Abstract:
A monolithically integrated delay circuit is provided that comprises a gate coupled for receiving a digital input signal. The output of the gate is capacitively loaded whereby the output signal has a sloping downward transition. A line receiver has a first input coupled to said gate and a second input coupled for receiving an analog signal for comparing the analog signal with the output of the gate and for providing a digital output signal that is delayed with respect to the digital input signal.

Description:
This application is a continuation of application Ser. No. 892,513, filed Aug. 4, 1986, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to a programmable delay circuit and, more particularly, to a circuit that provides an accurate delay of a digital input signal that has a large adjustable range wherein the delay may be programmed quickly. 
     BACKGROUND OF THE INVENTION 
     Delay circuits for delaying a signal for a desired time period are well known. Most of these known delay circuits typically may be used in a variety of applications. One such application is the testing of integrated circuits. A crucial step in the manufacturing of integrated circuits is the testing of those circuits after device fabrication. As with each of the steps involved in integrated circuit manufacture, it is advantageous to perform this testing with as little cost as possible. Therefore, high speed, automated integrated circuit testers are necessary. 
     As integrated circuits become more complex and require more pins for interconnection to circuit boards and the like, the apparatus necessary for testing the integrated circuits becomes much more complex. Current DC testers have multiple force and measure units, each applying a predetermined forcing condition of voltage and/or current to a pin of a test head and measuring the response thereto. Each pin of the test head is connected to a pin of the integrated circuit under test. Typically, a minicomputer contains the testing and measuring routines and is responsible for managing the testing process. 
     Current AC testers provide test signals from the tester to the integrated circuit under test that are delayed for providing setup and hold time, timing edges, etc. The delay circuit must be accurate, have a large adjustable range and be able to change delays in a few nanoseconds. Portions of previously known delay circuits were on chip but required external comparators and ramp circuits and did not provide the desired accuracy, range and quick change rate required for efficient high speed testing. 
     Thus, what is needed is an accurate on-chip delay circuit that has a large adjustable range. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved on-chip delay circuit. 
     Another object of the present invention is to provide an on-chip delay circuit that compares digital and analog input signals for providing a delayed output signal. 
     Yet another object of the present invention is to provide an on-chip programmable delay circuit that has a large range from which a delay may be selected. 
     A further object of the present invention is to provide an on-chip programmable delay circuit that has small selectable incremental delays. 
     Still a further object of the present invention is to provide an on-chip programmable delay circuit in which the time of the delay may be quickly changed. 
     In carrying out the above and other objects of the invention in one form, there is provided a monolithically integrated delay circuit including a gate coupled for receiving a digital input signal. The output of the gate is capacitively loaded whereby the output signal has a sloping downward transition. A line receiver has a first input coupled to said gate and a second input coupled for receiving an analog signal for comparing the analog signal with the output of the gate and for providing a digital output signal that is delayed with respect to the digital input signal. 
     The above and other objects, features, and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the preferred embodiment of the present invention. 
     FIG. 2 is a schematic of one gate within the preferred embodiment. 
     FIG. 3 is a waveform of the output of the gate of FIG. 2. 
     FIG. 4 is a block diagram of an integrated circuit test system that includes the preferred embodiment. 
     FIG. 5 is a more detailed block diagram of a first portion of FIG. 1 including an input circuit, counter, comparator, two OR gates, and one latch. 
     FIG. 6 is a more detailed block diagram of a second portion of FIG. 1 including an oscillator and one delay circuit. 
     FIG. 7 is a more detailed block diagram of a third portion of FIG. 1 including two multiplexers, a second delay circuit and a second latch. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates in block form the programmable delay circuit according to the present invention which is suitable to be fabricated in monolithic integrated circuit form. NAND gate 1 has an input coupled for receiving digital input signal V D  and an output connected to one input of line receiver 2. Line receiver 2 has another input coupled for receiving analog input signal V A  and an output for providing delayed signal V D . Gate 1 may actually be any type of gate such as an AND gate, OR gate or a NOR gate. FIG. 2 illustrates the configuration of the output transistor of gate 1. Transistor 3 has a collector coupled for receiving voltage V CC , a base coupled for receiving digital input signal V D  through the output of the logic in gate 1, and an emitter coupled for providing the input to gate 2 and to voltage V EE  by both current source 4 and parasitic capacitance 5. The shape of the waveform of digital input signal V D  and the output of gate 1 are shown in FIG. 1. The output of gate 1 has a sloping negative going transition due to parasitic capacitance of the metal connecting runs on the integrated circuit. This capacitance may be varied by shortening or lengthening the metal run for providing a fast rise time and a slow fall time so that the outputs may be compared with analog signal V A . A shallower slope would give a greater delay range and a steeper slope would give a smaller delay range (see FIG. 3). The length of the delay is a function of analog signal V A . Additional embodiments are illustrated included in the circuits in FIGS. 6 and 7 and will be discussed in greater detail hereinafter. 
