SELF CONTAINED ARRAY TIMING

Abstract: An on-chip timing control apparatus (19) for generation of timing signals for a large scale integrated (LSI) chip or semi­ conductor memory array (12) is disclosed. This apparatus re­ quires only an off-chip trigger pulse input (16) and it may be used both during the production testing of the memory and during normal functional operation. In the testing environment it allows use of much less expensive peripheral test equip­ ment, while also providing for much greater accuracy in deter­ mination of whether or not the memory array meets its timing specification. Use during normal functional operation (subse­ quent to use in the test environment) provides for a guarantee of defect free operation.

Description: Field of the Invention This invention relates to an integrated circuit chip containing a memory array according to the preamble of claim 1 and methods of testing the same. It is effective whether or not the memory array is embedded in logic. For purposes of discussion the term "embedded" is defined as that condition of a memory array, circuit element, or even a circuit function on an LSI chip, wherein it is surrounded by other circuitry on the chip, such that the memory array, circuit element, or circuit function is not directly accessible, either in whole or in part, from the input and output terminals or pads of the chip. A prime problem associated with the manufacture of memory arrays is the assurance that the device meets its performance specification. A prime problem in the use of memory arrays is the assurance that the control signals provided to the device fall within this same performance specification. As the performance of these devices is improved it becomes more and more difficult to insure that these problems are resolved in an economical manner. As the level of integration increases, the circuit designer and component manufacturer are able to offer more and more complex memory arrays to the logic designer and system manufacturer, with higher and higher performance levels. This in turn requires more and more sophisticated circuitry external to the memory device to properly test and utilize it. Without an inexpensive means to guarantee ultimate performance the potential improvements in device design will be lost due to imprecision in the manufacturing test equipment and in the manufacturing tolerances of the supporting circuitry. Cross-Reference to Related Patent U.S. Patent 4,441,075 entitled "Electronic Chip In-Place Test (ECIPT) Structure and Method". Background of the Invention and Prior Art Since memory arrays exist and are in common use it is obvious that the problem of testing them has been widely addressed. It is common in the industry to utilize a special purpose tester that provides logic and timing signals to the device under test with high accuracy. Equipment of this sort is commercially available from a number of vendors. However, improvements in the performance of these memory devices are difficult to match without significant increases in cost for these special purpose testers. Over the years there has been a lot of work in the field of memory array testing to reduce the cost of this special test equipment. Many efforts have been in the area of generation of the logical signals to be applied to the device under test. Benton (U.S. Patent 4,195,770) and Shimizu (U.S. Patent 4,293,950) each describe an apparatus to develop such logic signals. Chesley (U.S. Patent 4,038,648 and U.S. Patent 4,055,754) includes an apparatus of this nature in the memory array itself. Efforts have also involved generation of the timing signals to be applied to the device under test. Staiger (U.S. Patent 4,263,669) and Hilker (U.S. Patent 4,290,137) have each described an apparatus designed to resolve particular problems with generation of specialized timing signals for particular needs of particular memory arrays. The current art is so dependent on this external application of timing signals that Hurley (U.S. Patent 3,961,251), Eichelberger (U.S. Patent 3,961,252), and Cavaliere (U.S. Patent 3,961,254) have described various methods where memory arrays embedded inside a semiconductor device can be accessed by an external tester to provide these timing signals during the testing operation. Some work has been done to bring the timing generation circuitry onto the same semiconductor device as the memory array. A discussion can be found by Hnatek (A User's Handbook of Semiconductor Memories), John Wiley and Sons, Inc., 1977, pp. 444-450. However, in all previous work including the above, the manufacturing test operation has required specialised external test equipment for timing operation. JP-A-58 159293 (Fujitsu) (English language abstract published in the Patent Abstracts of Japan, vol 7, no 286 (P-244) [1431], 21 December 1983) teaches a chip with both a memory array and timing generation circuitry. A control signal of different lengths is passed to the timing generation circuitry and, depending on the pulse width of the control signal, refresh, read or write operations are carried out on the memory array. The timing generation circuitry comprises a sample pulse generating circuit and a clock generator circuit. The sample generator circuit generates first, second and third delay pulses that are ANDed with the control signal