Abstract:
A novel a method for determining the internal operation of an integrated circuit (IC) includes measuring electromagnetic (EM) emissions from the integrated circuit chip and analyzing the EM emissions. In a particular method, the EM emissions from the IC are measured using an RF close end probe. In a particular method, the electromagnetic emissions are measured with the IC configured in various ways. In the normal operating mode, the emissions are measured while the IC is provided with power and any external clock signal(s). After measuring the emissions of the IC in normal operating mode, the IC is reconfigured by disabling the external clock signal(s) to the IC and remeasuring the emissions. The external clock signal is disabled by disconnecting the power to the IC, disabling the external clock signal, and then reconnecting power to the IC. In yet a third test mode, the external clock signal is reenabled while power continues to be supplied to the IC. Information about the presence and/or proper functioning of internal clocks of the IC can be determined by analyzing the spectral scan data obtained in one or more of the three test modes.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/733,397 entitled “Method for Determining Information About the Internal Workings of a Chip Based on Electro-Magnetic Emissions Therefrom,” filed Nov. 4, 2005 by the same inventor, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to integrated circuits, and more particularly to techniques used to determine the internal workings of an integrated circuit through analysis electro-magnetic emissions. 
     2. Description of the Background Art 
     Integrated circuits (IC) are miniaturized electronic circuits consisting mainly of semiconductors and passive devices. Due to their relatively small size, high performance, and low cost, ICs have become quite common in almost every electronic device manufactured today. In most instances, ICs range in size from only a few square millimeters up to around 250 square millimeters. While this small size is great for packaging within electronic devices, it often makes troubleshooting and quality control very difficult. 
     The testing of ICs is typically done at least twice during the manufacturing process; once at the wafer stage, and once as the individual ICs are packaged in an electronic device. Automated test equipment (ATE) such as wafer probes are the most common means for testing ICs. A wafer prober uses a probe card as an interface between the electronic test equipment and an IC. The problem with wafer probing is that it requires highly sophisticated and expensive test equipment, and it is very time consuming. Additionally, the equipment must also be calibrated such that it does not damage the IC during testing. 
     In order to reduce the time and costs associated with IC testing, ICs are often designed with testability features. The problem with testing only the testability features is that it is often difficult to determine if the IC is indeed operating properly by only testing a select few features. This type of testing more typically filters functional devices from non-functional devices. 
     One of the largest obstacles in IC testing is the testing of clock signals. Integrated circuits often include one or more clock signals for coordinating the actions of two or more circuits. These clock signals oscillate between a high and low state at a predetermined clock rate (cycles per second, as measured in Hertz). The clock signal(s) are used to synchronize different parts of the circuit, and also to account for delays in transmission. As ICs become more complex, a particular IC may have many clock signals present, with various different clock rates. It is often difficult to determine if the clock rates are operating properly via known test procedures. 
     As may be appreciated, if a clock signal is operational, but is either inaccurate or not properly synchronized, certain functions or calculations may not be performed correctly by the IC, resulting in a functional, yet defective device. Such devices are often difficult to diagnose or identify through known test procedures. 
     Additionally, often times manufacturers may find it necessary to reverse engineer products. Whether it be their own ICs, competitor&#39;s ICs, or vendor&#39;s ICs, reverse engineering is a useful tool in designing, diagnosing, and improving IC&#39;s. This practice up until now has included much of the previously described IC testing procedures, as well as some dissection type procedures. The problem has been that in addition to the expensive test equipment required, the process is typically destructive, and may require multiple trained professionals to decipher functions from parts. Indeed, this destructive type reverse engineering may be particularly troublesome if there is only one sample to work with. 
     What is needed, therefore, is a non-destructive system and method for testing the operation of internal clocks of integrated circuits. 
