Abstract:
A pre-conditioner for enabling high-speed time interval measurements in an ATE system having a relatively low-bandwidth pathway between a UUT and a timer/counter includes a frequency divider and a D flip-flop located near the UUT. The frequency divider receives a first input signal from the UUT and produces a first output signal having a frequency equal to 1/N times the frequency of the first input signal. The first output signal connects over the low-bandwidth pathway to a first channel of the timer/counter. The first output signal also connects to the D input of the D flip-flop. The pre-conditioner receives a second input signal from the UUT that drives the CLOCK input of the D flip-flop. The Q output of the D flip-flop supplies a second output of the pre-conditioner. The second output connects over the low-bandwidth pathway to a second channel of the timer/counter. The time interval between successive rising edges of the output signals precisely match the time interval between successive rising edges of the input signals. Although the pre-conditioner preserves the edge locations of its input signals, the frequency of the signals it sends to the timer/counter is 1/N times the input frequency. The pre-conditioner thus enables to the timer/counter to measure closely spaced, consecutive edges of inputs over a low-bandwidth path. Multiplexors, inverters, and additional frequency dividers may be included with the pre-conditioner to enhance its functionality.

Description:
This application relates generally to electronic time measurement, and more particularly to making accurate time measurements in electronic test systems. 
     BACKGROUND OF THE INVENTION 
     Many electronic products have time-critical specifications that manufacturers verify before shipping the products to customers. Manufacturers typically use automatic test equipment (ATE) to verify these specifications. 
     FIG. 1A illustrates a conventional ATE configuration for testing time-critical devices. As shown in FIG. 1A, an ATE system  100  is connected to a unit under test (UUT)  120  via a plurality of channels  110  attached to test points of the UUT  120 . Each channel  110  includes comparators  124  and  126  that detect timing events at the UUT. The comparators generate time-sensitive signals that change in response to the timing events at the UUT. The time-sensitive signals typically follow a long path before they reach a measurement instrument, for example as a timer/counter  118 . The signals pass through a first signal pathway  112  to a multiplexor  114 . The multiplexor selects a pair of signals from one of the plurality of channels  110 . The selected signals pass through a second signal pathway  116  and on to the timer/counter  118 . 
     A timer/counter typically determines time intervals by measuring the delay between first and second signal edges, designated as a “start” event and a “stop” event. The start event initiates the time measurement, and the stop event terminates the time measurement. The start and stop events are typically provided at CH 1  and CH 2  of the timer/counter, respectively. 
     FIG. 1B shows the appearance of test signals that originate at the UUT  120 , at opposite points along the signal pathway between the channels  110  and the timer/counter  118 . Waveform  132  depicts an ideal, rectangular test signal as it emerges from a channel  110 . Waveform  124  shows the same signal as it arrives at the timer/counter  118 . In contrast with the rectangular, steep-edged signal  132  at the output of the channel, the signal  134  reaching the timer/counter is slow and distorted, and its amplitude is reduced. It would be difficult for the timer/counter  118  to determine accurately the timing characteristics, for example the pulse width, of the initial signal  132  by measuring the arriving signal  134 . Unlike the initial signal  132 , wherein pulse width is constant, the pulse width of the arriving signal  134  varies with the threshold voltage applied. As pulse width shortens, so too does the height of the pulse at signal  134 . Eventually, the pulse height becomes so short that the timer/counter cannot respond. 
     Another way in which the signal path between the channels  110  and the timer/counter  118  distorts test signals is by differentially delaying rising and falling edges. For example, if the signal path delays falling edges more than it delays rising edges, the signal path will erroneously lengthen positive pulse widths and shorten negative pulse widths. These errors further add to the difficulties of measuring time intervals using the configuration of FIG.  1 A. 
     Measuring high-speed events at the UUT would be simplest if the signal pathway between the UUT and the timer/counter could maintain high bandwidth. Unfortunately, however, maintaining high bandwidth proves to be expensive, particularly if the number of channels  110  is large. As ATE systems commonly include hundreds or even thousands of channels  110 , providing high bandwidth between each channel  110  and the timer/counter would excessively raise system cost. Consequently, ATE manufacturers have sought alternatives for measuring high-speed eventsin test systems having low-bandwidth paths. 
