Abstract:
The system contains a circuit apparatus and method for monitoring the status of components within a digital system, the apparatus having a digital device. A power-line is supplied to the digital device. A second signal is rendered from the power-line. At least one mask pulse is generated from a third signal. An extraction device is situated to extract a component of the power-line. A filtering device is situated to receive the extracted component. An extracted signal is output by the filtering device, wherein the extracted signal is a correlated result having characteristics corresponding to the status of components in a digital system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application entitled, “Power Line I/O Bit Stream Correlation,” having Ser. No. 60/964,586, filed Aug. 14, 2007 which is entirely incorporated herein by reference. 
    
    
     This application was made in part with Government support under contract N68335-07-C-0172 awarded by NAVAIR. 
    
    
     FIELD OF THE INVENTION 
     This invention is generally related to detecting performance variations and is more particularly related to detecting performance variations using power-line input/output signals and bit stream correlations. 
     BACKGROUND OF THE INVENTION 
     As a new field, prognostics or predictive diagnostics, is concerned with monitoring and assessing the operational status of electronic devices. The goal, beyond predicting the overall lifecycle of a device, is to determine the cause or causes of the eventual failure as well as the point in time where performance begins decreasing. To accomplish this, electronic prognostics rely on precursor signatures. These signatures indicate changes in operation that become metrics used to determine the “health status” of a digital device. Part of the on-going growth and maturation of the prognostics field involves identifying characteristics of an operating device that are predictive of performance, current health status, and remaining useful life. Once a predictive characteristic has been identified, a method must be developed that accurately and reliably extracts this characteristic for processing into a metric. 
     The best precursor signatures are those that can be correlated with failure but detected before performance is compromised. These sub-critical variations in performance give the most warning that makes them particularly useful as inputs for a prognostic health management (PHM) analysis platform or application. 
     At this time, prognostics or predictive diagnostics is a new field and in the process of discovery and maturation. The number of proven and reliable metrics is very limited. Examples of two existing metrics are Remaining Useful Lifetime (RUL) and State of Health (SoH). Prior efforts have involved destructive or invasive methodology to statistically forecast an expected device lifetime rather than monitor devices and gather the real-time data needed to determine actual lifecycles for specific devices in the field. 
     Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide an apparatus and method for detecting sub-critical variations in a digital system. Briefly described, in architecture, one embodiment of the system, among others, can be implemented as follows. The system contains a circuit apparatus for monitoring the status of components within a digital system, the apparatus having a digital device. A power-line is supplied to the digital device. A second signal is rendered from the power-line. At least one mask pulse is generated from a third signal. An extraction device is situated to extract a component of the at least one mask pulse. A filtering device is situated to receive the extracted component. An extracted signal is output by the filtering device, wherein the extracted signal is a correlated result having characteristics corresponding to the status of components in a digital system. 
     The present invention can also be viewed as providing a method of monitoring the status of components within a digital system. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: supplying a digital device with a power-line; rendering a change in the power-line into a second signal; generating at least one mask pulse from a third signal; extracting a component of the power-line; filtering the extracted component to generate an extracted signal; and determining a correlated result from the extracted signal, the correlated result having characteristics corresponding to the status of components in a digital system. 
     Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is an illustration of a system for monitoring current on a power-line, in accordance with the first exemplary embodiment of the present invention. 
         FIG. 2  is an illustrative graph of known and unknown quantities from which a desired result is attained, in accordance with a first exemplary embodiment of the present invention. 
         FIG. 3  is an illustration of a digital bit signal and a transient signal, in accordance with the first exemplary embodiment of the present invention. 
         FIG. 4  is an illustration of a positive signal and positive mask as well as a negative signal and negative mask, in accordance with the first exemplary embodiment of the present invention. 
         FIG. 5  is an illustration of a simplified mixing operation for a rising edge signal and an e+ mask producing a positive result, in accordance with the first exemplary embodiment of the present invention. 
         FIG. 6  is an illustration of a simplified mixing operation for a falling edge signal and an e+ mask producing a negative result, in accordance with the first exemplary embodiment of the present invention. 
         FIG. 7  is an illustration of signal processing of the results of  FIG. 5  and  FIG. 6  using a low pass filter, in accordance with the first exemplary embodiment of the present invention. 
         FIG. 8  is an illustration of integration, of the signals processed in  FIG. 7 , over time that results in an overall positive value, in accordance with the first exemplary embodiment of the present invention. 
         FIG. 9  is an illustration of the changes in magnitude of the correlation result that form the basis for the amplitude metric, in accordance with the first exemplary embodiment of the present invention. 