     SUMMARY 
     The present invention overcomes the problems associated with the prior art by providing a non-destructive system and method for testing internal clocks of integrated circuits by measuring electromagnetic emissions. The invention facilitates easier identification of faulty ICs by providing a non-destructive means for testing and identifying clock signals, as well as aiding in the reverse engineering of a particular IC through spectrum analysis. 
     According to one aspect of the present invention, a method for determining the internal operation of an integrated circuit (IC) includes measuring electromagnetic (EM) emissions from the integrated circuit chip and analyzing the EM emissions. In a particular method, the EM emissions from the IC are measured using an RF close end probe. The measurements can be taken over several different locations with respect to the IC. 
     In a particular method, the electromagnetic emissions are measured with the IC configured in various ways. In the normal operating mode, the emissions are measured while the IC is provided with power and any external clock signal(s). After measuring the emissions of the IC in normal operating mode, the IC is reconfigured by disabling the external clock signal(s) to the IC and remeasuring the emissions. The external clock signal is disabled by disconnecting the power to the IC, disabling the external clock signal, and then reconnecting power to the IC. In yet a third test mode, the external clock signal is reenabled while power continues to be supplied to the IC. 
     Various methods for analyzing the EM emissions are also disclosed. According to one method, the EM emissions are analyzed to determine whether they are below a predetermined radio frequency interference (RFI) level. According to another particular method, the EM radiation is measured over a range of frequencies, and the step of analyzing the EM emissions includes identifying frequencies corresponding to amplitudes greater than a predetermined amplitude (e.g., identifying frequency peaks in the spectrum). The identified frequencies can then be compared to a predetermined set of frequencies to determine whether the clock(s) of the IC are operating properly. In an alternative analysis, the identified frequencies are compared to sets of frequencies associated with known devices to determine whether any of the known devices are embedded in the IC. In yet another alternate analysis, the identified frequencies from the emissions of the IC in one test mode can be compared with the emissions of the same IC in another test mode to determine whether any internal clocks are synching with an internal clock or with the external clock signal. 
     The methods of the present invention can be implemented with an electronically readable medium having code embodied therein for causing an electronic device to perform and or facilitate any of the methods of the present invention. 
     An apparatus for determining the internal operation of an IC is also described. One embodiment of the apparatus includes a probe, a spectrum capture device, and a spectrum analyzer. The probe senses EM radiation from the IC. The spectrum capture device is operative to convert EM radiation sensed by the probe into electronic data. The spectrum analyzer is operative process the electronic data to provide information about the internal operation of the IC. 
     In one embodiment, the apparatus includes a display. The spectrum analyzer displays the captured spectrum data on the display in, for example, an amplitude versus frequency graph. In this embodiment, the data can be interpreted directly by the user simply by viewing the data on the display. 
     Another embodiment includes a processing unit for executing code and processing data, and memory for storing the data and code. The code includes a spectrum capture routine and a spectrum analyzer routine. The spectrum capture routine captures the electronic spectrum data and stores the data as records indicative of an EM spectrum emitted by the IC. Optionally, the spectrum capture routine can control the position of the probe and/or the configuration of the IC being tested. For example, the spectrum capture device can selectively interrupt the electrical power to the IC and/or selectively enable/disable the external clock signal. 
     The spectrum analyzer routine is operative to analyze the spectrum data captured by the spectrum capture routine. For example, the spectrum analyzer can compare sets of spectrum data captured from an IC during different test modes to ascertain relevant differences in the spectrum data. As another example, the spectrum analyzer can compare the captured spectrum data to spectrum data indicative of the proper operation of an internal clock of a known device to determine if the IC is operating properly. As yet another example, the spectrum analyzer can compare the captured data with sets of data associated with known electronic devices to determine whether any of the known devices are embedded in the IC. 
     Novel data structures are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described with reference to the following drawings, wherein like reference numbers denote substantially similar elements. 
