HIGH-SPEED TEST SYSTEM FOR A MEMORY DEVICE

Abstract: A memory device requires a minimum of two input/output lines from an external testing device to be coupled thereto. A first DQ line from the memory device provides a direct data path from the array so that the external tester can read data from the array at the maximum speed of the memory device. Test mode circuitry for multiplexing and comparing multiple DQ lines during address compression mode is coupled to two or more DQ lines, including the first DQ line. The compression mode testing circuitry can include on-chip comparators that compare the data simultaneously written to, and read from, the memory device. The comparison circuitry outputs a data test flag indicating whether or not the data read from the memory device matches. The test flag is output through a multiplexer to a second DQ line. As a result, the speed of the device can be tested from the first DQ line, while the results of on-chip comparison can be sampled at the second DQ line. The compare circuitry compares not only bits of a given data word, but also at least one bit from another data word. Therefore, rather than employing two compare circuits that compare first and second data words, and a third compare circuit that compares the results of the first two compare circuits, the present invention avoids the need for the third compare circuit by comparing the first data word in a first compare circuit with at least one bit from the second data word.

Description: Description HIGH-SPEED TEST SYSTEM FOR A MEMORY DEVICE Technical Field This invention relates generally to computers, and more particularly to memory devices and methods of testing such devices. Background of the Invention Computer designers desire fast and reliable memory devices that will allow them to design fast and reliable computers. Manufacturers of memory devices, such as random access semiconductor memories, must test a full range of functionality and timing characteristics of the memory devices in order to provide a reliable product to their customers. Because each memory cell of the device must be tested, the time and equipment necessary for testing memory devices having increasing density represents a significant portion of the overall manufacturing cost of such devices. Any reduction in the time to test each unit will reduce manufacturing costs. Semiconductor manufacturers have developed fast testing routines to allow a greater number of chips to be tested simultaneously using a given testing device. One known testing routine, Jedec, simply compares the data written to a memory device with the data read from that memory device, and assigns a 1 value to one or more memory cell addresses if the data matches (passes), or a 0 if the data does not match (fails). While the Jedec routine is fast, it does not output the actual data written to the memory device. As a result, if the tester outputs a continuous string of Is, indicating that the memory device passes, a technician is unsure whether the device actually passes, or if an error has occurred in the device, or at some point along the path from the device to the tester, to cause such an output. To compensate for this shortcoming of the Jedec routine, a Micron Test Mode Routine provides three outputs. The Micron Routine outputs the actual data, as a 0 or a 1, and a mid-level tri-state value therebetween. If the tri- state value is output, rather than a 1 or a 0, the technician recognizes that an error has occurred. Urifortunately, while the Micron Routine provides superior testing of most semiconductor devices, the routine typically cannot bias the output back to the tri-state value before the beginning of the next read/write cycle rapidly enough to allow current high-speed memory devices to be tested at their normal operating speed. As a result, such high-speed memory devices must be tested at speeds slower than their typical operating speed. To save testing time and cost, manufacturers of memory devices increasingly automate the testing procedure so that a tester applies the testing routine simultaneously to several chips. Automated testing is most easily accomplished after the memory device has been packaged as a semiconductor chip, because the chip can be automatically inserted into a test socket using pick and place machinery. Automated testing circuitry then performs the testing routine by applying predetermined voltages and signals to the chip, writing test data patterns to the memory, reading data, and analyzing the results to detect memory speed, timing, failures, etc. The more chips that can be tested simultaneously, the greater testing time savings per chip. Most testers used in testing semiconductor chips are expensive. For example, a current tester manufactured by Teradyne has 128 input/output ("I/O") lines. To maximize the number of chips that this tester can test simultaneously, the on-chip data input/output lines, or "DQ lines," are multiplexed so that fewer I/O lines from the tester are required to be coupled to each chip. For example, the tester writes a predetermined data pattern simultaneously to multiple locations in each memory device and then accesses the written data during a read cycle. Comparator circuits fabricated on-chip compare the data read from the multiple locations and indicate whether all the data read matches the data written. If the chip has 32 DQ lines (DQ0-DQ31), on-chip 4: 1 multiplexers and testing circuitry compress data onto only 8 of the 32 DQ lines. As a result, only 8 of the 128 lines of the tester are required for each chip. Consequently, the tester's 128 I/O lines can simultaneously test 16 chips. In another solution, certain semiconductor memory devices, manufactured by Micron Technology, Inc., provide on-chip test mode circuitry that helps compensate for such delays during testing of devices. Under such test mode circuitry, the external testing device writes data to the chip during a first interval, and then writes the same data again to the DQ lines during a second interval. During the second interval, while the data is written again to the DQ lines, the data previously written to the memory device is read therefrom and latched. On-chip comparators then compare the latched data to the data written during the second interval. If the latched data equals the data written during the second interval, then the chip passes. Such a device can rapidly analyze the read data written to the device. While the above solutions can detect for typical cell-to-cell defects and functionality of the chip, they cannot accurately test the speed of the chips. As semiconductor memory chips provide increasingly faster data VO rates, particularly with synchronous DRAMs, data is required to be transferred to and from the chips in as little as 9 nanoseconds or less, based on a 10-nanosecond or faster clock cycle. As a result, such chips provide only a 1 -nanosecond margin of error. Today's increasingly fast memory devices require highly precise generation of timing signals and precise measurement of the memory device's response thereto. Gate delays caused by the multiplexing circuitry required during testing cause the data to be read from the chips in greater than 10 nanoseconds. As a result, the tester cannot determine if the chip accurately output data within the required 9 nanoseconds. In other words, the on-chip testing circuitry prohibits the tester from testing the speed of such chips. Obviously, it is desirable to determine the performance, and thus the