Counter-Address Memory for Multi-Channel Timing Signals

This invention relates to a timing signal generating apparatus which generates timing signals of various timing for many channels for testing semiconductor integrated circuits.

In testing a semiconductor integrated circuit, timing signals of variable delay times are required.

In a conventional timing signal generator of this kind each channel has a counter which is reset by a reference signal, and after being reset each counter starts counting clock signals which are commonly provided to all of the counters. Registers are also provided for storing the data corresponding to the periods from the reference signal to each of the various timing signals to be generated for all the channels. The counted data of each counter is compared by a comparator provided for each channel with the data stored in the register and when both data coincide with each other, a timing signal is generated at the output of the comparator.

In this conventional timing signal generator, since a counter, a register, and a comparator are required for each channel, the total number of components required increases as the number of channels increases, and thus it becomes high in cost and large in size.

It is an object of the present invention to provide a timing signal generator which is capable of generating required timing signals for many channels by a simple construction.

According to this invention a mark memory is provided which stores a mark 1 in an address corresponding to a time interval from a reference signal to the timing signal that is desired to be generated. First a counter is preset, for example to 0, by the reference signal and then the counter counts clock signals provided thereto for a fixed time after the reference signal. The mark memory is accessed in succession by the counted data from the counter so that the mark is read out from the specified address of the mark memory and the mark that is read out is used for generating a timing signal for each channel.

Accordingly, by changing addresses wherein marks are stored, it is possible to change timings of the timing signals to be generated with respect to the reference signal with ease. Since registers or comparators are not required in this invention, a timing signal generator can be provided with fewer components that in the prior art.

FIG. 1 is a circuit diagram showing an embodiment of a timing signal generating apparatus in accordance with the present invention.

FIGS. 2A to 2G are timing charts for explaining the operation of the embodiment of FIG. 1.

FIG. 3 is a circuit diagram of a modification of the embodiment of FIG. 1, for eliminating the uncertainty caused by the access time of a mark memory.

FIG. 4 is a circuit diagram of an embodiment of a timing signal generating apparatus for generating timing signals of different timings for a channel in accordance with the present invention.

FIGS. 5A to 5C are timing charts for explaining the operation of the circuit of FIG. 4.

FIG. 6 is a circuit diagram showing a timing signal generating apparatus for reducing the capacity of a mark memory of the present invention.

FIG. 7 is a circuit diagram of a timing signal generating apparatus which results from combining the apparatus of FIG. 6 with that of FIG. 4.

FIGS. 8A to 8F are timing charts for explaining the uncertainty of operation that occurs when the clock signal and the reference signal are closely spaced to each other.

FIG. 9 is a circuit diagram showing a timing signal generating apparatus of the present invention for eliminating the uncertainty of the operation explained in reference to FIG. 8.

FIG. 10 is a circuit diagram for a configuration of a timing signal generating apparatus which results from applying the feature in FIG. 9 to the embodiment of FIG. 6.

FIG. 11 is a circuit diagram of one example of construction of a timing signal generating apparatus obtained by combining the feature shown in FIG. 4 with the circuit of FIG. 10.

FIG. 12 is a circuit diagram of a timing signal generating apparatus of the present invention wherein a low speed memory is used as the mark memory for reducing the cost.

FIG. 13 is a circuit diagram of a modification of the embodiment of FIG. 12.

FIGS. 14A to 14H are timing charts for explaining the occurrence of a timing signal being delayed beyond the next reference signal.

FIG. 15 is a circuit diagram of a timing signal generating apparatus of the present invention for generating timing signals after the occurrence of the next reference signal as shown in FIG. 14.

FIG. 16 is a circuit diagram of a timing signal generating apparatus which results from applying the feature shown in FIG. 15 to the apparatus shown in FIG. 6 or FIG. 7.

FIG. 17 is a circuit diagram of a variation of the timing signal generating apparatus shown in FIG. 15.

FIGS. 18A to 18D are timing charts for explaining the operation of the embodiment of FIG. 17.

FIG. 19 is a circuit diagram of a timing signal generating apparatus which results from combining the apparatus of FIG. 17 with the apparatus of FIG. 6.

FIG. 20 is a circuit diagram of a timing signal generating apparatus which results from combining the apparatus of FIG. 17 with the apparatus of FIG. 12.

FIG. 21 is a circuit diagram of a modification of the embodiment of FIG. 20.

FIG. 22 is a circuit diagram showing a configuration for writing data into the mark memory.

FIG. 23 is a circuit diagram showing a configuration for writing data into the mark memory of the present invention.

FIG. 24 is a circuit diagram showing an embodiment of the present invention for changing addresses while writing the marks into the mark memory.

Timing signal generators in accordance with the present invention are now described with reference to the drawings.

FIG. 1 shows a basic concept of the timing signal generating apparatus in accordance with the present invention. A k-bit counter 11 is preset by a reference signal Tp provided from a terminal 12 and starts counting the subsequent clock signal Tc provided from a terminal 13. A mark memory 14 is accessed by the counted data of the counter 11. The mark memory 14 has 2.sup.k storage locations and a word of each storage location has n bits, where n is the number of channels required in a timing signal generating apparatus. In other words, the mark memory 14 is comprised of n memories M.sub.1 to M.sub.n which correspond to channels CH1 to CHn, respectively. Each time an address is accessed by the address data provided from the counter 11, an n-bit word specified by the address data is read out at output terminals 1.sub.1 to l.sub.n of the mark memory 14. In other words, for each channel CH1 to CHn, the mark memory 14 has 2.sup.k addresses with a 1-bit data in each address.