         FIG. 10  is an illustration of a circuit for determining optimal shapes of mask functions, in accordance with the first embodiment of the present invention. 
         FIG. 11  is an illustration of a circuit to produce a correlation result, in accordance with the first exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The transition of a digital bit from a high to low state or from low to high will cause a fluctuation in the supply current along a power-line feeding any device that drives that bit onto a signal line. The exact nature of the fluctuation will depend on the characteristics of the bit driver as well as the characteristics of the line loads that are driven by the bit. Thus, time-dependent transfer functions that relate a bit transition to the associated power-line fluctuation can be useful for prognosticating health of devices attached to the power-line. Two specific metrics, gain and phase shift, can be extracted from the transfer functions and utilized to predict health of individual devices and the overall system. 
     Many digital bit stream sequences appear random unless correlated against an exact replica (or suitable transform) of themselves, in which case they have a large and sharply defined autocorrelation peak. Thus, power-line fluctuations caused by a given bit may be extracted from a noisy power supply line on a device that is driving many ports simultaneously. Similarly, a single representation of a current on the power-line can be correlated against many bit streams simultaneously through parallel architectures. 
       FIG. 1  is an illustration of a system for monitoring current on a power-line, in accordance with the first exemplary embodiment of the present invention.  FIG. 1  identifies the basic signals of the system: the power-line  24  carrying the transient signal  20 , the parallel bus  64  carrying the aggregate bit stream  22  and signal line  63  carrying the bit signal for at least one load. and the correlation result  72  which is the output for the circuit.  FIG. 1  also identifies the basic circuit components such as the power supply  50 , digital device  60 , filter  78 , mixer  74 , load  62 , and mask pulse generator  68 .  FIG. 10  shows the system for monitoring current on a power-line as a similar circuit including two inputs delivered to an oscilloscope  66 : a first input is a transient signal  20  from the power-line  24  and a second input is the bit stream  22  from the load  62 . The transient signal  20  is taken from the power-line  24  so the current fluctuations can be correlated to the sampled bit stream  22 . 
     Correlating a signal requires a mask. If a signal is a corrupted digital signal which tracks a given bit stream, the correlation may be performed by a digital multiplication of the signal and the mask signal. If the signal and the mask have an approximate linear relationship, the correlation will give an indication of phase shift. If the signal is digitized with a higher sampling rate than the bit rate and a resolution greater than binary, the correlation will indicate amplitude and distortion of the signal. In each case, the correlation will be insensitive to unrelated bit streams superimposed on the signal, such as by a bus, so the effects of a given signal can be located with selection of a proper mask. 
     Normally in a digital device  60 , such as that shown in the exemplary embodiment of  FIG. 1 , the signal along the power-line  24  and the signal line  63  do not have a linear relationship. Rather, maximum power draw occurs during the logic transitions of the digital signal  30 , when voltage levels pass through the linear ranges of the driver circuits. To exploit this effect, the power-line current may be sampled only during intervals in time approximately following bit transitions and the mask, rather than being the bit pattern itself, can be a pulse synchronized with the bit edges and shaped to match anticipated rising and falling current transitions. 
     When the digital device  60  drives a significant non-reactive load  62 , the power-line  24  current will have a prominent component that is a linear reproduction of the bit stream  22 . That is, clock-cycle-wide pulses that are either in-phase or inverted copies of the bit stream  22 , as opposed to edge transients. As propagation delays are small compared to clock width, simply multiplying the bit stream  22  against the current waveform along the power-line  24  provides a simple correlation. Thus, a two-pronged approach may include a simple correlation to monitor bus load levels and a more sophisticated edge-transient correlation to monitor delays and switching characteristics. 
     Returning to  FIG. 1 , an averaging function within the oscilloscope  66  may be triggered by the rising edge of the signal  30  to average the transient signal  20  from the power-line  24  and the bit stream  22  from the load  62 . Averaged over many signal  30  transitions, the oscilloscope  66  should reveal the power-line  24  transient characteristic for the rising edge, creating a mask. The process may be repeated for the falling edge. The two masks may be adjusted to provide positive correlations with the associated transients and negative correlations with complementary transients and minimal correlation with random fluctuations. 
