Device for synchronizing the output pulses of a circuit with an input clock

Abstract: A device for synchronizing the output test pattern signals (14) of a test circuit with the clock signal (14) of a device under test (DUT). The invention uses a programmable delay (56) in the feedback loop of a phase locked loop system (20) to adjust the phase of the test pattern signals (14) to be synchronized with the clock (12) of the device under test (DUT).

Description: Background of the Invention 1. Field of the Invention. The present invention pertains generally to measuring and testing devices and more particularly to a device which is capable of synchronizing test pattern generator pulses with an input clock signal from a board under test. 2. Description of the Background. In the process of performing in-circuit testing of components on a printed circuit board, it is often times necessary to produce a series of output pattern pulses from a pattern generator that are sufficient to properly test the component. These predetermined patterns can be produced from a test pattern generator which contains the desired patterns for testing the components. The output of the pattern generator is normally connected to a series of drivers which produce the desired output pulses for testing the components on the board. To perform proper testing of the component, it is necessary to synchronize the output signals from the test pattern generator with the clock pulses from the device under test (DUT). However, in many cases the clock cannot be disabled because of the nature of the circuit. Additionally, the output test pattern pulses are normally retarded in phase from the board clock pulses by up to 100 nanoseconds or more because of inherent delays in the circuitry. This phase difference between the DUT clock pulses and the output signals can result in faulty test readings of the device under test. Hence, it is desirable to synchronize the output signals with the DUT clock pulses to ensure that proper test results are obtained for the DUT. To date, in-circuit board testers have been unable to provide an adequate means for synchronizing the output signals with the DUT clock pulses. Reference is made to GB-A-2073515 which describes a frequency-locked loop based on a phase-locked loop. The circuit includes a phase comparator, a low-pass filter and a voltage controlled oscillator for generating the loop output. A multiplexer and step-adjustable delay line are coupled between the output and the phase comparator for adjusting the phase of the return signal in the loop. The delay is controlled by an up/down counter, and by a phase-shift detector which is connected to the output of the phase comparator. The phase delay circuit is enabled by a lock detector operable only in the synchronised state of the loop. Reference is also made to LU-A-68713 which illustrates a frequency controlled loop, wherein a flip-flop is used as a phase detector. SUMMARY OF THE INVENTION The present invention overcomes the disadvantages and limitation of the prior art by providing apparatus for use with an output device which produces an output signal in response to a reference clock signal produced by the apparatus the apparatus including a phase lock loop that synchronizes the phase of the output signal with an input clock signal produced by an external component, the apparatus comprising: phase detector means for generating a difference signal proportional to the difference in phase between said input clock signal and a delayed reference clock signal produced by delaying said reference clock signal by a predetermined amount; filter means coupled to said phase detector means for integrating said difference signal and producing a voltage signal proportional to an integrated value of said difference signal; voltage controlled oscillator means coupled to said filter means for producing said reference clock signal having a frequency proportional to said voltage signal produced by said filter means; programmable delay means coupled to said voltage controlled oscillator means for delaying said reference clock signal by said predetermined amount in response to a programmable delay signal to produce said delayed reference clock signal; phase state indicator means coupled to said input clock signal and said output signal for producing a phase state signal that indicates the phase relationship of said input clock signal and said output signal; processor means for varying said programmable delay signal in response to said phase state signal to alter the phase of said reference clock signal until said phase state signal produced by said phase state indicator means changes state indicating that said input clock signal and said output signal are in-phase. The advantages of the present invention are that it provides a simple and inexpensive manner of synchronizing the DUT input clock pulses with output test pattern signals in a manner which is independent of the circuitry that produces the delays. The present invention uses a unique method of varying the phase of the output signal of a phase locked loop by