These mark memories M.sub.1 to M.sub.n are commonly accessed by the data provided from the counter 11. A mark 1 is selectively stored in the addresses of mark memories M.sub.1 to M.sub.n corresponding to the timing to be generated for each channel. The counter 11 is incremented by the clock signal Tc from the terminal 13 which is generated a fixed time after the occurrence of the reference signal Tp as illustrated in FIGS. 2A to 2G.

In the case that the timing signal in the channel CH1 is desired to be generated, for instance, 112 nano-seconds after the reference signal Tp, and the time period of the clock signal Tc is, for example, 16 nano-seconds, a mark 1 is stored in the address 7 of the mark memory M1. Thus, when the counted result becomes 7, the address data which represents the 7th storage location is provided to the mark memory 14 and thus the mark is read out as shown in FIG. 2C and outputted at the output terminal 1.sub.1 as a timing signal. Concerning the second channel CH2, a mark 1 is stored in the address 3 of the mark memory M.sub.2, so that, as illustrated in FIG. 2D, a timing signal is generated at a time when the third clock signal Tc is counted by the counter 11. Namely, in the second channel, the timing signal is obtained 48 nano-seconds later than the reference signal Tp.

As described above, in this embodiment the timing signals for n channels can be generated by using the counter 11 and the mark memory 14, and the construction of the timing signal generating apparatus is greatly simplified. For changing the timing of a timing signal, it is sufficient to change an address of the mark memory 14 wherein a mark is stored.

In accessing a memory, an access time, that is, the time that is required for obtaining an output signal from a memory after an address of the memory is specified by the address data, varies according to the characteristics of each storage location in the memory. Thus, timing signals generated from the mark memory have slight variations in time according to the access time of the storage locations. FIG. 3 shows a circuit diagram of an embodiment in accordance with the present invention for eliminating the uncertainty caused by the difference of access time of storage locations of the mark memory. In FIG. 3 AND gates A.sub.1 to A.sub.n are provided at the outputs of mark memories M.sub.1 to M.sub.n. Either the clock signal or the reference signal is provided to the AND gates A.sub.1 to A.sub.n through an OR gate 15 and a delay circuit 16. The delay time of the delay circuit 16 is selected for opening the AND gates A.sub.1 to A.sub.n after the signal from the storage location with the longest access time is provided thereto. Accordingly, timing signals are generated as shown in FIGS. 2F and G in synchronism with the delayed clock signals shown in FIG. 2E without being affected by differences in the access times.

In semiconductor integrated circuit testing, it is often desired to generate in a channel a plurality of timing signals each of which has a different delay time with respect to a reference signal. In this case, timing set data Ts as shown in FIG. 4 is provided to the mark memory 14 from a terminal 17. The mark memory 14 is comprised of a plurality of memory blocks 14.sub.1 to 14.sub.4 each of which has the same memory construction as FIG. 3, for example, and each of which is addressed commonly by the counted data from the counter 11. Further, the upper bits of the address can be considered to be supplied by the timing set data Ts to select one of the memory blocks 14.sub.1 to 14.sub.4. In each memory block 14.sub.1 to 14.sub.4, marks are stored in the addresses correponding to the delay times for generating desired timing signals. Thus it is possible to change the timing of the timing signal in one channel in real time by changing the timing set data T.sub.s for every occurrence of the reference signal.

The operation of the embodiment of FIG. 4 is further explained in reference to FIGS. 5A to 5C. Each reference signal Tp1, Tp2 and Tp3 is provided as illustrated in FIG. 5A and timing set data 1, 2 and 3 is provided, respectively, for each reference signal for sequentially selecting the memory blocks 14.sub.1, 14.sub.2 and 14.sub.3 as indicated by FIG. 5B. Regarding the first reference signal Tp1, the memory block 14.sub.1 is selected because the timing set data indicates 1, and a timing signal is generated at the output terminal which is delayed by the time period T.sub.1 from tne reference signal Tp1 as illustrated in FIG. 5C. Similarly, for the second reference signal Tp2, the memory block 14.sub.2 is selected by the timing set data 2 resulting in a timing signal being generated at the time T.sub.2 after the occurrence of the reference signal Tp2. Regarding the next reference signal Tp3, the memory block 14.sub.3 is selected by the timing set data to generate a timing signal at time T.sub.3 after the reference signal Tp3.

As has been described, different timing signals are thusly generated for the same channel. Although the timing set data Ts is supplied to the mark memory 14 as the upper bits of the address in this example, it is possible to supply the timing set data as the lower bits of the address.

In the embodiment described above, as the number of time steps is increased or as a longer delay time is required for generating the timing signals, the required capacity of the mark memory becomes quite large. For instance, if a timing signal delayed by 1.048576 milli-seconds is desired to be generated, then 65,536 storage locations are required in the mark memory when the period of the clock signal is 16 nano-seconds. Further, a high speed memory of about 10 nano-seconds access time is necessary for being operated by such a 16 nano-second clock signal. However, a memory with both high speed and large capacity is quite expensive.

An embodiment for reducing the required capacity of the mark memory in such cases, by dividing up the mark memory, is illustrated in FIG. 6. In FIG. 6 the counter 11 is divided into four counters 11a to 11d each of which has a plurality of bits. Mark memories 14a to 14d are also provided corresponding to the counters 11a-11d, respectively. Each mark memory 14a to 14d has n-bit words for n channels in the same way as the embodiments described above, but the total number of addresses are reduced since the addresses provided from the counters 11a to 11d are divided. The data that is read out from corresponding channels of the mark memories 14a to 14d are respectively supplied to AND gates 2.sub.1 to 2.sub.n whose outputs are connected to the output terminals 1.sub.1 to l.sub.n respectively.