       FIG. 2  is an illustrative graph of known quantities  10  and unknown quantities  12  from which a desired result is attained, in accordance with a first exemplary embodiment of the present invention. The known quantities section  10  includes a clock cycle  25 , a selected bit stream  22 , and a power-line transient signal  20 . The unknown quantities section  12  includes eight bit streams A-H and eight corresponding transient signals from the other loads fed from the same source as the selected bit stream  22 . A goal section  14  includes a goal transient signal  21  specific for the selected bit stream  22 . Each of the transient signals of the unknown bit streams A-H combine with the goal transient signal  21  to form the power-line transient signal  20 . The goal transition signal  21  needs to be extracted from the power-line transient signal  20 . 
     As  FIG. 2  illustrates, extracting the goal transient signal  20  attributable to a bit stream  22  for a specific load of a power-line  24  may be a challenge. Unknown quantities section  12  illustrates a plurality of transient signals that cloud the power-line transient signal thereby giving insight towards the complexity of extracting the goal transient signal  20  from a power-line  24  with the power-line transient signals  20  impacted by the transient signals eight other bit streams A-H. 
       FIG. 3  is an illustration of a digital bit signal  30  and a transient signal  20 , in accordance with the first exemplary embodiment of the present invention. The bit signal  30  has a rising edge  32  and a falling edge  34 . The rising edge  32  may be characterized as a positive edge and may indicate the transition of a bit signal  30  from a low state to a high state, or in typically, from a zero value to a one value. The falling edge  34  may be characterized as a negative edge and may indicate the transition of a bit signal  30  from a high state to a low state or from a one value to a zero value. 
       FIG. 3  also includes a transient signal  20  having a phase shift  36  and amplitude  38 . Amplitude  38  is the peak-to-peak value of the transient signal  20  while phase shift  36  is the lag between the rising edge  32  of the bit signal  30  and the transient signal  20 . Amplitude  38  and phase shift  36  may be considered metrics targeted for extraction to detecting variations in a digital system. The phase shift  36  and amplitude  38  may also be considered fluctuations in the transient signal  20 . These fluctuations may be caused by, in part, a bit signal  30  shift in a supply current. This shift may be caused by the characteristics of input/output (I/O) drivers and loads. In the past, these fluctuations created by bit signal  30  shifts have hindered a reliable prognosis and extraction of a digital system. 
     The transition of a bit signal  30  from a high-to-low state or from low-to-high state causes a fluctuation in the supply current feeding any device driving that bit signal  30 . The exact nature of a fluctuation in the supply current  50  may depend on the characteristics of an I/O bit driver and the load associated with a circuit. I/O bit driver characteristics may commonly be slew rate, internal series resistance and leakage or pull-up/pull-down resistance, and internal capacitance. Line load characteristics may be interconnects, printed circuit board (PCB) traces, and I/O buffer inputs on other devices. The time-dependent transfer function relating an I/O bit transition to the associated driver transient signal  20  may be a good prognostic indicator for the health of a device, a driver and a load attached to a given signal line or bus. The two metrics, amplitude  38  and phase shift  36 , may be extracted from that time-dependent transfer function with a correlation operation and thereby may be ideal inputs for a Prognostic Health Management (PHM) system. 
     As may be seen in  FIG. 2 , many typical bit stream  22  sequences have a random or Pseudo-Random Noise (PRN) characteristic, only appearing random until correlated against a suitable signal which is a linear replica of themselves. When a PRN bit stream  22  is compared to its mask, the result is a large, sharply defined autocorrelation peak. This may allow transient signals  20  caused by a given I/O bit stream  22  to be extracted from a noisy power-line  24  on a device, which is driving many I/O ports simultaneously. Likewise, a single digitized representation of a supply current on the power-line  24  may be correlated against many I/O bit streams  22  simultaneously through parallel architectures, providing a very high number of prognostic indicator channels per device. 
     The signal extracted from a noisy power-line may be characterized as a low-frequency signal or a high-frequency signal. This characterization may depend on the frequency of the signal as compared to a baud rate. A low-frequency signal may be a signal with a frequency that is less than a baud rate whereas a high-frequency signal may be a signal with a frequency that is greater than a baud rate. Currently, the baud rate may be found to range from a low of 100 kHz to a high of 2 MHz, however further baud rates may fall within other ranges and are anticipated to do so. The high-frequency signal may be as high as possible, ideally 10 MHz or on the order of the inverse of a slew rate of a bit stream  22 . The low-frequency signal may generally be less than the baud rate, currently 100 kHz. 