     One alternative to providing high-bandwidth paths would be to include one complete timer/counter within each channel. As timer/counters themselves tend to be expensive, however, it is not feasible to include one timer/counter per channel as part of a reasonably priced ATE system. 
     Another alternative to providing high bandwidth paths is shown in FIG.  2 A. According to FIG. 2A, a tester includes channels  210  that are identical to the channels  110  of FIG. 1A, except that each channel  210  includes a pair of frequency dividers  212  and  214 . The frequency dividers  212  and  214  are respectively connected in series with the comparators  124  and  126 . As shown in FIG. 2B, the signal  252  at the output of each frequency divider  212 ,  214  has a period eight times longer than the period of the signal  250  at its input. The substantially longer period of the output signal  252  means that the signal  254  arriving at the timer/counter has more time to settle and thus can be measured more accurately. The arrangement of FIG. 2A thus enables the timer/counter to measure accurately the frequency of test signals from the UUT, even if the bandwidth of the test signals exceeds the bandwidth of the signal path. 
     Although the configuration of FIG. 2A marks a significant improvement over the configuration of FIG. 1A, it provides limited functionality. Because this technique sends only frequency-divided signals to the timer/counter, the timer/counter cannot measure time intervals between successive edges of an input signal. Nor can the timer/counter measure time intervals from falling edges, or between edges of different slope. Similarly, the timer/counter cannot directly measure pulse widths or single-shot periods using this technique. 
     SUMMARY OF THE INVENTION 
     With the foregoing background in mind, it is an object of the invention accurately and easily to measure time intervals between successive edges of high frequency signals. 
     It is another object of the invention to measure high-frequency time intervals using a relatively low-bandwidth transmission path. 
     To achieve the foregoing objects and other objectives and advantages, a timing circuit conveys timing relationships of UUT signals to a measurement instrument over a bandwidth-limited pathway. The timing circuit has first and second input nodes, for receiving at least one input signal from the UUT, and first and second output nodes, for connecting to the measurement instrument. The timing circuit includes a frequency divider that has an input coupled to the first input node and an output coupled to the first output node. The timing circuit also includes a clocked memory device that has a first input coupled to the output of the frequency divider and a second input coupled the second input node. The clocked memory device also has an output coupled to the second output node of the timing circuit. 
     In accordance with another aspect of the invention, a pre-conditioner conveys timing relationships of UUT signals to a measurement instrument. The pre-conditioner has first and second input nodes, for receiving at least one input signal from the UUT, and first and second output nodes, for respectively providing output signals to the measurement instrument. The pre-conditioner is divided into first and second circuit branches. The first circuit branch includes a first frequency divider having an input coupled to the first input node and an output coupled to the first output node. The second circuit branch includes a second frequency divider having an input coupled to the second input node and an output selectably coupled to the second output node. The pre-conditioner further includes a clocked memory device. The clocked memory device has a first input coupled to the output of one of the first and second frequency dividers and a second input coupled to the input of the second frequency divider. The clocked memory device also has an output selectably coupled the second output node. 
     In accordance with yet another aspect of the invention, a tester for determining whether a UUT is operating properly includes a plurality of channels connectable to the UUT and each including at least one voltage comparator. The tester includes a measurement instrument for measuring timing characteristics of the UUT and a signal pathway for conveying timing signals from the plurality of channels to the measurement instrument. Each of the plurality of channels further includes a pre-conditioner for conveying timing signals between the at least one comparator and the measurement instrument via the signal pathway. The characteristics of the pre-conditioner are set forth in the preceding paragraph. 
     In accordance with still another aspect of the invention, a method is disclosed for accurately measuring time intervals between first and second timing events at a UUT over a bandwidth-limited pathway. The method includes dividing the frequency of the first timing events to generate a first frequency-divided signal. The edges of the first frequency-divided signal are coincident with alternate first timing events. The method further includes generating a second frequency-divided signal having edges coincident with alternate second timing events. The second frequency-divided signal changes state at most once for each change of state of the first frequency-divided signal, and only after the first frequency-divided signal changes state. In accordance with this aspect of the invention, the first and second frequency-divided signals are conveyed to a measurement instrument. The measurement instrument determines the time interval between the first and second timing events by measuring the time interval between the first and second frequency-divided signals. 