Consider that the period of the clock signal Tc is 16 nano-seconds and that the maximum delay time required is 1.048576 milli-seconds, so that the counter 11 of FIG. 4 would need 16 bits for addressing the mark memory. In FIG. 6 the counter 11 is divided into four counters connected in series, each of which has 4 bits, for example. Since the counters 11a to 11d are 4-bit counters, the counter 11a counts up to 256 nano-seconds, the next counter 11b counts up to 4,096 nano-seconds, and the counter 11d counts up to 1.048576 milli-seconds. For the case of generating a timing signal delayed 1 millisecond with respect to the reference signal for the channel CH1, the output data of the counter 11 should be 1111010000010100. Since the counter 11 is divided into four counters in this embodiment, the output of the counter 11a should be 0100, and thus the corresponding mark memory 14a has a mark 1 in the fourth storage location of the channel CH1. A mark 1 is written into the first storage location of the channel CH1 of the mark memory 14b since the 4-bit data from the counter 11b is 0001. Similarly, the mark memory 14c stores the mark 1 in the address 4 of the channel CHl because the 4-bit data from the counter 11c is 0100, and the channel CH1 of the mark memory 14d stores the mark in the address 15 since the data from the last counter 11d is 1111. Thus when the counted data of the counter 11 becomes 1111010000010100 as a result of counting the clock signals after being preset by the reference signal, the signals that are read out from all the channels CH1 of the mark memories 14a-14d all become 1, so that the output signal delayed by 1 milli-second with respect to the reference signal is generated at the AND gate 2.sub.1.

In the embodiment shown in FIG. 1, the mark memory 14 needs storage locations for generating a 1 milli-second timing signal, whereas in the embodiment of FIG. 6, the mark memory 14 needs 64 storage locations. Generally, a high speed memory of about 10 nano-seconds access time is not so expensive if its capacity is small such as 256 bits. It is especially effective to apply this invention to the embodiment of FIG. 4, since the capacity of the mark memory is thusly further increased for being able to select the memory blocks by the timing set data Ts. FIG. 7 shows one of the examples wherein the embodiments of FIGS. 4 and 6 are combined.

In generating timing signals delayed by specified numbers of the clock period from the reference signal, it may happen that the last clock signal and the next reference signal are closely spaced with each other, which can cause an erroneous counting of the clock signal. For example, consider the case as illustrated in FIGS. 8A to 8F. In FIG. 8A, the first reference signal Tp1 and the next reference signal Tp2 are illustrated. In FIG. 8B, a group of clock signals Ta following the first reference signal Tp1, and a group of clock signals Tb following the second reference signal Tp2, are closely spaced with each other with respect to time. Thus there is the possibility that, before the desired output of the data from the mark memory accessed by the address data according to the clock count Tca can occur, the counter might be preset by the reference signal Tp2, thus resulting in the address data being preset and thereby preventing the required timing signal from being obtained from the mark memory.

Further, in the testing of semiconductor integrated circuits, a timing signal generator with high resolution, for example of around 100 pico-seconds, is required. In this case, since the mark memory is not operable at such a high speed, variable delay circuits (not illustrated) are provided in series with the output terminals 1.sub.1 to 1.sub.n for obtaining time differences of 100 pico-seconds.

For avoiding improper operation caused by the coincidence of the clock signal and the next reference signal as described above, and also for generating timing signals of high resolution, the circuit diagram of in FIG. 9 illustrates a preferred embodiment of the present invention.

In FIG. 9, a register 18 is provided which indicates whether or not a timing signal with a small delay, that is, of less than a clock period, is to be generated. When a timing signal of less than a clock period delay from the reference signal is desired to be generated, data 1 is stored in the respective bit of register 18 for each channel. The data from the register 18 and the reference signal from the terminal 12 are supplied to AND gates 19.sub.1 to 19.sub.n. The outputs of the AND gates 19.sub.1 to 19.sub.n are provided to the output terminals 1.sub.1 to 1.sub.n via OR gates 21.sub.1 to 21.sub.n, respectively. The clock signal from the terminal 13 is provided to the counter 11 through a delay circuit 22. Further, the counter 11 is preset by the reference signal to 1 in this embodiment. Each output from the channels CH.sub.1 to CH.sub.n of the mark memory 14 is supplied to the OR gates 21.sub.1 to 21.sub.n via AND gates A1 to An and via delay circuits 23.sub.1 to 23.sub.n.

To generate a timing signal for a time interval which starts at the reference signal Tp1 and which is less than the period of the clock signal Tc in FIG. 9 for the channel CH1, the data 1 is stored in the first bit 18 of the register 18. Before providing the clock signal to the counter 11, a signal D is generated at the output terminal as in FIG. 8D, through the AND gate 19.sub.1 in synchronism with the reference signal Tp1, since the data from the register address 18.sub.1 is 1 so that the AND gate 19.sub.1 is open. Thus by delay circuits (not shown) which have time differences of 1200 pico-seconds, for instance, connected in series with the output terminal 1.sub.1, a timing signal which has a required delay time within one clock period and a high resolution is obtained.

In the case of a clock pulse occurring very close to the next reference signal Tp2, as mentioned above, the embodiment of FIG. 9 works as follows. Since the counter 11 is preset to 1 by the first reference signal Tp1, the counter becomes 2 by the first clock pulse Tc1 of FIG. 8C and becomes 3 by the second clock pulse Tc2 and so on. Namely, the addresses 1, 2 and 3 of the mark memory are accessed before generation of the clock pulses Tc1, Tc2 and Tc3, respectively. In other words, in this embodiment, the addresses of the mark memory 14 are accessed one clock pulse prior to that of the embodiments of FIGS. 2, 3, 6 and 7. Thus in the case where a mark 1 is stored in the address 2 of the n-th channel CHn of the mark memory 14 as in FIG. 9, the mark 1 in the address 2 is read out by the first clock pulse Tc1 of FIG. 8E, so that a timing signal is generated at the output terminal 1.sub.n as shown in FIG. 8F in synchronism with the next clock signal Tc2.