to perform the refresh, read and write operations together with identification and control operations. The output of the clock generating circuit activates an address buffer, a gate circuit, a row decoder, a column decoder, a data input buffer and a data output buffer at the required times. Using this circuitry, it will be possible to control the operation of a memory array. However testing of the timing signals will require the specialised external test equipment mentioned above. This type of tester has an accuracy of around 200ps which is not be accurate enough for the type of measurements required. Measurements of the array set up time, time required for a write-thru operation in the memory array, pulse width and the cycle time for critical paths through the logic and the memory array cannot be easily made using the circuitry taught in this document. JP-A-58-201149 (Toshiba) (English language abstract published in the Patent Abstracts of Japan, vol 8, No 51, (P-259) [1488], 8 March 1984) discloses an integrated circuit in which the output buffers of the integrated circuit are connected together to form a recirculating loop. By placing a signal on a test terminal, the loop enters a self-oscillating state and the delay time of the output buffers can be calculated by measuring the frequency of self-oscillation. In this second Japanese patent application, however, no timing control network is disclosed which controls the logic internal to the integrated circuit. There is therefore no indication that the self-oscillating recirculating loop can be used to measure the delay time of the timing control network. In particular, since the integrated circuit taught in this document is not necessarily a memory array, there is no discussion about why measurements of the delay time of the timing control network are critical to the correct operation of the integrated circuit. Summary of the Invention The object of the invention is to provide an improved integrated circuit chip of high circuit density containing a memory array that is readily testable. This object is solved by providing on the chip a timing control network which includes a trigger signal responsive recirculating tapped delay line with means for initiating a test mode for said delay line and a test output terminal for providing a test signal. The invention also includes a method for testing the timing control network of the integrated circuit chip comprising the steps of triggering said timing control network; recording the signal produced at the test output means and measuring the frequency of oscillation of said produced signal using a counter and time source. In order to test the integrated circuit chip, the memory array must be isolated from external timings. This is done by placing latches in series with the address and data inputs and the data outputs. The overhead from inclusion of these latches may be minimized by incorporating them into the circuitry of the off-chip receivers and drivers, or of the memory array driver circuits or sense amplifiers. Clock signals to control these latches, as well as the actual array write clock, are developed on-chip using the timing control network. These control signals are triggered by the leading edge of an external clock, with all relative timings, including pulse width, controlled internally. For example, for a write cycle, the clock signal to the address latches is turned on first, then after just enough time for the operation of the address decode, the memory array write clock is turned on. Timed relationships between external signals are not required, and only a single edge (rather than the width) of the external triggering clock is important. Clocks for the data input latches, and for the data output latches in read mode, are developed with similar appropriate timings. These clocks are all developed by combining suitably delayed transitions triggered from the external clock. The NOR of one falling signal and one later rising signal supplies a positive clock pulse whose offset and duration is determined by the delays of the two signals. In the preferred embodiment of the invention, the timing control network, as needed for the latch and memory array circuitry, can be developed from a serial chain of inverter circuits. The operation of the array is critically dependent on the timing control network. Its timing can be tested in manufacturing using the method of claim 6, even with relatively inexpensive and unsophisticated test equipment, by adding a feedback control circuit to turn it into a recirculating loop. Loop frequency measurement is extremely accurate compared even with expensive delay measurement equipment. If the natural loop frequency would be too high for the test environment, a frequency divider should be added before the output driver instead of adding stages to the loop, to minimize measurement error of the significant circuitry. Since the chip functions by racing one path (through the memory array itself) against another (through the timing control network), delay defects in the slower path will cause the operation to fail due to a resulting gross DC functional error. Delay defects in the timing control network of overall process shift will be detected by RLF (recirculating loop frequency) measurement. While there is a possibility of some small