     Additional objects, advantages and novel features of the invention will become apparent from a consideration of the ensuing description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by reference to the following more detailed description and accompanying drawings in which 
     FIGS. 1A and 1B are a block diagram and associated timing diagram of a distributed test system architecture in which time measurements can be made according to the prior art; 
     FIGS. 2A and 2B are a block diagram and timing diagram illustrating a prior art improvement to the test system of FIG. 1; 
     FIG. 3A is a simplified schematic diagram of a timer/counter pre-conditioner according to the invention; 
     FIGS. 3B and 3C are timing diagrams of signals within the timer/counter pre-conditioner of FIG. 3, wherein the pre-conditioner is respectively configured for time interval and period measurements; 
     FIG. 4 is a simplified schematic of the preferred embodiment of the timer/counter pre-conditioner according to the invention; and 
     FIG. 5 is a simplified schematic of an alternate embodiment of the timer/counter pre-conditioner according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Structure 
     FIG. 3A illustrates a timing circuit  300  according to the invention. The timing circuit  300  includes a logic gate  310 , for example an AND gate. The AND gate  310  has a first input connected to a first input node  316 , and has an output. The timing circuit  300  also includes frequency divider  312  having an input connected to the output of the AND gate  310  an output connected to a first output node  322  of the timing circuit  300 . 
     The timing circuit  300  further includes a clocked memory device  314 , for example a flip-flop. The flip-flop has a D input connected to the output of the frequency divider  312  and a CLOCK input connected to a second input node  318  of the timing circuit. The flip-flop  314  also has a Q output connected to a second output node  324  of the timing circuit. 
     The first and second input nodes  316  and  318  respectively convey first and second input signals to the timing circuit. The first and second output nodes  322  and  324  respectively convey first and second output signals from the timing circuit. 
     An enable input  320  of the timing circuit called “FRAME” is connected to a reset input of the frequency divider  312  (“R”). It is also connected to a clear input of the flip-flop  314  (“CLR”), and to a second input of the AND gate  310 . A FRAME signal conveyed by the FRAME input  320  enables and disables the timing circuit  300 , and synchronizes the timing circuit with events at the UUT. 
     The inclusion of the AND gate  310  in the timing circuit helps to prevent metastability of the frequency divider  312 . As is known, metastability can occur if a frequency divider is released from its reset state too closely in time to receiving an active edge at its signal input. The AND gate  310  prevents metastability by holding off changes at the signal input of the frequency divider for one gate delay after FRAME changes, long enough for frequency divider to change from a reset state to an operating state. 
     Within the ATE system, the first and second input nodes  316  and  318  of the timing circuit  300  are respectively coupled to the outputs of the comparators  124  and  126  (See FIG.  1 ). The first and second output nodes  322  and  324  of the timing circuit  300  are respectively coupled, through the signal path, to the first and second channels of the timer/counter  118 . 
     Operation 
     The timing circuit  300  begins to operate when the signal at the FRAME input  320  changes state from low to high. When FRAME goes high, the AND gate  310  allows the first input signal to propagate to the frequency divider  312 . Immediately upon receiving a first rising edge of the first input signal after FRAME goes high, the frequency divider  312  generates a first rising edge at the first output signal. The frequency divider  312  generates subsequent rising edges every N rising edges of the input signal thereafter, as long as FRAME remains high. The frequency of the first output signal therefore equals 1/N times the frequency of the first input signal. From the timing circuit  300 , the first output signal propagates to the first channel of the timer/counter  118 , where it may provide a start event for a time measurement. The first output signal also propagates to the D input of the flip-flop  314 . The second input signal propagates to the CLOCK input of the flip-flop  314 . Immediately upon a rising edge of the second input signal, the flip-flop  314  clocks the logic level at its D input to its Q output. The Q output propagates to the second output node  324  of the timing circuit  300 . From the timing circuit, the second output signal propagates to the second channel of the timer/counter  318 , where it may provide a stop event for a time measurement. 