Accordingly, even though the last clock signal Tca is closely spaced to the next reference signal Tp2 as shown in FIGS. 8A and 8B, the data corresponding to the clock signal Tca is already read out from the mark memory 14 by the clock signal Tca-1, and a timing signal as shown in FIG. 8F is generated at the output terminal in synchronism with the next clock signal Tca. Thus although the counter 11 may be preset by the reference signal Tp2, the timing signal required is already outputted and thus there arises no improper operation as mentioned above.

The delay circuit 22 is inserted for preventing the output signal from being generated at the output terminal in synchronism with the clock signal being applied at that time to the counter, and instead the output signal is provided at the next clock signal. Thus the delay circuit 22 is not always necessary because a certain length of time has usually elapsed, from the time of providing the clock signal to the counter 11 until the data is read out from the mark memory, so that the signals at the AND gates A.sub.1 to A.sub.n are generated in synchronism with the next clock signal. The delay circuits 23.sub.1 to 23.sub.n are used for adjusting the time differences between the output signals from the AND gates A.sub.1 to A.sub.n and the output signals from the AND gates 19.sub.1 to 19.sub.n, so that the timing signal generated from for example the address 1 of the mark memory 14 occurs one clock period after the signals provided from the AND gates 19.sub.1 to 19.sub.n.

The circuit configuration for generating timing signals delayed for shorter periods than the clock period as mentioned above in FIG. 9 is applicable as illustrated in FIG. 10 to the embodiment of FIG. 6, wherein the counter and the mark memory are divided for reducing the memory capacity. Also, the means described in FIG. 9 can be applied to the embodiment of FIG. 7 as illustrated in FIG. 11. Corresponding parts of the figures have the same reference characters. In FIG. 11, a memory of n-bits per word is used as the register 18 of FIG. 9, and this memory is addressed by timing set data Ts provided from a terminal 17.

In the embodiments of FIGS. 6, 7, 10 and 11, respectively, the mark memories 14a to 14d have to supply signals to the AND gates 2.sub.1 to 2.sub.n for each channel at the same time. For doing this, the mark memories 14a to 14d need to operate with the same access times. For instance, if the period of the clock signal T.sub.c is 16 nano-seconds, all the mark memories 14a to 14d must operate with about 10 nano-seconds of access time, but such high speed memories are expensive.

FIG. 12 shows an embodiment in accordance with the present invention for reducing the cost of the mark memories by using low speed memories. If the clock period is 16 nano-seconds, an address sequence to the mark memory 14.sub.b is 256 nano-seconds or more, at least 4,096 nano-seconds for the mark memory 14.sub.c and at least 65.536 micro-seconds for the mark memory 14.sub.d. Thus by applying the idea shown in FIG. 9 wherein the counter 11 is preset to 1 by the reference signal for obtaining one clock period margin, it is possible to comprise a timing signal generating apparatus with low speed memories for the mark memories 14.sub.b, 14.sub.c and 14.sub.d and with a high speed memory for the mark memory 14.sub.a.

In FIG. 12, the mark memory 14.sub.a is a high speed memory, for example, of about 10 nano-seconds access time, and the other memories 14.sub.b, 14.sub.c and 14.sub.d are low speed memories of, for example, about 200 nano-seconds access time, while the period of the carry signal from the counter 11.sub.a to the counter 11.sub.b is 256 nano-seconds in this case, since 11.sub.a is a 4-bit counter. A register 25 is provided in this embodiment, for indicating whether a timing signal is desired to be produced in each channel within the time interval of the carry signal from the counter 11.sub.a, that is, within the first 256 nano-seconds.

For generating a timing signal of shorter delay with respect to the reference signal than the time period of the carry signal, the data 1 is stored in the position corresponding to each respective channel of the register 25. When the data of the register 25 is 1, a timing signal is generated at a time when a mark 1 is read out from the high speed mark memory 14.sub.a. If the data of the register 25 is not 1, a timing signal is generated at a time when an AND signal of all the data read out from the mark memories 14.sub.b to 14.sub.d is obtained. Thus, as illustrated in FIG. 12, for the channel CH.sub.1 the outputs of the low speed memories 14.sub.b to 14.sub.d are connected to an AND gate 26.sub.1. The carry signal generated by the counter 11.sub.a is provided from a terminal 27 to a flip-flop 28.sub.1 to take an output signal from the AND gate 26.sub.1 into the flip-flop 28.sub.1. Also, AND gates 29.sub.1 and 31.sub.1 are provided for the channel CH.sub.1 for setting and resetting the flip-flop 28.sub.1. The data from the register 25 and the reference signal T.sub.p are provided to the AND gate 29.sub.1 whose output signal sets the flip-flop 28.sub.1. An inverted output of the register 25 and the reference timing signal T.sub.p are provided to the AND gate 31.sub.i whose output signal resets the flip-flop 28.sub.1. The Q output of the flip-flop 28.sub.1 is connected to an AND gate 32.sub.1. The clock signal T.sub.c and signals from the high speed memory 14.sub.a are provided to the other input terminals of the AND gate 32.sub.1.

In the case that, for example, a timing signal is to be generated in the channel CH1 which is delayed for a time shorter than the carry signal period, 256 nano-seconds in this example, the flip-flop 28.sub.1 is set by the signal from the AND gate 29.sub.1 and thus its Q output becomes high level. Accordingly, when the mark is read out from the channel CH.sub.1 of the high speed memory 14.sub.a, the mark is outputted from the AND gate 32.sub.1 in synchronism with the clock signal T.sub.c and generated as a timing signal at the output terminal 1.sub.1 through the OR gate 21.sub.1.