         FIG. 1  illustrates an apparatus for testing integrated circuits according to one embodiment of the present invention; 
         FIG. 2  is a block diagram showing the computer system of  FIG. 1  in greater detail; 
         FIG. 3  is a diagram showing an example data structure for device EM spectrum data; 
         FIG. 4  is a diagram showing an example data structure for known device data; 
         FIG. 5  is a flowchart summarizing one particular method for determining the internal operation of an integrated circuit (IC) via electromagnetic emissions from the IC; 
         FIG. 6  is a flowchart summarizing one example method for configuring and reconfiguring an integrated circuit for an EM emissions scan; 
         FIG. 7  is a flowchart summarizing one example method for configuring the integrated circuit for an EM emissions scan without an external clock source; 
         FIG. 8  is a flowchart summarizing one example method for measuring EM emissions from an integrated circuit; 
         FIG. 9  is a flowchart summarizing one example method for analyzing EM emissions to determine the proper operation of an internal clock of a known integrated circuit; 
         FIG. 10  is a flowchart summarizing one example method for analyzing EM emissions to determine the presence and character of an internal clock on an unknown integrated circuit; 
         FIG. 11  is a flowchart summarizing one example method for measuring an EM emissions spectrum and writing the associated data to spectrum files; and 
         FIG. 12  illustrates an example spectrum analysis graph as displayed on the display of the computer of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention overcomes the problems associated with the prior art, by providing a system and method for the non-destructive testing of clock signals of integrated circuits. In the following description, numerous specific details are set forth (e.g., example testing methods) in order to provide a thorough understanding of the invention. Those skilled in the art will recognize, however, that the invention may be practiced apart from these specific details. In other instances, details of well known circuit testing practices (e.g., circuit connections, testing equipment, etc.) have been omitted, so as not to unnecessarily obscure the present invention. 
       FIG. 1  shows an apparatus  100  for testing an integrated circuit (IC)  102  according to one embodiment of the present invention. Testing apparatus  100  includes a general purpose computer system  104 , a robotic arm  106 , an electromagnetic emission sensing probe  108 , a power and clock signal generator  110 , and an IC interface  112 . Robotic arm  106  is electrically coupled to and controlled by computer system  104  via cable  114 , and power and signal generator  110  is coupled to and controlled by computer system  104  via cable  116 . Cables  114  and  116  are representative of any type of communication cable, including but not limited to a USB cable, a firewire cable, an ethernet cable, and so on. Robotic arm  106 , under the control of computer system  104 , positions probe  108  during the measurement of EM emissions from IC  102 . Power and clock signal generator  110  provides electrical power and clock signals, via electrical connections  118  and interface  112 , to IC  102  to place IC  102  into several different test modes. 
     In one test mode, power and clock signal generator  110  provides IC  102  with power and an external clock signal, such that IC  102  operates in a normal mode of operation. During normal operation internal clocks (not visible) in integrated circuit  102  will emit electromagnetic (EM) radiation  120 . EM emissions  120  are sensed by probe  108  as robotic arm  106  moves probe  108  over integrated circuit  102 . EM emissions  120  sensed by probe  108  are converted to electrical signals and transmitted to computer system  104  via cable  114 , where the electrical signals are analyzed to discern information regarding the presence and/or operation of clocks in IC  102 . The electrical signals can be processed by software running on computer  104  and or displayed graphically on a display  122  of computer  104 . The particular testing procedures will vary based on the purpose of the testing, and further details of the equipment and specific test procedures will be outlined in later figures. 
     In a particular embodiment, probe  108  is a radio frequency close end probe with a bandwidth of 30 Mhz to 1 Ghz or DC to 30 Mhz connected via an external 25 db gain amplifier (not shown) into a spectrum analyzer (in computer  104 ) with a bandwidth capability of DC to 7 Ghz. By using a close end type probe  108 , emissions may be localized to the tip of probe  108 , thereby eliminating the need to use an external screen room in order to localize the emissions leakage to the system component without having exterior emissions compromising the results. 