     The second output signal does not respond to the second input signal until the first output signal changes state, i.e., the Q output of the flip-flop  314  always follows the D input. Therefore, the second output signal changes state a maximum of one time for each change of state of the first output signal. Therefore, the frequency of the second output signal at most equals the frequency of the first output signal—1/N times the frequency of the first input signal. Although the timing circuit reduces the frequencies of the first and second input signals by a factor of N, the positions of the rising edges of the first and second input signals remain intact. 
     FIG. 3B illustrates timing relationships between input and output signals of the timing circuit  300 , for a frequency divider value N equal four. Waveforms  330  and  332  respectively represent the first and second input signals, and waveforms  334  and  336  respectively represent the first and second output signals. As shown in FIG. 3B, the frequency of the first output signal  334  equals one-fourth the frequency of the first input signal  330 . 
     The first output signal  334  goes high immediately upon receiving the first rising edge of the first input signal  330  that follows the assertion of FRAME. It goes high again every N=4 rising edges of the first input signal later, as long as FRAME remains high. 
     Once the first output signal  334  assumes a high state, the first subsequent rising edge of second input signal  332  clocks the flip-flop  314  and causes the second output signal  336  to assume a high state. Subsequent edges of the second input signal  332  have no effect on the second output signal  336 , as long as the first output signal  334  remains high. When the first output signal  334  goes low, the first subsequent rising edge of the second input signal  332  again clocks the flip-flop  314 , and causes the second output signal  336  to go low. Subsequent edges of the second input signal  332  have no effect on the second output signal  336 , as long as the first output signal remains low. 
     The timing circuit  300  delivers significant advantages over the divider circuit of FIG.  2 . Using the timing circuit  300 , a timer/counter can determine precisely time intervals between successive rising edges of the input signals  330 ,  332  by measuring time intervals between rising edges of the output signals  334 ,  336 . The measured intervals are not averaged, or aggregated values taken over N periods of an input signal, but actual, instantaneous time intervals between immediately subsequent rising edges of the first and second input signals  330 ,  332 . This is true although the frequency of the output signals is at least N times slower than the frequency of the first input signal. 
     Using the timing circuit  300  according to the invention, a timer/counter can measure accurately time intervals, even over a relatively low-bandwidth signal path. Because the timing circuit  300  converts all time-sensitive events at its inputs into rising edges at its outputs, the timer/counter can measure time intervals unaffected by differential delays between rising and falling edges through the signal path. In addition, the timer/counter can measure time intervals unaffected by distortion. Although the signal path distorts the shape of output signals from the timing circuit  300 , distortion from any channel to the timer/counter affects only the net delay between the timing circuit  300  and the timer/counter. The effects of distortion can therefore be removed by known techniques for equalizing path delays, such as software calibration or hardware alignment. FIG. 3C illustrates how a timer/counter can use the timing circuit  300  to measure a single-shot period. As shown in FIG. 3C, INPUT 1 =INPUT 2 , i.e., a common input signal  350  drives both the first and second input nodes  316  and  318  of the timing circuit  300 . The frequency divider  312  generates a first output signal  352 , having a frequency equal to one-fourth the frequency of the common input signal  350 . The first output signal  352  goes high immediately upon receiving the first rising edge of the common input signal  350  that follows the assertion of FRAME. The output of the frequency divider propagates to the D input of the flip-flop, and the common input signal  350  propagates to the CLOCK input. Because the frequency divider  312  has a finite propagation delay, the edge from the common input signal reaches the flip-flop sooner than the edge from the frequency divider—too soon to clock a new value for the current clock edge. Upon the next rising edge of the common input signal, however, the flip-flop  314  clocks the high level at the D input to the Q output, and the second output signal  354  changes state from low to high. The time between the first rising edge of the first output signal  352  and the first rising edge of the second output signal  354  equals the single-shot period of the common input signal  350 . The timer/counter can measure and report this period. Again the result obtained is not an average measurement taken over N periods, but an instantaneous value based upon immediately subsequent rising edges of the input signal. 
     Implementation 
     Within the scope of the invention, the construction of the components that make up the timing circuit  300  can varied in a number of ways. For example, the frequency divider  312  can be constructed by cascading a desired number of D flip-flops each configured as divide-by-two circuits. According to this technique, the inverting output of each D flip-flop is connected back to the D input of the flip-flop. An input signal to the circuit drives the CLOCK input of the flip-flop, and an output signal from the circuit is taken from the non-inverting, Q, output. Divide-by-two circuits can be cascaded to achieve a desired division factor by connecting in series the Q output of one stage and the CLOCK input of the next stage. Other ways of constructing the frequency divider  312  are readily known to those skilled in the art. 