     FIG. 4 illustrates in block form another embodiment of the programmable delay circuit which is suitable to be fabricated in monolithic integrated circuit form. Input circuit 11 is connected to input terminal 12 for receiving digital input signal V IN . Input circuit 11 is further connected to counter 13 for providing thereto resetting signal V RESET  and clocking signal V CLOCK , respectively. Counter 13 is connected to comparator 14 for providing a plurality of timing signals V COUNT  representing a period of time. Latch 15 is coupled to receive programming signals V MSB  representing the most significant bit and is connected to comparator 14 and OR gate 16 for providing latched programming signals V MSB  thereto. Comparator 14 compares timing signals V COUNT  and latched programming signals V MSB  and provides to OR gate 17 a plurality of comparator output signals V COMPARE  representing a period of time from the change in state of input signal V IN  till the time when signal V COUNT  equals latched signal V MSB . OR gate 17 further has an input connected to input circuit 11 for receiving enable signal V ON . The output of OR gate 17 is connected to input circuit 11 and to delay circuit 18 for providing clocking signal V DONE  thereto that indicates when counter 13 has completed counting. The output of OR gate 16 is connected to input circuit 11, multiplexer 19, and delay circuit 18 for providing signal V ZERO  thereto that indicates whether specific further delay increments are desired. 
     Oscillator 21 has inputs connected to input circuit 11 for receiving signal V ON  for enabling oscillator 21, to terminal 22 for receiving calibration analog signal V CALIB , and coupled for receiving input timing calibration signals V Time  A and V Time  B for calibrating the timing of oscillator 21. Delay circuit 18 is connected to oscillator 21 for receiving oscillator output signal V OSC , and input timing calibration signals V Time  A* and V Time  B, and is connected to the output of OR gate 16 for receiving signal V ZERO . Multiplexer 19 has inputs connected to input terminal 12 for receiving input signal V IN  and to delay circuit 18 for receiving signal V DELAY . Signal V DELAY  provides delays greater than 10 nanoseconds, as explained further hereinafter. Multiplexer 19 provides either of signals V DELAY  or V IN  to delay circuit 23 depending on the state of signal V ZERO . Delay circuit 23 is connected to terminal 24 for receiving analog signal V PROG . Latch 25 is coupled for receiving programming signal V LSB  representing the least significant bit. Latch 15 and latch 25 are coupled for receiving strobe signal V STROBE  for latching signals V MSB  and V LSB , respectively. Multiplexer 26 is connected to latch 25 for receiving latched programming signals V LSB  and to delay circuit 23 for receiving a plurality of outputs therefrom. Multiplexer 26 is connected to output terminals 27 and 28 for providing signals V OUT  and V OUT , respectively. The output of multiplexer 26 is selected from one of the plurality of signals from delay circuit 23 in accordance with the state of latched programming signals V LSB . 
     The delay between input signal V IN  and output signal V OUT  is determined by the number of flip-flops, gates, etc., within the circuit. For the embodiment specifically illustrated in FIG. 4, an input signal may be delayed in 20 picosecond increments for the range of 0.0 nanoseconds to 640 nanoseconds plus the minimum circuit propagation time. The circuit may be used to delay a signal for a variety of applications, but is specifically usable to delay the output signal of an automated integrated circuit tester. For example, the circuit is responsive to a sequenced test signal from the tester for providing a delayed signal to the test head. The test head comprises a plurality of pins, each coupled to a pin of the integrated circuit being tested. 
     Delay circuit 23 provides a delay of 1.25 nanoseconds or less. Multiplexer 19 forwards input signal V IN  to delay circuit 23 when specified by instructions provided by programming signals V MSB   through OR gate 16. Multiplexer 26 determines the incremental delay up to 10 nanoseconds in 1.25 nanosecond steps in accordance with instructions received from programming signals V LSB  through latch 25. Input circuit 11, counter 13, comparator 14, OR gate 17, oscillator 21, and delay circuit 18 provides for additional delays in 10 nanosecond steps up to 640 nanoseconds, which may be summed with the delay of less than 10 nanoseconds provided by delay circuit 23. The operation of each of the elements of the delay circuit will be explained in greater detail in reference to FIGS. 5, 6, and 7. 