On the other hand, in the case that the channel CH.sub.1 is not indicated to generate a timing signal within the carry signal period, an output signal from the AND gate 31.sub.1 resets the flip-flop 28.sub.1, since the inverted data of the register 25 is at high level, and thus the AND gate 32.sub.1 is kept closed from the first clock signal until 256 nano-seconds has elapsed. The counter 11 is incremented further by the clock signal, and when the marks provided from the low speed memories 14.sub.b, 14.sub.c and 14.sub.d coincide with each other, the AND gate 26.sub.1 generates a coincidence signal at its output. The coincidence signal is taken into the flip-flop 28.sub.1 by the carry signal from the terminal 27 immediately following the coincidence signal and its Q output becomes high level to open the AND gate 32. Thus, when the mark is read out from the channel CH.sub.1 of the high speed memory 14a, a timing signal with longer delay time than the carry signal period is generated at the output terminal 1.sub.1 via the AND gate 32.sub.1.

In the embodiment of FIG. 12, the least significant bit of the counter 11.sub.b is preset to 1 by the reference signal Tp. As a result, the low speed memory 11.sub.b is accessed one carry signal before the number of the carry signals supplied to the low speed memory 11.sub.b to obtain one carry signal period margin in access time. Thus, as mentioned above, the low speed memories 14.sub.b to 14.sub.d which operate with a larger access time than the clock signal period can be used in this embodiment so that the embodiment becomes low in cost.

For example, in generating a timing signal 304 nano-seconds after the reference timing signal T.sub.p, a mark 1 is stored in address 3 of the high speed memory 14.sub.a, since 304-256=48 and 48/16=3. Also a mark 1 is stored in address 1 of the low speed memory 14.sub.b. Further, low speed memories 14.sub.c and 14.sub.d each store a mark 1 in the 0th address. Accordingly, when the reference timing signal is supplied to the counter 11, the marks are read out from each of the low speed memories 14.sub.b, 14.sub.c and 14.sub.d, since the counters 11.sub.b, 11.sub.c, and 11.sub.d are preset to 1, 0 and 0, respectively, resulting in a high level at the output of the AND gate 26.sub.1. Thus the output Q of the flip-flop 28.sub.1 becomes high level by the carry signal from the terminal 27 generated 256 nano-seconds after the reference timing signal T.sub.p so that the AND gate 32.sub.1 is opened. Accordingly, by providing three clock signals to the counter 11.sub.a, that is 48 nano-seconds after the first carry signal, the mark 1 is read out from the address 3 of the high speed memory 14.sub.a. As a result, a timing signal delayed by 304 nano-seconds with respect to the reference signal T.sub.p appears at the output terminal 1.sub.1 since the AND gate 32.sub.1 is open.

FIG. 13 shows another circuit configuration which can be used in the embodiment of FIG. 12. In FIG. 13, an OR circuit 33.sub.1 is provided between the flip-flop 28.sub.1 and the AND gate 32.sub.1. Data from the register 25 is supplied to one input of the OR circuit 33.sub.1, whose other input is provided with the Q output of the flip-flop 28.sub.1. The AND gate 29.sub.1 and the AND gate 31.sub.1 shown in FIG. 12 are eliminated in FIG. 13.

With regard to the embodiments of FIG. 12 and 13, it is possible to apply the timing set data as mentioned above in reference to FIG. 4, for selecting memory blocks to obtain a plurality of different timing signals with regard to the same channel.

At times it is desired to generate a timing signal delayed with respect to a reference signal beyond the occurrence of the next reference signal. For example, as shown in FIG. 14A, the reference timing signals Tp1, Tp2, Tp3 and Tp4 are generated in turn, and corresponding timing signals Tm1, Tm2, Tm3 and Tm4 are desired to be generated, as in FIG. 14B. In this case, the timing signal Tm2 corresponding to the reference signal Tp2 is generated after the occurrence of the next reference timing signal Tp3.

For obtaining a timing signal delayed beyond the next reference signal, a circuit diagram of a timing signal generating apparatus in accordance with the present invention is illustrated in FIG. 15. In the embodiment shown in FIG. 15, the reference signals are divided into two groups, such as Tp1, Tp2 and Tp3, Tp4 as in FIGS. 14C and 14E, respectively, and the clock signals corresponding to the divided reference signals are generated as shown in FIGS. 14D and 14F, respectively.

In FIG. 15, the reference signal provided from the terminal 12 is supplied to a toggle flip-flop 33 whose Q and Q outputs are connected to AND gates 34.sub.o and 34.sub.e, respectively. The other terminals of the AND gates 34.sub.o and 34.sub.e are provided with the reference timing signal from the terminal 12 to generate divided reference timing signals at the outputs of the AND gate 34.sub.o and 34.sub.e as shown in FIGS. 14C and 14E. Clock signal generators 35.sub.o and 35.sub.e are triggered by the divided reference timing signals. The clock signal generators 35.sub.o and 35.sub.e generate clock signals Tco and Tce having equal periods, after receiving the reference signals as shown in FIGS. 14D and 14F, respectively. Counters 11.sub.o and 11.sub.e are provided corresponding to the divided reference signals Tpo and Tpe, that is, the counter 11.sub.o receives the reference signal Tpo and the clock signal Tco, and the counter 11.sub.e receives the reference signal Tpe and the clock signal Tce.