     The logic gate  310  need not necessarily be implemented using an AND gate. Any combination of logic gates or discrete components arranged to provide an AND function can be used. Although the AND gate enhances performance by helping to prevent metastability of the frequency divider  312 , it is not essential to the functionality of the timing circuit  300 . Therefore, the logic gate  310  can alternatively be omitted altogether. 
     Polarities of the signals and components can be reversed within the scope of the invention. For example, the AND gate can be replaced with a NAND gate and the frequency divider constructed to respond to falling edges. The frequency divider can be constructed to provide an inverting output signal to the first output node  322 , and the flip-flop can be connected to provide its inverting output to the second output node  324 . FRAME can be provided as a negative-true logic signal, if the inputs to which it connects are made to respond to a negative-true signal. 
     The measurement instrument need not necessarily be a timer/counter. Rather, the timing circuit can be used with any instrument or receiving circuit, or in any application in which it is critical to preserve signal edge positions. 
     The implementation of the clocked memory device  314  need not necessarily be a D flip-flop. Other types of flip-flops or clocked memory devices can be used. Any device capable of generating an output signal that equals, or is the complement of, an input signal, as defined by a clock signal, serves the purposes of the flip-flop  314 . 
     According to one such variation, a counter is used in place of the D flip-flop  314  of FIG.  3 A. Thecounter has an ENABLE input and a CLOCK input respectively connected in place of the D input and CLOCK input of the D flip-flop of the timing circuit  300 . The counter has an active-low RESET input, connected to the FRAME input  320 , and a TERMINAL COUNT output connected in place of the Q output of the D flip-flop. In addition, the counter has a COUNT input that receives a plurality of signals for presetting a counting value, M, and a LOAD COUNT input for clocking the counting value M into the counter. 
     Prior to operation, the counter is loaded with the counting value M. When ENABLE is low, the counter remains stable, regardless of clock edges at the CLOCK input. After ENABLE goes high, however, the counter counts down one count in response to each subsequent rising edge at its CLOCK input. TERMINAL COUNT goes high immediately upon the clock edge that reduces the counter&#39;s value to zero. 
     If M is preset to one, the counter behaves like the D flip-flop. It generates a rising edge upon the first rising edge of the second input signal that follows a first rising edge of the first output signal after FRAME is asserted. If M is preset to a value greater than one, however, the counter waits until the M-th rising edge of the second input signal before asserting the second output signal. By setting M to various values, the timing circuit  300  can vary the particular rising edge of the second input signal that forms the stop event of a time measurement. The counter can therefore be used to expand the flexibility of the timing circuit  300 . 
     Preferred Embodiment 
     FIG. 4 illustrates the preferred embodiment of the invention. A timer/counter pre-conditioner  400  includes a timing circuit  300 , like the one shown in FIG.  3 . Within the pre-conditioner  400 , the first input node  316  of the timing circuit  300  is coupled to the output of a first multiplexor  414 , and the second input node  318  is coupled to the output of a second multiplexor  416 . The first and second multiplexors  414  and  416  each have inputs coupled to a pair of input buffers  410  and  412 . The input buffers  410  and  412  in turn receive their inputs from the comparators  124  and  126  of a tester channel  110 . FIG. 4 identifies the outputs of the comparators  124  and  126  respectively as “DATA HI” and “DATA LO.” The buffers  410  and  412  can alternatively be omitted, and the outputs of the comparators connected directly to the multiplexors  414  and  416 . 
     The pre-conditioner  400  preferably employs buffers  410 ,  412  and multiplexors  414 ,  416  having differential construction, to maintain accurate timing between the comparators  124 ,  126  and the timing circuit  300 . Alternatively, the pre-conditioner  400  employs single-ended input buffers and multiplexors in less critical applications. 
     As with the timing circuit  300 , the FRAME input  320  enables and disables the pre-conditioner  400 , and synchronizes its operations. The pre-conditioner  400  also includes a SELECT input  438 . The SELECT input conveys a plurality of control signals to the multiplexors of the pre-conditioner to flexibly configure them for various modes of operation. 