     Referring to FIG. 5, input circuit 11, counter 13, comparator 14, latch 15, and OR gates 16 and 17 are illustrated in block form. Input circuit 11 includes flip-flops 30 and 31 having their D inputs coupled to the output of OR gate 16 for receiving signal V ZERO . The clock input of flip-flop 30 is connected to input terminal 12 for receiving input signal V IN . The clock input of flip-flop 31 is connected to the output of OR gate 17 for receiving clock signal V DONE . Output Q of flip-flop 30 is connected to an input of OR gate 17 for providing signal V ON  and to the input of OR gate 32 for providing clock signal V CLOCK  to counter 13. Output Q of flip-flop 31 is connected to the reset input of flip-flop 31 by OR gate 33 and to the reset input of flip-flop 30. Output Q of flip-flop 31 is coupled to counter 13 by NOR gate 34 for providing reset signal V RESET . 
     Counter 13 includes flip-flops 35-40 having their reset and clock inputs connected to input circuit 11 for receiving reset signal V RESET  and clock signal V CLOCK , respectively. Flip-flop 35 has its D input and Q output connected to an input of exclusive OR gate 45 and its output Q connected to a clock input of flip-flop 36 and to an input of each of NAND gates 41-44. The date input D and output Q of flip-flops 36-39 are each connected to an input of NAND gates 41-44, respectively. The outputs of NAND gates 41-44 are each connected to clock inputs of flip-flops 37-40, respectively. Outputs Q of each of flip-flops 36-40 are connected to an input of exclusive OR gates 46-50, respectively. The output of NAND gates 42 and 43 are further connected to an input of NAND gates 43 and 44, respectively. 
     Latch 15 includes flip-flops 51-56, each having a data input coupled for receiving one each of programming signals V MSB , respectively. Flip-flops 51-54 have enable inputs connected to an inverting output of NOR gate 57 and flip-flops 55-56 have enable inputs connected to a non-inverting output of NOR gate 57. Flip-flops 51, 53, and 55 have output Q connected to an input of exclusive OR gates 45, 47, and 49, respectively, and each to a separate input of OR gate 16. Flip-flops 52, 54, and 56 have output Q connected to a separate input of OR gate 16 and output Q connected to an inverting input of exclusive OR gates 46, 48, and 50. NOR gate 57 is coupled to receive strobe signal V STROBE . 
     Programming signal V MSB  is provided to OR gate 16 when incremental 10 nanosecond delays are desired, which results in signal V ZERO  being provided to the &#34;D&#34; input of flip-flops 30 and 31. The Q output of flip-flop 30 clocks flip-flops 35-40 of counter 13 by signal V CLOCK . Counter 13 counts in a manner known to those skilled in the art, providing outputs V COUNT  from each flip-flop 35-40 to inputs of exclusive OR gates 45-50 of comparator 14. These outputs are compared in comparator 14 with latched programming signals V MSB , providing an output from each of exclusive OR gates 45-50 to OR gate 17; thereby providing signal V DONE  representing when signals V MSB  and V COUNT  are the same. 
     Referring to FIG. 6, oscillator 21 includes gates 61-65, each having an input coupled for receiving analog signal V CALIB . Gate 61 has an inverting input connected to multiplexer 66 and to input circuit 11 for receiving signal V ON , and gates 62-65 have an inverting input connected to the output of the preceding gate. The inverting outputs of gates 62 and 64 and the non-inverting outputs of gates 63 and 65 are connected to multiplexers 66 and 68. Multiplexers 66 and 68 have an input coupled to receive input timing calibration signal V TIME  A* through OR gate 67 and input timing calibration signal V TIME  B. Multiplexer 66 provides signal V ON  to one of the outputs to multiplexer 68 depending on the state of signals V TIME  A* and V TIME  B. Multiplexer 68 provides one of the inputs from multiplexer 66 as output V OSC  depending on signals V TIME  A* and V TIME  B. 
     AND gate 69 has one inverting input connected to output V OSC  of multiplexer 68 and another inverting input connected to output Q of flip-flop 70. Flip-flop 70 has its data input and clock input coupled for receiving signals V ZERO  and V DONE , respectively. Flip-flop 70 further has its reset input connected to the output of AND gate 69 and its Q output connected to the input of NOR gate 71 and to multiplexer 77. NOR gates 71-75 have their outputs connected to the inputs of the following NOR gates in series and the outputs of gates 72, 74, and 76 are connected to multiplexer 77. Multiplexer 77 is coupled to receive input timing calibration signals V TIME  A* and V TIME  B. The output of multiplexer 77 is signal V DELAY . Multiplexer 77 provides one of the inputs from flip-flip 70 or gates 72, 74, 76 as signal V DELAY  depending on the state of signals V TIME  A* and V TIME  B. 