defects in the timing control network escaping this test, the probability is relatively small. The path delay of the timing control network must be designed to be a little longer than that through the memory array to compensate for the tracking error. To avoid cutting into the yield, the fastest delay through the timing control network must be slower than the slowest delay through the memory array, as the timings vary due to manufacturing tolerances. Since both paths are on the same chip, contain many stages, and include both directions of transition, this tracking error should be minimal. It is currently estimated that this error will be smaller than the tester accuracy errors for manufacturing test equipment for the foreseeable future. The timing control network does not affect application of test patterns to the memory array; all of the necessary traditional sequences can be used. While it may still be necessary for the tester to apply these patterns at a high repetition rate to detect disturb defects, the tester need to provide precise relative timings. This can reduce tester cost by about a third. Additionally, some of the assembly level testers with very high pin counts, currently unable to do any delay test, can do at least some performance testing. In system use, presence of the latches in the address and data input paths can provide a significant reduction in cycle time for paths through the memory array. Since the logic values on the chip inputs are strobed at the beginning of the memory array operating cycle, and then held internally during completion of the cycle, address and data changes for the next cycle may be initiated during completion of the first cycle. As an example, it is common to have a tolerance for skew between the clock and the data at the input to a latch. If there is a 4nS skew, an overlap of 3nS for the next cycle's data with the current cycle's operation is not unreasonable. For an array operating in a 10nS path, this reduction to 7nS represents a 30% cycle time reduction. Overlap on a read cycle requires that the system data flow makes use of the address latches. The design strategy has been shown to offer advantages in chip testing (test accuracy and cost), assembly testing (accuracy and cost, ability to perform a delay test without a high speed buffer, and failure analysis), and system operation (faster cycle time, simpler clock generation, and statistical delay design). It may be further applied to embedded memory arrays. Brief Description of the Drawings The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings, wherein: FIG. 1 is a block diagram of an LSI memory array chip or semiconductor device in accordance with the invention. FIG. 2 shows idealized waveforms referred to hereinafter in the explanation of the operation of the memory array chip in accordance with the invention. FIGS. 3, 4 and 5 are respectively block diagrams referred to hereinafter in the explanation of the control network of the memory array chip in accordance with the invention. FIG. 6 is a block diagram of the circuit of a control network which may be employed in the memory array chip (FIG. 1) in accordance with the invention. FIGS. 7A, 7B and 7C respectively disclose a number of idealized waveforms referred to in the explanation of the operation of the invention (FIG. 1) employing the control network of FIG. 6. FIG. 7A, 7B and 7C respectively disclose in detail the relative timing and waveforms for a "write operation", a "read operation", and a "test operation". Description of the Preferred Embodiment Referring now to FIG. 1 there is illustrated in block diagram form an LSI chip or semiconductor device 11 in accordance with the teachings of the present invention. Chip 11 typically includes a memory array 12, a control network 19, data input latches 20, address latches 21, and data output latches 22. The memory array 12 may typically be 64 x 8, meaning 64 addressable locations, or words, each containing 8 bits. The invention is not size limited, however, and is applicable to either smaller or larger arrays as well. The memory array 12 is assumed to be a complete basic array, incorporating the normal address decoders, word and bit drivers, and sense amplifiers in addition to the memory cells. The data input latches 20, address latches 21, and data output latches 22 may be constructed in any of the forms as found commonly in textbooks on logic design, and further may incorporate the Level Sensitive Scan Design (LSSD) technique of Eichelberger, U.S. Patent 3,761,695, if desired. The data inputs 15, address inputs 14, and data outputs 13 may be connected directly to primary inputs and outputs to the chip, or more generally may connect to logic or other memory arrays on the chip as required for the system function. The inputs 16, 17 and 18 to the control network 19 must be connected directly to chip primary inputs. If it were not for the functional blocks on the chip (control network 19, and latches 20, 21, and 22) a manufacturing test would require generation of a number of signals with a complex timing interrelationship. With these functional blocks, however, very simple operation is possible where only switching order, and not actual times, must be controlled. This permits use of a much simpler tester, saving significant costs in the manufacturing process. The