The mark memories 14.sub.o and 14.sub.e are accessed by the count values provided from the counters 11.sub.o and 11.sub.e, respectively. The mark memories 14.sub.o and 14.sub.e are constructed with n-bits per word, where n is the number of channels required. The total number of storage locations of the memories 14.sub.o and 14.sub.e are decided by the maximum delay time with respect to the reference signal required for generating timing signals. Thus, in the event of generating a timing signal whose delay time extends beyond the next reference timing signal, the maximum number of storage locations in each of the mark memories 14.sub.o and 14.sub.e is selected for achieving a delay time longer than the time interval between the reference timing signals. Further, the timing set data T.sub.s from a terminal 17 is provided to the mark memories 14.sub.o and 14.sub.e to select respective blocks of the mark memories 14.sub.o and 14.sub.e, as required by the alternating reference signals. Output signals from both the mark memories 14.sub.o and 14.sub.e are thusly provided to OR gates 37.sub.1 -37.sub.n for each channel 1.sub.1 to 1.sub.n.

For generating a timing signal as shown in FIG. 14B in the channel CH.sub.1, the counter 11.sub.o is preset by the reference signal T.sub.p1 and then starts counting the clock signals so that the mark memory 14.sub.o is accessed to generate a timing signal as shown in FIG. 14G. Since the reference signals are divided into two groups as described above, the counter 11.sub.o is not preset by the reference signal Tp2. Likewise, the counter 11.sub.e is not preset by the reference signal Tp3 and continues counting the clock signals. Accordingly, as shown in FIG. 14H, a timing signal corresponding to the reference signal Tp2 is generated by reading out the mark from the mark memory 14.sub.e at a time later than the reference timing signal Tp3. The data read out from the mark memories 14.sub.o and 14.sub.e are combined at the OR gate 37.sub.1 to obtain the timing signals of FIG. 14B at the output of the OR gate 37.

If it is not known beforehand which timing signals will have to be generated by which of the two mark memories 14.sub.0 and 14.sub.e, then the same data must be stored in respective memory blocks of both mark memories. That is, the addresses wherein the marks are stored are identical in respective memory blocks of both mark memories, and each memory block is independently selected for each reference signal that is provided to each of the two mark memories.

The present invention explained above in reference to FIG. 15 is applicable to the embodiment of FIG. 7, wherein the counter 11 and the mark memory 14 are divided for reducing the capacity of the mark memory 14 as illustrated in FIG. 16. Also, the embodiment of FIG. 15 is applicable to the embodiments shown in FIGS. 10 and 12. Further, although the reference timing signals are divided into two groups in the embodiment of FIG. 15, it is possible to divide the reference signals into three or more groups, for generating timing signals delayed beyond two subsequent signals, by providing a corresponding number of counters and mark memories.

In the embodiment mentioned above in reference to FIG. 15, a plurality of counters are arranged for accessing the mark memories 14.sub.o and 14.sub.e. It is possible to provide a timing signal generating apparatus which uses only a single counter, for generating timing signals identical to those of the timing signals explained with regard to the embodiment of FIG. 15.

To accomplish this, a circuit diagram of another embodiment in accordance with the present invention is illustrated in FIG. 17. In FIG. 17, the mark memories 14 and 14' are commonly accessed by the counted data of the counter 11, and a timing set data from the terminal 17 is provided to both mark memories 14 and 14'. In this embodiment the timing set data to the mark memory 14' is provided one period of the reference signal later than that of the timing set data to the mark memory 14. For doing this, the timing set data T.sub.s from the terminal 17 is provided to a data terminal of a D-type flip 38, and the reference timing signal from the terminal 12 is provided to the clock terminal of the flip-flop 38. The timing set data taken in the flip-flop 38 is provided to the memory 14'. Thus the timing set data supplied to the mark memory 14' is shifted in phase and thereby delayed by one period of the reference signal. The mark memories 14 and 14' are constructed in the same way as the memories mentioned above, for example, the mark memories 14.sub.o and 14.sub.e. However, the mark memory 14' is used only for obtaining a timing signal delayed with respect to a reference signal beyond the occurrence of the next reference signal. In this embodiment a mark is stored in the address corresponding to the time interval between the next reference signal and the timing signal to be generated, that is, corresponding to the time interval which starts at the occurrence of the next reference signal and ends at the generation of the desired timing signal.

In FIGS. 18A to 18D a timing chart is illustrated for explaining the operation of the embodiment of FIG. 17. In FIG. 18A the reference signals Tp1, Tp2, Tp3 and Tp4 are provided to generate corresponding timing signals S.sub.1, S.sub.2, S.sub.3 and S.sub.4. As shown in FIG. 18B the timing signal S.sub.2 which corresponds to the reference signal Tp2 is generated after the next reference signal Tp3 by the time interval D.sub.2. Thus in the mark memory 14' a mark 1 is stored in the address corresponding to the time interval D.sub.2. The data read out from the mark memories 14 and 14' is combined by OR gates 37.sub.1 to 37.sub.n for each channel and provided to output terminals 1.sub.1 to 1.sub.n in synchronization with the clock signal by the AND gates A.sub.1 to A.sub.n.

When the reference signals shown in FIG. 18A are supplied to the counter 11 from the terminal 12, the counter 11 is preset and then starts counting the clock signal Tc that is provided from the terminal 12. After the generation of the reference signal Tp1, the timing set data 1 as shown in FIG. 18C is provided to the mark memory 14 and also to the flip-flop 38 so that the mark memory 14 is selected to generate a timing signal S.sub.1. By the second reference signal Tp2, the timing set data 1 is taken into the flip-flop 38 as shown in FIG. 18D and the counter 11 is preset again. Thus, during the time interval between the reference signals Tp2 and Tp3 the timing set data 1 is provided to the mark memory 14' and timing set data 2 is provided to the mark memory 14. In this example, since the timing signal S.sub.2 is desired to be generated after the third reference signal Tp3, the marks are not read out from either the mark memory 14 or 14' during this time interval. When the third reference signal Tp3 is supplied to the counter 11 and the flip-flop 38, the counter 11 is preset and the timing set data 2 is taken into the flip-flop 38. Thus, after generation of the reference signal Tp3, the timing set data 3 is provided to the mark memory 14 whereas the timing set data 2 is provided to the mark memory 14'. Thus when the counted data of the counter 11 becomes the address corresponding to the delay time D.sub.2, the mark is read out from the mark memory 14' to generate the timing signal S.sub.2. Also, when the counter 11 accesses the address corresponding to the timing signal S.sub.3, the mark is read out from the mark memory 14 and outputted at the terminal 1.sub.1 via the OR gate 37.sub.1.