utilizing a programmable delay in the feedback path of the phase locked loop so that the output signals are adjusted in phase to correspond to the phase of the DUT input clock signal. This is achieved by adjusting a programmable delay line in the feedback path of the PLL (phase locked loop) until the input and output phases are synchronized. Brief Description of the Drawings Figure 1 comprises a schematic illustration of the device of the present invention. Figure 2 comprises a schematic illustration of the pattern generator clock illustrated in Figure 1. Detailed Description of the Preferred Embodiment of the Invention Figure 1 is a schematic drawing of the device 10 of the present invention. The device under test (not shown) includes a digital component to be tested which has a clock signal (DUT clock) that cannot be disabled during the test of the component. Hence, to properly test the component, the test pattern signals must be synchronized with DUT clock. Hence, the purpose of the device 10 of the present invention is to receive the clock signal 12 from the DUT and synchronize the output test pattern pulses 14 with the DUT clock 12. This is accomplished in the following manner. Input component clock 12 is received by clock receiver 16, comprising a threshold comparator, to produce a receiver clock signal 18 which, in turn, is applied to reference clock generator 20, comprising the phase locked loop circuit illustrated in Figure 2. The output of the phase locked loop 20 comprises a reference clock signal 22 which functions as a clocking signal that is applied to the test pattern generator 24 that synchronously generates a series of output test patterns which are used to test the component for proper operation. The test pattern generator 24 may comprise a Random Access Memory (RAM) device wherein the desired test pattern is stored at sequential address locations. A counter, (not shown) forming part of the test pattern generator 24, accumulates the reference clock pulses which are used as a sequential address to provide the desired test pattern output. The test pattern output from test pattern generator 24 is then used to stimulate the DUT. Output signals 14 are also applied to the flip flop clocking input 28 of calibration flip flop 30. Alternatively, the reference clock 22 can be applied to the calibration flip flop 30 by way of path 26 and a computed offset can be introduced to compensate for the delay of test pattern generator 24 within predicted limits. Additionally, the DUT clock 12 is applied to the data input 32 of calibration flip flop 30. As a result, a phase state signal 34 is produced at the output of the calibration flip flop 30. The phase state signal 34 is high only when data input 32 is high before the flip flop clocking input 28 goes high. Hence, as long as the input clock 12 occurs prior to output pulse 14, the phase state signal 34 will remain high. However, when the output signal 14 occurs prior to input clock 12, the phase state signal 34 will be low. In this manner, the phase state signal 34 can provide an indication of the cross-over point at which the phase of the output signals 14 match the phase of the input clock 12. Alternatively, the data input and clock input can be reversed without affecting the functionality of the invention. The phase state signal 34 is applied to a processor 36 which detects the phase state signal 34 and generates a programmable control signal 38 which is applied to the reference clock generator (PPL) 20 to adjust the phase of the reference clock 22 in the manner set forth below. The adjustment of the programmable control signal 38 by processor 36 comprises a simple iterative process of increasing or decreasing the value of the programmable control signal until the state of the phase state signal 34 changes, thereby indicating substantial synchronization between input clock 12 and output signal 14. Figure 2 is a schematic block diagram of the reference clock generator 20 illustrated in Figure 1 which varies the phase of the reference clock 22 in response to the programmable control signal 38 produced by processor 36. Referring to Figure 2, the receiver clock signal 18 is input to a divide by M circuit 40 which divides the receiver clock signal 18 by a preselected number M. The output of the divide by M circuit 40 is applied to a phase detector 42 which produces a phase detector difference signal 44 that has a pulse-width that is proportional to the difference in phase between the input clock signal received from divide by M circuit 40 and the delayed reference clock signal 46. The phase detector difference signal 44 is applied to filter 48 which comprises an integrator for integrating the phase detector difference signal 44 and producing a voltage signal 50 having a voltage magnitude that is proportional to the difference in phase between the receiver clock signal 18 and the delayed reference clock signal 46. The voltage signal 50 is applied to a voltage controlled oscillator 52 which produces reference clock 22 having a frequency proportional to the voltage of voltage signal 50. Reference