key to this apparatus is the control network 19, which performs the following functions: The control network provides all the timing signals to the array during testing operations. It may also be used to provide all timing signals to the array during system operation as well. The control network also signals the mode of operation (read or write) to the array. The control network initiates the proper timing sequence to the array when instructed by an external trigger signal 16. A performance test may be executed in the write mode by the following sequence of events: Select the write mode for the array by placing the appropriate signals on the READ/WRITE 17 and control 18 primary inputs. Place the desired address location on the address input signal lines 14. Place the desired data bits on the data inputs 15. Supply a trigger signal on the trigger primary input 16. The write timing sequence will automatically be generated by the control network 19, causing the selected data to be written into the selected address. The stored data may be read out from the array to verify that the data was written within the generated time period. A performance test may be executed in the read mode by the following sequence of events: Store the appropriate data into the appropriate words in the array. Load an address different from that desired into the address latches 21. Select the read mode for the array by placing the appropriate signals on the READ/WRITE 17 and control 18 primary inputs. Place the desired address location on the address input signal lines 14. Supply a trigger signal on the trigger primary input 16. The read timing sequence will automatically be generated by the control network 19, causing an address change at the input to the array 12. After the proper period the output latches 22 are gated off preventing the data from being stored if the array is not read within the guaranteed time period. What has been described is that in either the read mode or in the write mode all timings for the array 12 and the latches 20, 21, and 22 surrounding it are generated by the control network. Any desired arbitrary times, matching the specification for the array, can be generated as described later. All times are referenced to an arbitrary trigger signal which starts a fixed sequence of operation. External signals, such as from a tester in the manufacturing area, may be applied without concern for their timing accuracy. It must further be shown how the timings are generated by the control network 19. Generally, the control network 19 comprises a tapped delay line. In one embodiment, as shown in FIG. 3, all timing signals are generated from an inverter chain. This includes those to inhibit and to release data or addresses from the input and output latches. In this very simplified example only two timing pulses are developed; the generated waveforms are shown in FIG. 2. The I (inverter) and A (AND) gate delays control the pulse widths and timings. The gates shown in FIG. 3 provide pulse widths equal to the delay through an odd number of stages. As shown in FIG. 4, delays for an even number of stages may easily be developed. In addition, loading such as added capacitance and resistance can be used to fine tune the delays and thus the pulse widths and timings. Since the inverter chain is used to provide all array timing, it can be considered to be an extension of the tester. This circuitry must therefore be tested to insure that it is functioning at its rated speed before using it to test the array. Finally, this test must be within the capabilities of an inexpensive tester or the previously mentioned advantages of the present invention will be wasted. FIG. 5 shows a simple addition to the inverter chain. It provides for a special test mode where the inverter chain is converted into a recirculating loop. DC logic testers that are not suitable for high precision timing tests can often make frequency measurements with high degrees of accuracy. By measuring the natural frequency of oscillation of the recirculating loop the actual performance of the control network may be determined. If the natural frequency of the loop is designed above the tester limit, a frequency divider may be included in the buffer amplifier of FIG. 5 without loss of accuracy. (Reference is made to U.S. Patent 4,332,028.) Since the control network is on the same LSI chip as the array, a "fast" chain will coincide with a "fast" array. Little yield loss should occur from process variations. A "fast" chip containing a delay defect in the array might be within the product specification but fail when tested by its associated "fast" control network. There may be some debate about whether this should be 35 considered a yield loss; it probably reduces long term problems. The implementation of this circuitry does not preclude use of the latches in system mode, nor does it preclude flush through operation in system mode. However, operation in the flush through mode would reduce the probability that an array that functioned properly in the manufacturing tester would also function properly in the system, as in some cases a delay defect in the control network could mask a defect in the array, while still allowing it to pass the test. This circuitry can co-exist with other circuitry that makes use of input and output latches to