set the initial conditions of each shift register unit. It is easily seen that in the example random bit pattern generator circuit  10 , the seed circuit  100  of FIG.  6 ( b ) sets the initial condition of the five registers to a bit pattern ‘10101’ when the set signal  22  is high, however, it should be understood that the initial conditions may be different for each random generator circuit  10 ,  10 ′ and  10 ″ in the embodiments of  FIGS. 3 and 4 . It is important note all ‘0’s and all ‘1’s are prohibited for the initial condition signals  111 - 115 , however, any other binary combinations are acceptable. 
   Referring back to  FIG. 5 , each initial condition signal  111 - 115  is fed into a respective clock complement signal “{overscore (CLK)}” input  147  of a respective shift register. The output of each register (O 1 -O 5 ) is respectively fed together with the output of the second previous stage shift register into a respective XOR gate. For instance, the XOR gate  103  receives a signal O 4  output of shift register SR- 4  and a signal O 2  output of shift register SR- 2  and generates an output  106  which is fed to the input of Shift Register SR- 4 ; likewise, XOR gate  107  receives a signal O 5  output of shift register SR- 5  and a signal O 3  output of shift register SR- 3  and generates an output  110  which is fed to the input of Shift Register SR- 5 ; and so on. At the same time, the output of each shift register O 1  to O 5  are forwarded to one or more inverters  111 - 115  to generate the final random digits herein labeled RN 1  to RN 5  corresponding to shift registers SR- 1  to SR- 5 , respectively. It is these signals are input to the delay circuits and ring oscillator circuits for generating random signal patterns to provide random noise on a chip. It should be understood that other configurations of the above described random pattern generator circuit  10  are possible. For example, the XOR gate  103  may receive a signal O 4  output of shift register SR- 4  and a signal O 2  output of shift register SR- 2  and generates an output which is fed to the input of Shift Register SR- 3 . It is understood that such configuration changes directed to XOR gate inputs and output connections to the serial shift registers may control the degree of pseudo-randomness in the random pattern generator. 
     FIG. 7  illustrates an example HSPICE simulation of the random pattern generator circuit  10  showing the output signals RN 1  to RN 5  of respective shift registers SR- 1  to SR- 5  after a number of clock cycles  146 . 
   While the invention has been particularly shown and described with respect to illustrative and preformed embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention which should be limited only by the scope of the appended claims.