     The pre-conditioner  400  expands the capabilities of the timing circuit  300 . It provides rising edges to the inputs of the timing circuit  300  in response to both rising and falling edges at DATA HI and DATA LO. The pre-conditioner  400  buffersnrising edges at DATA HI and DATA LO and passes them directly to the timing circuit. It converts falling edges into rising edges by inverting (crossing) the differential inputs to a multiplexor  414 ,  416 . Specifically, each input multiplexor has four differential inputs, numbered  1 - 4 , which respectively provide a rising edge to the timing circuit  300  in response to the following conditions: 
     1. a rising edge of DATA HI 
     2. a falling edge of DATA HI 
     3. a rising edge of DATA LO 
     4. a falling edge of DATA LO. 
     The pre-conditioner  400  also includes a second frequency divider  420  and a second AND gate  418 . The second frequency divider  420  divides the frequency of a signal from the second multiplexor  416  to mirror the division of the signal from the first multiplexor  414  by the first frequency divider  312 . Preferably, the first and second frequency dividers  312  and  420  have the same divider factor N, to supply comparable signals to the two channels of the timer/counter. 
     The pre-conditioner  400  still further includes a second AND gate  418 . The second AND gate  418  helps to prevent metastability of the second frequency divider  420 , in the same manner described above for the first AND gate  310 . The first and second AND gates  310  and  418  perform a second function as well in the embodiment of FIG.  4 —they gate signals from the first and second multiplexors  414 ,  416  to the outputs of the pre-conditioner. As described below, the pre-conditioner  400  can be configured to directly supply its input signals to its outputs without frequency division. Under these circumstances, the AND gates  310  and  418  selectively pass and block input signals to the outputs of the pre-conditioner, in response to FRAME. 
     The pre-conditioner  400  always conveys the first output signal of the timing circuit  300  to the first channel of the timer/counter. It can convey either of two signals, however, to the second channel of the timer/counter—the second output signal of the timing circuit or the output of the second frequency divider  420 . The pre-conditioner  400  can thus assume two distinct modes. In the first mode, the pre-conditioner generates immediately subsequent edges of the timing circuit&#39;s inputs for critical edge-to-next-edge measurements. In the second mode, the pre-conditioner operates as a pair of simple dividers. 
     A bridging multiplexor  426  selects between the first and second modes. The bridging multiplexor has a first input connected to the Q output of the flip-flop  314  and a second input connected to the output of the second frequency divider  420 . The output of the bridging multiplexor is coupled to the second channel of the timer/counter. In response to control from SELECT, the bridging multiplexor  426  connects either of its inputs to its output, and therefore selects one of the first and second modes. 
     The timer/counter pre-conditioner  400  also includes third and fourth frequency dividers  428  and  430  to further divide the frequencies of signals to the timer/counter. The third and fourth frequency dividers  428  and  430  can be used for exceptionally high-frequency input signals, as well as for very low-bandwidth signal paths between the pre-conditioner and the timer/counter. The divider factor of the third and fourth frequency dividers is shown to be four; however, this value can be varied to suit individual circumstances. 
     For added flexibility, the pre-conditioner includes bypass multiplexors  422 ,  424 ,  432 , and  434  to selectably jumper over each of the frequency dividers  312 ,  420 ,  428 , and  430  of the pre-conditioner  400 . Bypass multiplexors preferably operate in pairs. For example, multiplexors  422  and  424  operate together, both either passing or bypassing the frequency divider to which it is connected. Multiplexors  432  and  434  operate together in a similar fashion. When a division factor of four is used for each of the frequency dividers  312 ,  420 ,  428 , and  430 , the pre-conditioner  400  can divide input frequencies by either 1, 4, or 16. 
     As an alternative to providing two pairs of frequency dividers each having a discrete division factor, a single pair of variable frequency dividers can be used. Variable frequency dividers can be programmed to provide a wide range of frequency divisions suitable for measuring various time intervals. If fixed frequency dividers are used, division factors should be selected in accordance with expected testing conditions, with consideration given to the expected range of input frequencies and the bandwidth of the signal path to the timer/counter. More stages of frequency dividers and bypass multiplexors can be included, as desired. 