The invention explained above in reference to FIG. 17 is applicable to the other embodiments mentioned above. For example, one of the embodiments derived by applying the invention of FIG. 17 to the embodiment of FIG. 6 is illustrated in FIG. 19. In FIG. 19 the timing set data Ts from the terminal 17 is provided to the mark memories 14.sub.a to 14.sub.d and the timing set data Ts' delayed by the flip-flop 38 by one period of the reference signal is provided to the mark memories 14.sub.a ' to 14.sub.d '. The mark memories 14.sub.a, 14.sub.a ' to 14.sub.d to 14.sub.d ' are accessed by the contents of the counters 11a to 11d, respectively. Output signals from the mark memories 14.sub.a to 14.sub.d are provided to the AND gate 2.sub.1 and output signals from the mark memories 14.sub.a ' to 14.sub.d ' are provided to the AND gate 2.sub.1 ', respectively, for the channel CH.sub.1. The outputs of the AND gates 2.sub.1 and 2.sub.1 ' are combined by the OR gate 37.sub.1 to generate a timing signal at the terminal 1.sub.1 via the AND gate A.sub.1.

For generating a timing signal that is delayed, for example, by 1.5 milli-seconds with respect to the reference signal, while the time interval of the reference signal is 1 milli-second and the period of the clock signal Tc is 16 nano-seconds, the embodiment of FIG. 20 operates as follows. In this case a fraction of the delay time corresponding to the time interval D.sub.2 shown in FIG. 18 is 0.5 milli-seconds. The binary data which represents this 0.5 milli-seconds is 0111101000001010, and this binary data is divided into four, that is, as 1010, 0000, 1010 and 0111, each of which is provided to the corresponding mark memories 14.sub.a ' to 14.sub.d ' as address data. Each mark memory 14.sub.a ' to 14.sub.d ' stores the mark 1 at the corresponding location represented by the divided address data. As a result, a timing signal is generated 0.5 milli-seconds after the next reference signal.

Further, as is illustrated in FIG. 20, the feature shown in FIGS. 17 and 19 is also applicable to the embodiment of FIG. 12. In FIG. 20, the mark memories 14.sub.a ' to 14.sub.d ' are provided in the same way as in the embodiment of FIG. 19, although the memory 14.sub.a ' operates at high speed and the other memories 14.sub.b ' to 14.sub.d ' operate at low speed. An AND gate 26.sub.1 ' is provided for detecting the coincidence of the signals from the low speed memories 14.sub.b ' to 14.sub.d '. A register 18' is provided for indicating whether or not the fraction of the delay time corresponding to D.sub.2 shown in FIG. 18 is greater than the time period of the clock signal T.sub.c. Both outputs of the registers 18 and 18' are supplied to the AND gate 19.sub.1 via an OR gate 39.sub.1. Further, a register 25' is provided for indicating whether or not the fraction of the delay time corresponding to D.sub.2, that is, from the next reference timing signal to the occurrence of the timing signal, is greater than the period of the carry signal from the terminal 27.

The output of the register 25' is provided to flip-flops 29.sub.1 ' and 31.sub.1 ', whose other terminals are provided with the reference signal in the same way as the circuit connection regarding the register 25. A flip-flop 28.sub.1 ' is set and reset by the output signals from the AND gates 29.sub.1 ' and 31.sub.1 ', respectively. An output of the AND gate 26.sub.1 ' is supplied to a data terminal of the flip-flop 28.sub.1 ' whose Q output is connected to an AND gate 32.sub.1 '. The signal that is read out from the mark memory 14.sub.a ' is provided to the other terminal of the AND gate 32.sub.1 '. The outputs of both AND gates 32.sub.1 and 32.sub.1 ' are supplied to the OR gate 37.sub.1 and provided to the OR gate 21.sub.1 through the AND gate A.sub.1. The timing set data from the terminal 17 is provided to the registers 25 and 18 and the timing set data delayed by the flip-flop 38 is provided to the registers 25' and 18'. Further detail of the operation of the embodiment of FIG. 20 is not explained in the following since it is understandable from the preceding explanation.

Although in the embodiment of FIG. 20 the registers 18, 18' 25 and 25' are employed, it is possible to reduce the number of registers as illustrated in FIG. 21. In FIG. 21, besides the registers 18 and 25, a register 43 is provided for indicating whether or not a timing signal is intended to be generated after the occurrence of the next reference signal. The register 43 is accessed by the timing set data from the terminal 17. Each data for the channel CH.sub.1 of the registers 25, 18 and 43 is supplied to the data terminals of D-type flip-flops 46.sub.1, 45.sub.1 and 44.sub.1, whose clock terminals are provided with the reference signals from the terminal 12 to take in the data from the registers 25, 18 and 43, respectively. Thus, at the Q outputs of the flip-flops 44.sub.1, 45.sub.1, and 46.sub.1, the data that indicates whether or not a timing signal is generated after the occurrence of the next reference signal, the data that indicates whether or not a timing signal is delayed within the clock signal period, and the data that indicates whether or not a timing signal is delayed within the carry signal period, are made to appear, respectively, with one reference signal period delay. Data from the register 43 is also provided to both an AND gate 47.sub.1 and the AND gate 32.sub.1 via an inverter 49, and the delayed data of the register 43 is provided from the flip-flop 44.sub.1 to both AND gates 48.sub.1 and 32.sub.1 '. The data from the register 18 and its delay data are supplied to AND gates 47.sub.1 and 48.sub.1, respectively. Both outputs of the AND gates 47.sub.1 and 48.sub.1 are supplied to the OR gate 39.sub.1 to generate combined data at the output of the OR gate 39.sub.1. Data from the flip-flop 46.sub.1 is provided to the AND gates 29.sub.1 ' and 31.sub.1 ' for operation as in FIG. 20.