       FIG. 2  is a block diagram showing computer system  104  in greater detail. Computer system  104  includes non-volatile data storage  202 , one or more processing units  204 , working memory  206 , user I/O devices  208 , and EM probe and test mode interface  210 , all intercommunicating via an internal bus  212 . Non-volatile data storage  202  stores data and code that are retained even when computer system  104  is powered down. Typical examples of non-volatile data storage include read only memory (ROM), hard disk drives, optical disk drives, and other types of removable media. Processing unit(s)  204  impart functionality to computer system  104  by processing executable code stored in non-volatile data storage  202  and memory  206 . Working memory  206  provides temporary storage for data and code being processed by processing unit(s)  204 . User I/O devices  208  provide a means for the user to interact with computer system  104 , and typically include such devices as a keyboard, display  122 , a printer, a pointing device, and so on. EM probe interface  210  is an interface that converts EM emissions received by electromagnetic sensing probe  108  into digital data that can be processed (e.g., displayed, stored, analyzed, etc.) by computer system  104 . 
     In order to clearly explain the operation of testing apparatus  100 , the functionality of computer system  104  is shown representationally as code blocks in memory  206 . Those skilled in the art will understand, however, that all of the code need not remain in memory  206  during the operation of computer system  104 . Indeed, processing unit(s)  204  will typically shuffle portions of the code into and out of memory  206  (e.g. to/from non-volatile data storage  202 , etc.), for execution as required during operation. Further, although the functional blocks in memory  206  are shown to be physically coupled, those skilled in the art will understand that they are actually processes that communicate by calling one another for execution. 
     As shown in  FIG. 2 , memory  206  includes an operating system  214 , one or more application programs  216 , a device database  218 , a spectrum analyzer  220 , a spectrum capture routine  222 , spectrum files  224 , a master control routine  226 , and calibration routine  228 . Operating system  214  is a low level program upon which the other programs run. Application program(s)  216  is representative of word processing programs, graphics programs, and the like, and is intended to show that computer system  104  is a general purpose computer system that can host other applications that can enhance or augment the operation of computer  104  within testing apparatus  100 . 
     Device database  218  is a database including records of manufacturers&#39; specifications or empirically determined data for various integrated circuits. The records of device database  218  are used in the analysis of test data as a means for comparison, in determining the proper operation of known devices or the identification of unknown devices. Spectrum capture routine  222  is a program for initializing and terminating a test and controls the collection of EM emission data via EM probe and test mode interface  210 . Spectrum analyzer  220  is a program that analyzes the collected spectrum data to provide information (e.g., existence and or proper operation of internal clocks, identification of unknown device, etc.) about the device being tested. Spectrum files  224  are files that store the data collected during a test scan and associate that data with the device tested. Spectrum files  224  can include, without limitations, data such as clock frequencies, clock circuit locations, chip architecture, and so on. Additionally, spectrum files  224  can be compared with device database  218  records to identify unknown devices, demonstrate the existence of an embedded processor, and so on. Master control  226  provides overall coordination and control of the testing features of computer  104  and, in particular, synchronizes the operations of robotic arm  106  and power and clock signal generator  110  with spectrum capture routine  222 . In addition, master control  226  provides data to and receives commands from a user via user I/O devices  208 . In general, master control  226  invoke the various other components of memory  206  in order to properly synchronize data collection, data display, data storage, data analysis, and so on throughout the testing procedure. Calibration routine  228  includes routines for calibrating EM sensing probe and amplifiers included in EM probe and test mode interface  210 . 
     It should be noted that the function and data groupings within memory  206  may vary without departing from the scope of the present invention. These example functional and data groupings should not be limitations or essential elements of the invention. Rather, they are provided as an illustration of one particular mode of implementing the present invention, as well as to provide a more thorough understanding of the invention. 