clock signal 22 is then applied to a divide by N circuit 54 which in turn applies the divided reference clock 22 to programmable delay line 56. Programmable delay line 56 functions to introduce a delay in accordance with a programmable control signal 38. As delays are added to the reference clock 22 by programmable delay line 56, the phase detector difference signal 44 momentarily increases thereby momentarily increasing voltage signal 50 causing voltage control oscillator 52 to momentarily increase its frequency to offset the difference in phase detected by phase detector 42. Hence, as more delay is added to programmable delay line 56 by way of programmable control signal 38, the phase of the reference clock is advanced. In this manner, as processor 36 (Figure 1) proceeds through an iterative process of increasing the programmable control signal, the phase of the reference clock 22 is advanced until output signals 14 are in-phase with the DUT clock 12. Referring to Figure 1, this causes the phase state signal 34 to go from high state to a low state signalling the processor to stop the iterative process of introducing delay through the programmable delay line 56 (Figure 2) so as to halt the further advance in phase of the reference clock signal 22. Hence, the output signal 14 can be synchronized with a high degree of accuracy with the input DUT clock 12 by introducing delays in the feedback circuit of a phase lock loop which generates the reference clock signals for the test pattern generator 24. Resolution dependent only on the resolution of the programmable delay line can be achieved with a high degree of accuracy without the necessity for complex and expensive control circuitry. The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to precise form disclosed, and other modifications and variations may be possible in light of the above teachings. For example, the present invention need not be restricted to testing devices or instrumentation, but can be used with any circuitry where phase or time relationships must be controlled or measured, such as in radar systems, navigational systems, ultrasound imagers, etc. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated.

Claims: Apparatus (10) for use with an output device (24) which produces an output signal (14) in response to a reference clock signal (22) produced by the apparatus (10) the apparatus including a phase lock loop that synchronizes the phase of the output signal (14) with an input clock signal (12) produced by an external component, the apparatus comprising: phase detector means (42) for generating a difference signal (44) proportional to the difference in phase between said input clock signal (12, 18) and a delayed reference clock signal (46) produced by delaying said reference clock signal (22) by a predetermined amount; filter means (48) coupled to said phase detector means (42) for integrating said difference signal (44) and producing a voltage signal (50) proportional to an integrated value of said difference signal; voltage controlled oscillator means (52) coupled to said filter means (48) for producing said reference clock signal (22) having a frequency proportional to said voltage signal (50) produced by said filter means; programmable delay means (56) coupled to said voltage controlled oscillator means (52) for delaying said reference clock signal (22) by said predetermined amount in response to a programmable delay signal (38) to produce said delayed reference clock signal (46); phase state indicator means (30) coupled to said input clock signal (12) and said output signal (14) for producing a phase state signal (34) that indicates the phase relationship of said input clock signal and said output signal; processor means (36) for varying said programmable delay signal (38) in response to said phase state signal (34) to alter the phase of said reference clock signal (22) until said phase state signal (34) produced by said phase state indicator means (30) changes state indicating that said input clock signal (12) and said output signal (28) are inphase. The device of claim 1 wherein said phase state indicator means (30) comprises a flip flop (30) having a data input (32) connected to said input clock signal (12) and a clock input (28) connected to said output signal (14) so that whenever said output signal (14) is retarded in phase in comparison to said input signal (12), said phase state signal (34) produced by said flip flop (30) is in a high state, and whenever said output signal (14) is advanced in phase in comparison to said input signal (12), said phase state signal (34) is in a low state. An assembly comprising apparatus (10) as defined in claim 1 or 2, and an output device (24) coupled to said apparatus (10) for producing said output signal (14) synchronised to said input clock signal (12) in response to said reference clock signal (22) from said apparatus (10).

Inventors: PRATER DAVID M, REDIG MICHAEL J, PRATER, DAVID M., REDIG, MICHAEL J.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1988
Main classification: H03L7
IPC codes: H03L7, G01R31, H04L7