speed up or simplify testing. Although the present invention can eliminate the need for a manufacturing tester with close timing tolerances, certain types of defects or manufacturing throughput may dictate use of a tester with an extremely high test rate. Nothing in this invention precludes use of this type of tester; in fact, the design of such a tester would be greatly simplified by allowing wider timing tolerances at the chip inputs. One possible embodiment of the control network 19 of FIG. 1 is described in detail in FIG 6. The three modes of operation of this control network 19 are further described, showing the generation of the output waveforms of FIGS. 7A, 7B and 7C. Nonspecific time units, equal to individual circuit element block delays, are used in these descriptions as the exact timings may vary as needed for the exact performance of the integrated circuit memory array being implemented. Write Mode (FIGS. 6 and 7A) Referring now to FIG. 6 and FIG. 7A, the operation of the control network 19 of FIG. 1 will be explained. First, the READ/WRITE input 41 must be preset to a logic value of 0, and the control input 43 must be preset to logic 0. A positive transition (from 0 to 1) on the trigger input 42 initiates the operation. This causes a negative transition on the output of block 1 of the inverter chain at time 1, and a positive transition on the output of block 6 at time 6. The outputs of blocks 1 and 6 are combined by blocks 25 and 26 to generate pulses 5 time units wide starting at time 2 on both the address 45 and data input 46 outputs from the control network. In FIG. 1, these signals are shown to energize the address 21 and data input 20 latch clocks. Returning now to FIG. 6 and 7A we see that similar transitions at the output of blocks 3 and 10, combined by block 23, are used to generate a pulse 7 time units wide starting at time 4 on the write clock output 47. This signal is used to energize the write clock of the array 12 of FIG. 1. These are all of the control signals needed to complete the write sequence. Any time after time 7 the trigger input 42 may be returned to the logic 0 state to prepare for the next operation, either another write cycle or a read cycle. Read Mode (FIG. 6 and 7B) The operation of the control network in read mode is similar to that described above for the write mode. In this case, the READ/WRITE input 41 must be preset to a logic value of 1, as well as presetting the control input 43 to logic 0. Now when the positive transition on the trigger input 42 initiates the operation, no output pulse occurs on the data input 46 and write clock 47 outputs from the control network. However, the outputs of blocks 11 and 16 of the inverter chain now are combined by block 24 to provide a pulse 5 time units wide starting at time 12 on the data output 48 from the control network. This pulse is used to energize the data output 22 latch clocks of FIG. 1. As with the above write sequence, these are all of the control signals needed to complete the read sequence. Any time after time 5 the trigger input 42 may be returned to the logic 0 state to prepare for the next operation, either another read cycle or a write cycle. Test Mode (FIG. 6 and 7C) The test mode is much simpler than either of the above two operational modes. Preset values of 1 on the READ/WRITE input 41, 1 on the trigger input 42, and 1 on the control input 43 are applied by the tester. An oscillating loop is formed by the blocks 1 through 16 and 31, since they cause an odd number of signal inversions upon each recursion of a signal state through the entire loop. This signal is picked off by the buffer amplifier 32 and placed on the test output 44 for measurement by the tester. A simple counter controlled by an accurate time source can be used to determine the frequency of oscillation, and delay defects in the oscillating loop will be easily detected. In addition to the advantages previously mentioned the following additional advantages are listed: In between the time two addresses are selected in a system the addresses may be randomly changing. This may subject the array of current designs to stress, and subsequent loss of data (disturb problems). Since with the method of the present invention the address changes only affect the array when the trigger signal arrives, the array will be isolated from this situation. The complex timing relationships, previously required from the system design, have been simplified by the circuitry incorporated on the array chip. Embedded array testing is greatly simplified. It should be noted that while specific means have been shown in relation to the design of the control network 19, other means could be used. While the particular design for the control network 19 has been described for a single port array, it will be understood and appreciated by those skilled in the art that a design appropriate for a multiport array may be readily provided by following the teachings of the invention. Also, referring to FIG. 1 and the description thereof, it is to be appreciated that the "address latches", "data input latches" and "data output latches" may be incorporated into the array circuitry. Further still referring to FIG. 1 the "read control 17" and "write control 18" may in reality be a single READ/WRITE control input for receiving a multi-level input.