       FIG. 3  is a database table  300  illustrating one example of a data structure that is suitable for use for spectrum files  224  of  FIG. 2 . The records of table  300  includes a DUT No. field  302 , a Scan ID field  304 , a Frequency Peak field  306 , a Frequency Sweep Low Value field  308 , a Frequency Sweep High Value field  310 , a Peak Search field  312 , and a Spurious Frequency field  314 . DUT No. field  302  includes data indicative of a particular device under test. Scan ID field  304  includes data indicative of a particular scan (e.g., date and time). Frequency Peak field  306  includes data indicative of a particular frequency of EM radiation that exceeded a predetermined amplitude during the test scan, so as to be considered a frequency peak. Frequency Sweep Low Value field  308  includes data indicative of the lowest frequency measured during the scan. Frequency Sweep High Value field  310  includes data indicative of the highest frequency measured during the scan. Peak Search field  312  includes data indicative of the predetermined amplitude used to define a frequency peak. Finally, Spurious Frequency field  314  includes data indicative of whether or not the peak associated with the record appears to be just a spurious peak or the result of a clock operating on IC  102 . 
       FIG. 4  is a database table  400  illustrating one example of a data structure that is suitable for use for device database  218  of  FIG. 2 . The records of table  400  include a Manufacturer Name field  402 , a Manufacturer&#39;s Part No. field  404 , a Number of Bits field  406 , a Clock Speed field  408 , a Technology field  410 , and a Power/MHz field  412 . Manufacturer Name filed  402  includes data indicative of the manufacturer of the device associated with the record. Manufacturer&#39;s Part No. field  404  includes data indicative of a unique number used by the manufacturer to identify the device. Number of bits field  406  includes data indicative of the bit classification of the device. For example, is the device a 32-bit processor, a 64-bit processor, etc. Clock Speed field  408  includes data indicative of the clock speed(s) at which the device operates. Technology field  410  includes data indicative of the technology classification of the device. Finally, Power/MHz field  412  includes data indicative of power consumed by the device divided by the frequency at which the device operates. 
     The example fields shown in  FIGS. 3-4  are shown by way of example and without limitation. Each of these fields includes data that can help identify an unknown device or demonstrate the presence of a known device. It should be understood however, that additional or alternative types of data could be used for these purposes without departing from the scope of the invention. 
       FIG. 5  is a flowchart summarizing one example method  500  for testing an integrated circuit via electromagnetic emissions from the integrated circuit. In a first step  502 , an integrated circuit is provided for testing. Then, in a second step  504 , the integrated circuit is configured for a first EM emissions scan. Next, in a third step  506  EM emissions from the integrated circuit are measured. Then, in a fourth step  508 , the measured EM emissions are analyzed to determine the presence and/or operability of an internal clock. Next, in a fifth step  510 , it is determined whether additional measurements are required. If not, method  500  ends. Otherwise, method  500  returns to second step  504 , where the integrated circuit is reconfigured for the next EM scan. 
       FIG. 6  is a flowchart summarizing one example method  600  for performing second step  504  (configure/reconfigure IC) of method  500 . In a first step  602  it is determined whether a first scan of the IC has been completed. If not, then in a second step  604  the IC is configured for the first scan (test mode  1 ), and method  600  ends. If the first scan has already been completed, then in a third step  606  it is determined whether a second scan of the IC has been completed. If not, then in a fourth step  608  the IC is configured for the second scan (test mode  2 ), and method  600  ends. If, however, it is determined that the second scan has already been completed, then in a fifth step  610  the IC is configured for a third scan, and method  600  ends. 
     Note that in the example method  600  there are three possible configurations of the IC, which are referred to as test modes  1 - 3 . In test mode  1 , the IC is provided with power and connected to any necessary external clock signals, such that the IC operates normally as it was intended to operate in its normal operating mode. In test mode  2 , any external clock signals provided to the IC are disabled, but power is still provided to the IC. The IC operates, but without the benefit of the external clock signals. Finally, in test mode  3 , the disabled external clock signals are reenabled. Important information regarding the operation of any internal clocks can be determined by analyzing the EM emissions from an IC in each of the three test modes, as will be explained in greater detail below. 