In the case that the data from the register 43 is 0, signifying that a timing signal is to be generated before the occurrence of the next reference signal, the data from the register 18 is ouputted at the OR gate 39.sub.1 since the AND gate 47.sub.1 is opened by the inverted data of the register 43. The flip-flop 28.sub.1 is set by the data from the register 25 to open the AND gate 32.sub.1. Thus in this case the embodiment of FIG. 21 operates as described above in reference to FIG. 12, since the data from both the registers 25, 18 is ready to be supplied to the subsequent circuits.

In the case that the data of the register 43 is 1, the AND gates 47.sub.1 and 32.sub.1 are closed since the data from the register 43 is inverted, whereas the AND gate 48.sub.1 is opened by the next reference signal. Accordingly, the data from the register 18 that is delayed by one period of the reference signal is outputted at the output of the OR gate 39.sub.1 and the delayed data of the register 25 from the flip-flop 46.sub.1 sets the flip-flop 28.sub.1 ', and is provided at the output of the AND gate 32.sub.1 '. Accordingly, this embodiment works according to the operation described in reference to FIGS. 17 and 18, and thus a timing signal is generated after the next reference signal.

FIG. 22 shows a circuit diagram for writing the mark data into the mark memory. Channel select data for selecting channels wherein marks are stored is provided to a decoder 48 from a terminal 47. The channel select data selects a write enable terminal WE of one of the memories M.sub.l -M.sub.n. Data for presetting the counter 11 is provided to the counter 11 from a terminal 49. The preset data in the counter 11 is the address of the mark memory 14 corresponding to the delay time of a timing signal with respect to a corresponding reference signal. The data 1 is provided at the terminal 51, and the mark is stored in the selected address of the selected channel of the mark memory. The data in the other storage locations of the mark memories M.sub.l -M.sub.n should be 0. Thus before writing in a mark 1 as mentioned above, all the storage locations of the mark memory must be loaded with the data 0, since the addresses wherein the marks 0 and 1 were previously stored are not known. In this case, writing 0 into all the storage locations of the mark memory takes a long time.

FIG. 23 shows a circuit diagram for reducing the time required for changing storage locations wherein the marks are stored. In FIG. 23, a mark address memory 52 is provided for storing the address data of each mark previously stored in the channels of the mark memory 14. An address register 53 supplies new address data indicating a memory location wherein each mark is to be stored, this data being provided from the terminal 49 to the mark address memory 52. The address data stored in the mark address memory 52 is read out by the channel select data supplied from the terminal 47, and at the same time the new address data from the address register 53 is stored in the same channel of the mark address memory 52.

In writing 0 into all the addresses of the mark memory 14 in this embodiment, the address data which indicates the address of the mark memory 14 wherein a mark 1 is stored is read out by the channel select data and presets the counter 11 to access the mark memory 14. Also the channel of the mark memory 14 is selected by the channel select data provided by the decoder 48, and data 0 is provided at the terminal 51. Thus, the storage locations of the mark memory 14 wherein each mark 1 was written are addressed and changed to 0, resulting in the data in all the storage locations becoming 0. At the same time, the new address wherein the mark 1 is intended to be stored is provided from the address register 53 and is stored in the same channel of the mark address memory 52 which is being selected by the channel select data. This process is repeated for each channel until the marks of all the channels of the mark memory 14 are changed to 0 and also the new address data for all channels are stored in the mark address memory 52.

In writing the new marks into the mark memory 14, the new address data for each channel is sequentially read out from the mark address memory 52 by the channel select data. At the same time, the channel of the mark memory 14 which is identical to that of the mark address memory 52 is selected, and the terminal 51 is set to high level. Thus the new data corresponding to the delay time of the new timing signals for n channels will be stored by repeating this procedure n times.

Some devices under test, such as a micro-processor for example, operate with timings that are determined by a signal which the device itself generates, for instance, an address latch enable signal, and not by a clock signal provided from the timing signal generating apparatus. The time difference between the clock signal and the address latch enable signal generally varies from device to device. For generating timing signals for such devices, mark address data must be shifted by the corresponding time difference. However, generation of the mark address data including the information on the time differences by CPU operation requires additional operation time and thus the test speed is limited.

FIG. 24 shows a circuit diagram for providing mark addresses to the mark memory for generating timing signals for the devices mentioned above. In FIG. 24 mark address data that indicates the timings of the timing signals is provided from a CPU 54 to a register 55. The CPU 54 also provides channel select data to the terminal 47. A register 56 is provided for storing data indicating for each channel whether or not a compensation of the timing is required, and the stored data in the register is selected by the channel select data at a selector 57.

If compensation is required, the selector 57 opens an AND gate 58 whose other terminals are provided with offset data from a register 59. The offset data in the register 59 corresponding to the time required for compensation, for example, the time difference between the clock signal and the address latch enable signal. The offset is provided through the AND gate 58 to an adder 61 wherein the offset data is added to the mark address data. As a result, compensated mark address data is generated at the terminal 49. As has been described, by storing the offset data in the register 59, the mark address data is automatically compensated.