     The correlation operation,
 
 R ( t )=∫ m ( t+t )*[ s ( t )+noise] dt  
 
is a standard tool for extracting signals from noisy environments. If a mask signal, m(t) is identical to the signal, s(t), or merely has a matching time dependence, then its Fourier components will multiply constructively with the corresponding components of the signal s(t), producing an integral which is maximized when t˜0. The magnitude of the integral R(t) indicates the amplitude of s(t), and the value of t which maximizes R indicates the phase shift  36  between the signal and mask. If the integral is carried on over a sufficiently long time interval, the m(t)*noise term may contribute a negligible amount to the integral even if the ‘noise’ contains signals in the same spectral band as the signal.
 
     If the signal s(t) is a corrupted digital signal which tracks a bit stream  22 , such as the current waveform of an I/O device driving the data onto a bus, then the correlation may be performed by a digital multiplication of s(t) with a mask signal m(t). This may be seen as merely the bit stream  22 , itself. If s(t) and m(t) have an approximately linear relationship, this correlation will give an indication of phase shift  36 . If s(t) is digitized with a sampling rate much higher than the bit rate, and a resolution greater than binary, then the correlation result will also indicate amplitude  38  and distortion of the signal. In both cases the correlation, integrated over a sufficient time interval, will be relatively insensitive to the presence of unrelated bit streams  22  superimposed on the signal s(t), so the effects on a bit signal  30  within a bus can be selected by choice of the mask bit stream  22 . 
     Generally in a digital device, the signal  30  and the supply current  50  (shown in  FIG. 10 ) do not have an exactly linear relationship, but rather, the maximum power draw occurs during the logic transitions when the voltage levels pass through the linear ranges of the driver circuits. Exploiting this effect may result in a much higher sensitivity in the bit stream  22  correlations. To accomplish this, the supply current  50  may be sampled only during the intervals in time just following the bit transitions, and the mask function, rather than simply being the bit pattern itself, can be a pulse synchronized with bit edges and shaped to match the expected current transient signals  20  for rising and falling transitions. 
     On the other hand, when a significant non-reactive load (such as a termination resistor) is driven by a digital device, the supply current  50  may have a prominent component, which is a simple linear reproduction of the bit stream  22 . In other words, the clock-cycle-wide pulses are either in-phase or inverted copies of the bit stream  22  itself, as opposed to edge transient signals  20 . In the overall picture of these pulses, propagation delays are small compared to the clock width, and a correlation can be performed by multiplying the bit stream  22  against the current waveform as previously discussed. This may be understood as a two-pronged approach including a correlation to monitor bus load levels, and a more sophisticated edge-transient signal  20  correlation to monitor delays and switching characteristics. 
     Amplitude  38  is an analog signal and fluctuations in amplitude  38  form a metric useful in digital prognostics. Linear correlation may be used to extract amplitude  38  from the bit stream  22 . Since the rising edge  32  and falling edge  34  of bit signals  30  are unique and readily distinguishable from each other, they are ideally suited for characterizing bus load levels. A primary part of extracting the amplitude  38  is to generate masks  41  for the transient signals  20 , as discussed below with respect to  FIG. 4 . 
       FIG. 4  is an illustration of a positive signal  40  and positive mask  41  as well as a negative signal  42  and a negative mask  43 , in accordance with the first exemplary embodiment of the present invention.  FIG. 4  illustrates representations of rising edges  32  and falling edges  34 , a positive mask  41  and a negative mask  43  generated by a mask pulse generator  33  (shown in  FIG. 11 ). A rising edge  32  may be referred to as e+ and a falling edge  34  may be referred to as e−. The corresponding masks  41 ,  43  are referred to as e+ mask and e− mask, respectively. In a simple power-line transient correlator, the current power transient signals  20  are multiplied with the masks using a mixer. 
       FIG. 5  and  FIG. 6  illustrate simplified representations of the mixing operations that combine a transient signal with its mask.  FIG. 5  shows a mixing operation involving a rising edge transient  32  and an e+ mask  41 . Mixing a rising edge transient  32  and an e+ mask  41  results in a positive result.  FIG. 6  shows a mixing operation involving a falling edge transient  34  and an e+ mask  41 . Mixing a falling edge transient  34  and an e+ mask  41  results in a negative value. As will be discussed with respect to  FIG. 7 , additional signal processing may involve the use of a low pass filter  76 . 
       FIG. 7  is an illustration of signal processing of the results of  FIG. 5  and  FIG. 6  with a low pass filter  76 , in accordance with the first exemplary embodiment of the present invention.  FIG. 8  is an illustration of integration, of the signals processed in  FIG. 7 , over time that results in an overall positive value, in accordance with the first exemplary embodiment of the present invention. As shown in  FIG. 8 , the processed signal accumulates over time, to produce a correlation result with an overall value that is positive when rising edge transients  32  are predominantly coincident with e+ masks  41  and falling edge transients  34  are predominantly coincident with e− masks  43 , negative when those polarities are reversed, and near zero when there is no predominant coincidence. 