     Referring briefly back to  FIG. 1 , note that IC  102  can be placed in any of the three test modes by power and clock signal generator  110 , under the control of computer  104 . This would generally be the case when testing apparatus  100  is being used to perform quality control testing on known devices. However, when trying to determine the presence of internal clocks in an unknown device, perhaps an IC that is mounted on a printed circuit board with other devices, it may be necessary to configure the IC for testing manually. For example, it may be necessary to search for and physically interrupt any external clock signals being provided to the IC. 
       FIG. 7  is a flowchart summarizing one example method  700  for performing fourth step  608  (configure for scan  2 ) of method  600 . In a first step  1102 , electrical power is disconnected from the integrated circuit. Next, in a second step  1104 , the external clock source (e.g., an oscillator, crystal, ceramics, clock circuit, etc.) is disabled by shorting or interrupting the connection to the integrated circuit. Then, in a third step  1106 , power is reconnected to the integrated circuit, and method  700  ends. 
       FIG. 8  is a flowchart summarizing one example method  800  for performing third step  506  (measure EM emissions from IC) of method  500 . In a first step  802 , the electromagnetic sensing probe is positioned over the integrated circuit to be tested. As previously described herein, this may be accomplished via a robotic arm or by various other means such as manually holding the probe by hand. In a second step  804 , the EM emissions spectrum is measured, and the data is stored in associated spectrum files. Then, in a third step  806  it is determined whether the measurements are complete. If so, method  800  ends. Otherwise, method  800  proceeds to a fourth step  708  where the electromagnetic sensing probe is repositioned. Then, method  800  returns to second step  804  where the EM emissions spectrum is again measured. Method  800  facilitates scanning different areas an IC to locate potential clock signals. 
       FIG. 9  is a flowchart summarizing one example method  900  for performing fourth step  508  (analyze EM emissions) of method  500 . Method  900  is useful to verify that internal clocks of an integrated circuit are operating correctly. In a first step  902 , it is determined if any frequency peaks are detected in the EM emissions spectrum of the IC. If so, then in a second step  904  it is determined if the test data matches the known data for a properly operating device. If the test data does match the known data, then in a third step  906  it is concluded that the internal clock of the IC is operating properly, and method  900  ends. If, however, it is determined in first step  902  that no frequency peaks are detected, then method  900  proceeds to a fourth step  908  where it is concluded that the internal clocks of the IC are not operating properly, and method  900  ends. Similarly, if in second step  904  it is determined that the test data does not match the known data for the properly operating device, then method  900  proceeds to fourth step  908 . 
       FIG. 10  is a flowchart summarizing an example method  1000  of performing fourth step  508  (analyze EM emissions) of method  500 . Method  1000  is useful in determining the presence and types of internal clocks in an unknown integrated circuit. In a first step  1002 , the data D 1  acquired in test mode  1  (normal operating mode) is analyzed to determine whether there is at least one frequency peak. If so, then in a second step  1004  the data D 2  acquired in test mode  2  (external clock signal disabled) is analyzed to determine whether there is at least one frequency peak. If so, then in a third step  1006  the data D 2  is analyzed to determine whether the existing frequency peaks are harmonics of a fundamental frequency peak. If not, then in a fourth step  1008  the data D 3  acquired in test mode  3  (external clock signal reenabled) is analyzed to determine whether the existing frequency peaks are harmonics of a fundamental frequency peak. If so, then in a fifth step  1010  it is determined that there is an internal clock synching to the external clock signal, and method  1000  ends. 