Claims: An integrated circuit chip, containing a memory array (12), said memory array including: a plurality of memory elements for storing binary data, said stored binary data being arranged within said array at a plurality of addressable locations; controllable write circuit means for writing data into storage at a predetermined address within said array; controllable read circuit means for reading data stored at a predetermined address within said array; gated address latch circuit means (21) coupled to said controllable write circuit means and said controllable read circuit means, said gated address latch circuit means (21) providing said predetermined address to said write circuit means or said read circuit means; gated data input latch circuit means (20) coupled to said controllable write circuit means, said gated input latch circuit means providing data to be written into storage at said predetermined address provided by said gated address latch circuit means (21); gated data output latch circuit means (22) coupled to said controllable read circuit means, said gated output latch circuit means receiving the binary data read from the predetermined address by said controllable read circuit means; and timing control network means (19) coupled to and controlling said controllable write circuit means and said controllable read circuit means, said timing control network also being coupled to said gated address latch circuit means, said gated data input latch circuit means and said gated data output latch circuit means, said timing control network on said integrated circuit chip providing all timing control signals (45-48) for the operation of said memory array contained on said integrated circuit chip upon receipt of a trigger signal (16); characterised in that said timing control network (19) includes a trigger signal responsive recirculating tapped delay line with means for initiating a test mode for said delay line and a test output terminal (44) for providing a test signal. Integrated circuit containing a memory array as recited in claim 1 characterised in that said trigger signal responsive tapped delay line consists essentially of a serial chain of inverter circuits. Integrated circuit containing a memory array as recited in claim 2. characterised in that said tapped delay line consists of n serially connected INVERTER circuits (2-16), where n is an odd integer and having an input, an output and at least first, second, third and fourth (time displaced) spaced apart taps. Integrated circuit containing a memory array as recited in claim 3 characterised in that said timing control network (19) further comprises a single INVERTER circuit (27) having an input connected to said read/write input terminal (41) of said timing control network (19) and an output; a first AND-INVERTER circuit (28) having a first input connected to said trigger input terminal (42) of said timing control network (19), a second input, and an output connected to said input of said delay circuit formed by said n serially connected INVERTER circuits; a second AND-INVERTER circuit (31) having a first input connected to said control input terminal (43) of said timing control network (19), a second input connected to said output of said delay circuit formed by said n serially connected INVERTER circuits and an output connected to said second input of said first AND-INVERTER circuit (28); a first OR-INVERTER circuit (26) having a first input connected to said read/write input terminal (41) of said timing control network (19), a second input connected to said output of said first AND-INVERTER circuit (28), a third input connected to said second tap on said delay circuit formed by said n serially connected INVERTER circuits, and an output connected to said on-chip data input terminal (46) of said timing control network (19); a second OR-INVERTER circuit (25) having a first input connected to said output of said first AND-INVERTER circuit (28), a second input connected to said second tap on said delay circuit formed by said n serially connected INVERTER circuits, and an output connected to said on-chip address terminal (45) of said timing control network (19); a third OR-INVERTER circuit (23) having a first input connected to said read/write input terminal (41) of said timing control network (19), a second input connected to said first tap on said delay circuit formed by said n serially connected INVERTER circuits, a third input connected to said third tap on said delay circuit formed by said n serially connected delay circuits and an output connected to said on-chip write clock terminal (47) of said timing control network (19); and, a fourth OR-INVERTER circuit (24) having a first input connected to said output of said single INVERTER circuit (27), a second input connected to said fourth tap of said delay circuit formed by said n serially connected INVERTER circuits, a third input connected to said output of said delay circuit formed by said n serially connected INVERTER circuits, and an output connected to said on-chip data output terminal (48) of said timing control network (19). An integrated circuit device containing a memory array, as recited in claim 4, wherein said timing control network (19) further includes: buffer amplifier circuit means (32) having an input connected to said output of said delay circuit formed by said n serially connected delay circuits and an output connected to said test output terminal (44) of said timing control. Method for testing the timing control network (19) of the integrated circuit chip according to any of the above claims comprising the following steps: a) triggering said timing control network (19); b) recording the signal produced at said test output means (32, 44); c) measuring the frequency of oscillation of said produced signal using a counter and time source. Method for testing the timing control network (19) according to claim 6 wherein the delay of the timing control network is calculated from the frequency of the oscillation of said produced signal.

Inventors: KLARA, WALTER S., KWAP, THEODORE W., MARCELLO, VICTOR, RASMUSSEN, ROBERT A.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: G11C29
IPC codes: G11C11, G01R31, G11C29, H01L21, H01L27, G11C7