     The timer/counter pre-conditioner  400  according to the invention greatly enhances the ability of an ATE system to measure accurate time intervals over a relatively low-bandwidth path. Using the pre-conditioner  400 , the timer/counter can measure edge-to-next-edge time intervals from DATA HI or DATA LO. It can measure time intervals between any two consecutive edges of a single input signal, or can measure time intervals between a first edge of one input signal and a next edge of the other input signal. Using the pre-conditioner  400 , the timer/counter can measure time intervals for all edge combinations-rising-to-rising, falling-to-falling, rising-to-falling, and falling-to-rising. Either DATA HI or DATA LO can start a time measurement, and either can stop a time measurement. 
     In addition to edge-to-next-edge measurements, the pre-conditioner  400  can output its input signals directly to the timer/counter, or can output simple divisions of its input signals. 
     The timer/counter pre-conditioner  400  is preferably manufactured as part of an Application Specific Integrated Circuit (ASIC), in which it can operate at data rates exceeding 2.5 Giga-bits per second. The pre-conditioner  400  is not limited, however, to an ASIC implementation. Because its consists of readily available logic, the pre-conditioner  400  can also be constructed from discrete devices, or as part of a Field Programmable Gate Array (FPGA). 
     Alternatives 
     Having described one embodiment, numerous alternative embodiments or variations might be made. For example, FIG. 5 illustrates an alternative embodiment of the invention, in which a pre-conditioner  500  includes two separate timing circuits  512  and  514 . The timing circuits  512  and  514  are similar to the timing circuit  300  of FIG.  3 . The first and second input nodes  316  and  318  of each of the timing circuits  512 ,  514  connect together at a common input node. A differential buffer  510  receives an input signal DATA IN from a channel  110 . The differential buffer  510  has non-inverting and inverting output nodes respectively coupled to the common input nodes of the first and second timing circuits  512  and  514 . 
     During operation, the first timing circuit  512  responds to rising edges of DATA IN, and the second timing circuit  514  responds to falling edges of DATA IN. Because the first and second input nodes  316  and  318  of each timing circuit  512 ,  514  are connected together, the timing circuits  512 ,  514  behave in a manner similar to that described above for FIG. 3 c.  Each rising edge at the second output node  324  of either of the timing circuits  512 ,  514  always occurs exactly one period after the first rising edge at the first output node  322  of the timing circuit. 
     The pre-conditioner  500  also includes a first output multiplexor  516  and a second output multiplexor  518 . The first output multiplexor  516  selectably switches the first output node  322  of each timing circuit  512 ,  514  to the first channel of the timer/counter. The second output multiplexor  518  switches the same signals to the second channel of the timer/counter. The second multiplexor  518  also can switch the second output node  324  of both timing circuits  512 ,  514  to the second channel of the timer/counter. 
     The pre-conditioner  500  further includes third and fourth frequency dividers  524  and  526 , respectively coupled in series with the outputs of the output multiplexors  516 ,  518 . The third and fourth frequency dividers further reduce the frequencies of signals sent to the timer/counter, to allow the timer/counter to measure even higher frequency signals. 
     The pre-conditioner  500  can be flexibly configured to measure a wide range of time intervals in an ATE system  100 . For example, with the outputs  322 ,  324  of the first timing circuit  512  switched to the outputs  524 ,  526  of the pre-conditioner  500 , the timer/counter can measure single-shot periods between consecutive rising edges of DATA IN. Similarly, with both outputs of the second timing circuit  514  switched to the outputs of the pre-conditioner  500 , the timer/counter can measure single-shot periods between consecutive falling edges of DATA IN. The timer/counter can measure an averaged period of DATA IN from the first output node  322  of either timing circuit. It can also measure the pulse width of DATA IN. Pulse width can be measured as the interval between the first output signal of one timing circuit and the second output signal of the other timing circuit. For measuring positive pulses, the first timing circuit  512  starts the interval. For measuring negative pulses, the second timing circuit  514  starts the interval. 
     It should be understood, therefore, that the particular implementation of the invention disclosed herein could be widely varied by those skilled in the art within the scope of the invention. Therefore, the invention should be limited only by the spirit and scope of the appended claims.