     Over a longer period of time, this value will have a larger or smaller magnitude depending upon the behavior of the circuit elements involved in the creation of the power-line transient signals  20 . This is the amplitude metric  38  as shown in  FIG. 9 . Changes in the amplitude  38  are data that can be a vital signature in digital prognostics and therefore useful in monitoring variations in digital systems. Once masks  41  and  43  are generated for each edge  32  or  34 , the transient signals  20  and masks  41  and  43  are combined to create different characters of the power-line  24  current. Characters of the power-line  24  current may be used to make predictions in the health or status of a system. 
       FIG. 10  is an illustration of a circuit for determining optimal shapes of mask functions, in accordance with the first embodiment of the present invention.  FIG. 10  includes a digital device  60  connected to a load  62 . The connection may be by signal line ( FIG. 1 ) or a parallel bus  64  carrying a plurality of signal bits  30 . The load  62  can number from one to multiple.  FIG. 10  includes two inputs: a transient signal  20  from the power-line  24  and the bit stream  22  from a load  62 . The transient signal  20  is taken from the power-line  24  so the current fluctuations can be correlated to the sampled bit stream  22 . Through use of a mask pulse train synthesized from the bit stream  22 , phase shift  36  and amplitude  38  are extracted from transients  20  in the supply current  50  for a load  62 . 
     Transient signals  20  may be monitored by a digital oscilloscope  66  across a current sense resistor  68  or similar current sensor in series with the digital device  60 , which is programmed to generate a repeating toggle (square wave) on a single bit  30 . The oscilloscope  66 , with an averaging function, is triggered by the rising edge  32  of a bit signal  30 . Averaged over many transitions, the oscilloscope  66  waveform may reveal the power-line transient signal  20  that is characteristic of that edge. This process may be repeated for the falling edge  34  for the same result. The two masks  41  and  43  may be adjusted to provide positive correlations with the associated transients  20 , negative correlation with the complementary transients, and ideally, a zero correlation with random fluctuations. This need for symmetry may require the use of current monitors on both the source and drain power-lines of the device  60 , since the power-line  24  transient signals  20  will likely involve unbalanced currents. 
     This technique may be non-invasive and performed actively in real time. The prognostically-enabled devices or systems can be operational and fielded. The metrics may permit ongoing performance evaluation as conditions change and the stresses involved impact the operational envelope. The nature of the design allows for monitoring of individual loads  62  and extraction of prognostic data whether the device  60  or system is connected to any number of I/O loads  62 . 
       FIG. 11  is an illustration of a circuit situated to produce a correlation result  72 , in accordance with the first exemplary embodiment of the present invention.  FIG. 11  includes a generalized schematic diagram of a system  70  using optimal masks signals generated upon triggering from associated rising edges  32  or falling edges  34  of selected bits  30 . The mask pulses are generated in a mask pulse generator  68  triggered by the rising edge  32  and the falling edge  34  of the bit stream  22  and multiplied by the sensed current transient signal  20  at a mixing stage. The mixing stage may involve the use of a mixer  74 , which may be an RF mixer. An edge detector  76  may be used to detect the rising edge  32  or the falling edge  34 . A filter  78  may be included adjacent to the mixer  74 . The mixer  74  may output a signal to the filter  78  which may be a running correlation that is the correlation result  72 . In this figure, the current sensor  80  is a pulse transformer having a high enough RF bandwidth to faithfully transmit the current transients  20 , while blocking the DC component. The use of an analog RF mixer  74  may obviate the need to perform an ultra-high-bandwidth digitization of the current transient signals  20 ; instead, a dedicated direct digital synthesizer might generate the mask pulses. In this embodiment, the mask pulse generator  68  generates both the pulse shapes required for parity. The need for a balanced output may require that a similar correlator be fed from the negative supply conductor. 
     For parallel correlations of many bit streams  22 , the mask pulses may be fanned out to many mixers  74  one per bit  30 , and each bit stream  22  would have its own filter/accumulator. Other designs are possible for the present embodiment as well. For example, a single pair of mask pulse generators  68  and mixers  74  could generate rising edge  32  and falling edge  34  correlation terms which would then be gated into analog integrators—one per bit stream  22 —according to which transition had occurred in each.