     If, in first step  1002 , it is determined that the D 1  data does not show at least one frequency peak, then method  1000  proceeds to a sixth step  1012  where it is determined that there is no internal clock operating on the IC, and method  1000  ends. If, in third step  1006 , it is determined that the D 2  data shows that the existing frequency peaks are harmonics of a fundamental frequency peak, then method  1000  proceeds to a seventh step  1014  where it is determined that internal clocks on the IC are synching to an internal clock, and method  1000  ends. If, in fourth step  1008 , it is determined that the D 3  data does not show that the existing frequency peaks are harmonics of a fundamental frequency peak, then method  1000  proceeds to an eighth step  1016  where it is determined that the IC includes asynchronous clocks or the frequency peaks are not clock signals. Then, method  1000  ends. 
       FIG. 11  is a flowchart summarizing one example method  1100  for performing second step  804  (measuring EM emissions spectrum) of method  800 . In a first step  1002 , a frequency variable (f) and number of samples to be taken variable (n) are initialized. Then, in a second step  1004 , the amplitude of the EM emissions from an integrated circuit are measured at the initial specified frequency (f). Next, in a third step  1006  it is determined if the current frequency being scanned is the last frequency in the frequency range. If not, then in a fourth step  1108 , the frequency variable (f) is incremented and method  1100  returns to step  1004  to measure the amplitude of the next frequency. If, in third step  1106 , it is determined that the last frequency in the range has been measured, then method  1100  proceeds to a fifth step  1110  where it is determined whether the required number of scans have been taken. If not, then method  1100  proceeds to a sixth step  1112 , where the sample number variable (n) is incremented and the frequency variable (f) is reinitialized to facilitate the next scan. The loops of method  1100  are repeated until it is determined, in fifth step  1110 , that the required number of scans have been completed. Then, in a seventh step  1014 , spectrum files are written to store the measurement data taken. 
       FIG. 12  shows a qualitative example of an EM spectrum graph  1200  of frequency (horizontal axis  1204 ) versus amplitude (vertical axis  1206 ), as can be displayed on display  122  of computer  104 . Frequency axis  1204  includes a frequency sweep low value  1208  and a frequency sweep high value  1210  as limits within which a frequency sweep is performed. Graph  1200  shows several frequency peaks  1216  that rise above a noise floor  1222 . These frequency peaks can be indicative of internal clocks operating at the frequencies at which the peaks occur. 
       FIG. 12  is shown to illustrate that information regarding the presence and/or proper operation of internal clocks can be gleaned from inspection of graph  1200  and the observance of changes in graph  1200  as the IC being tested is configured in test modes  1 - 3 . For example, the approximately equal spacing of the peaks  1216  of graph  1200  indicates that one of peaks  1216  is a primary peak and the other equally spaced peaks are harmonics of the primary. This suggest a plurality of clocks operating at different frequencies, but synching to a primary clock signal. 
     When the IC is placed in test mode  2  (external clock signal disabled) the viewer can determine whether the internal clocks are synching with an internal clock or the external clock signal. If the internal clocks are synching with an internal clock, then graph  1200  should remain unchanged when the external clock signal is removed. However, if the peaks wander (i.e., the frequencies become unstable), that suggests that the internal clocks are attempting to synch to the external clock signal that has been disabled. If the frequency peaks stabilize when the IC is placed in test mode  3  (external clock signal is reenabled), then the internal clocks are synching to the external clock signal. This is essentially the same analysis performed by method  1000  of  FIG. 10 , and can be accomplished directly by observing graph  1200  as the IC is placed in the various test modes. 
     The description of particular embodiments of the present invention is now complete. Many of the described features may be substituted, altered or omitted without departing from the scope of the invention. For example, alternate means for recording EM spectrum data (e.g., manual data taking, alternate data structures, etc.) may be substituted for the computerize method and apparatus shown. As another example, although the invention is shown as implemented with software on a general purpose compute, it should be understood that the invention can be implemented in hardware, firmware, software, or any combination thereof. Indeed, as described with reference to  FIG. 12 , the invention can be implemented manually with a spectrum analyzer and an EM sensing probe. These and other deviations from the particular embodiments shown will be apparent to those skilled in the art, particularly in view of the foregoing disclosure.