     Gates 61-65 are responsive to signal V ON  and calibration analog signal V CALIB , providing output signals to multiplexer 66 and multiplexer 68. These output signals represent incremental delays due to gates 61-65. Signal V OSC  from multiplexer is determined by timing signals V TIME  A* and V TIME  B. Flip-flop 70 provides signals to multiplexer 77 in response to signals V OSC , V DONE  and V ZERO . These signals to multiplexer represent incremental delays due to NOR gates 71-76. The output signal V DELAY  from multiplexer 77 is determined by timing signals V TIME  A* and V TIME  B. 
     Referring to FIG. 7, multiplexer 19 has inputs coupled for receiving signals V DELAY , V IN , and V ZERO . The output of multiplexer 19 is connected to the inverting input of gate 80. The inverting input of gates 81-85 are connected to the output of the preceding gate and the non-inverting inputs of gates 80-85 are coupled to terminal 24 for receiving analog signal V PROG . Gate 86 has its inverting input and non-inverting input connected to the inverting and non-inverting outputs of gate 85, respectively. Gates 87-92 have their inverting and non-inverting inputs connecting to the non-inverting and inverting outputs, respectively, of the preceding gate. Multiplexers 93 has inputs connected to the non-inverting output of gates 85, 86, and 88 and the inverting output of gate 87. Multiplexer 94 has inputs connected to the non-inverting output of gates 90, and 92 and the inverting output of gates 89 and 91. 
     Flip-flops 97-99 of latch 25 have their data inputs D and clock inputs C coupled for receiving programming voltages V LSB  and strobe signal V STROBE , respectively, and are coupled to multiplexers 93 and 94 for selecting the desired input. OR gate 95 and NOR gate 96 ave their inputs connected to the outputs of multiplexers 93 and 94, respectively, and provide outputs V OUT  and V OUT , respectively. 
     In summary, incremental delays of 10 nanoseconds are provided by oscillator 21 and delay circuit 18. The total number of incremental delays is determined by counter 13 and comparator 14. A delay of less than 10 nanoseconds, that may be summed with these 10 nanosecond incremental delays, is provided by delay circuit 23 and multiplexer 26. 
     Multiplexer 19 selects either input signal V IN  or delay signal V DELAY  in response to signal V ZERO , providing the selected signal to gate 80. For a desired delay of less than 10 nanoseconds, input signal V IN  is directed through multiplexer 19 to delay circuit 23 by programming signals V MSB  (see FIG. 4). Referring again to FIG. 7, this input signal is compared in gates 80-85 with analog signal V PROG , providing a delay of 1.25 nanoseconds or less in increments determined by the gate delay of gates 80-85. This delayed signal may be further delayed by gates 86-92 in 1.25 nanosecond increments in accordance with programming signals V LSB  from latch 25. The output is ORed by gates 95 and 96 to provide outputs V OUT  and V OUT . 
     For desired delays greater than 10 nanoseconds, additional increments of 10 nanoseconds may be summed with the less than 10 nanosecond delay by the remainder of the circuit (FIGS. 5 and 6). Input signal V IN  clocks flip-flop 30, whose output Q, through OR gate 32, clocks flip-flops 35-40. Flip-flops 35-40 count in a manner known to those skilled in the art and provide output signals to exclusive OR gates 45-50. The desired output from one of exclusive OR gates 45-50 is selected by programming signals V MSB  through flip-flops 51-56 and provided to OR gate 17. Output V DONE  of OR gate 17 both clocks flip-flop 31 for resetting flip-flops 30 and 35-40, and for initializing delay circuit 18. 
     The inverting input of gate 61 of oscillator 21 receives signal V ON  of input circuit 11. The inverting inputs of gates 62-65 are each connected to the output of the preceding gate. These inputs are compared to analog signal V CALIB  and the results passed to multiplexers 66 and 68. This delayed signal may be further delayed by gates 86-92 in 1.25 nanosecond increments in accordance with programming signals V LSB  from latch 25. The output is ORed by gates 95 and 96 to provide outputs V OUT  and V OUT . 
     Further embodiments of the programmable delay circuit illustrated in FIG. 1 comprises gates 61-65 of oscillator 21 of FIG. 6 and gates 80-85 in delay circuit 23 of FIG. 7. The outputs of gates 61-64 are capacitive loaded by having long metal runs on the integrated circuit for providing a fast rise time and a slow fall time so that the outputs may be compared with analog signal V CALIB . Similarly, the outputs of multiplexer 19 and gates 80-84 are capacitive loaded by having long metal runs on the integrated circuit for providing a fast rise time and a slow fall time so that the outputs may be compared with analog signal V PROG . 
     By now it should be appreciated that there has been provided a programmable delay circuit that provides a quickly adjustable and accurate delay of an input signal.