speed of, semiconductor memory chips, especially high-speed chips. Additionally, because of manufacturing process tolerance and variations, one memory device of a particular design may be faster than another memory device of the very same design. Manufacturers therefore typically also desire to test the speed of such chips so that such chips can be sorted based on speed grades. To provide such speed testing, typical address compression mode testing, and on- chip multiplexing of DQ lines, must be abandoned. As a result, where 16 or more chips could previously be simultaneously tested using multiplexing, only 4 of such chips can be simultaneously speed tested because all 32 DQ lines of each chip must be coupled to the tester's I/O lines. As a result, there is a need to simultaneously speed test an increasing number of chips using a given tester. One solution has been to purchase a larger number of testers, or more expensive testers having a greater number of I/O lines. However, as noted above, such testers are quite expensive. Summary of the Invention The present invention provides a semiconductor device that can be efficiently speed tested, and which overcomes at least the shortcoπiings of the prior art discussed above. The memory device requires, at a minimum, only two I/O lines from an external testing device to be coupled thereto. A first DQ line from the memory device provides a direct data path from the array so that the external tester can read data from the array at the maximum speed of the memory device. Test mode circuitry for multiplexing and comparing multiple DQ lines in address compression mode is coupled to two or more DQ lines, including the first DQ line. The compression mode testing circuitry can include on-chip comparators that compare the data simultaneously written to, and read from, the memory device. The comparison circuitry outputs a data test flag indicating whether the data read from the memory device matches or whether the data does not match. The test flag is output through a multiplexer to a second DQ line. As a result, the speed of the device can be tested from the first DQ line, while the results of on-chip comparison can be sampled at the second DQ line. Importantly, the external testing device need not read the data from the first DQ line simultaneously with the test data flags from a second DQ line. The present invention also embodies a method of reducing the number of compare circuits required in on-chip test circuitry. To reduce the number of exclusive OR gates, and thereby realize increased surface area on the die, compare circuits compare not only bits of a given data word, but also at least one bit from another data word. Therefore, rather than employing two compare circuits that compare first and second data words, and a third compare circuit that compares the results of the first two compare circuits, the present invention avoids the need for the third compare circuit by comparing the first data word in a first compare circuit with at least one bit from the second data word. In a broad sense, the present invention embodies a semiconductor memory device having a plurality of memory cells, at least first and second output terminals, a control circuit, a test circuit and a data path or switch. The plurality of memory cells are operable to store data therein, and include first and second sub-arrays of memory cells. The first and second output terminals are coupled to the first and second sub-arrays, respectively. The control circuit is coupled to the first and second sub-arrays and is operable to transfer data from the first and second sub-arrays to the first and second output terminals, respectively. The test circuit is coupled to the control circuit and to the first and second sub-arrays. The test circuit, in response to a test mode signal from the control circuit, tests data written to the first sub-array and outputs a test signal based on the testing. The switch receives the test mode signal from the control circuit and couples the test circuit to the first output terminal in response thereto. As a result, the test signal can be provided to the first output terminal, while data stored in the second sub-array can be provided to the second output terminal during the testing. Additionally, the present invention embodies a method of testing a semiconductor memory device having at least first and second sub-arrays of memory cells. The method includes the steps of: (a) entering into a test mode; (b) writing data to the first and second sub-arrays; (c) testing the data written to the first sub-array; (d) providing a test signal indicating the results of me testing; (e) selectively providing the test signal to an output terminal when the memory device is in the test mode; and (f) reading the data from the second sub-array and providing the data to a second output terminal. Moreover, the present invention embodies a semiconductor memory device having a plurality of memory cells, a control circuit and a test circuit. The plurality of memory cells include first and second sets of sub-arrays of memory cells, each set having eight sub-arrays corresponding to a data word. The control circuit is coupled to the plurality of memory cells and is operable to write data to, and read data from, the plurality of memory cells. The test circuit has first and second compare circuits. The first compare circuit compares bits in a first data word written to the first set of sub- arrays to each other and outputs a pass value for a test signal if all of the bits in the first data word match. The second compare circuit compares bits in a second data word written to the second set of sub-arrays to each other and to at least one bit from the first data word, and outputs a pass value for the test signal if all of the bits of the second data word and the one bit from the first data word match. Brief Description of the Drawings Figure 1 is a block diagram showing a memory device having a test mode circuit according to the present invention. Figure 2 is a block diagram showing details of the test mode circuit and associated circuitry of the memory device of Figure 1. Figure 3 is schematic diagram of the test mode circuit and related circuitry of the memory device of Figure 1. Figure 4 is a block diagram of an alternative embodiment of the test mode circuit of the memory device of Figure 1. Figure 5 is a part isometric view, part block diagram of a testing station for testing a chip containing the memory device of Figure 1. Figure 6 is a block diagram of a computer system containing the memory device of Figure 1. Detailed Description of the Preferred Embodiment Figure 1 shows a block diagram of the preferred embodiment of a memory device 10 according to the present invention. The memory device 10 is a synchronous dynamic random access memory (SDRAM) device that includes as its central memory element left and right memory array banks 1 1A and 1 IB. Each of the memory arrays I IA and 1 IB includes a plurality of memory cells (not shown) arranged in rows and columns. A control logic circuit 12 controls the data transfer steps associated with a read or write access to the memory cells in the arrays 11 A and 1 IB. In one embodiment, each of the arrays 11 A and 1 IB has memory cells arranged in 512 rows by 256 columns by 32 bits. The memory device 10 has a width of 32 bits — meaning that each memory address (combined bank, row and column address) corresponds to a respective 32-bit memory cell in one of the arrays 1 1A and 1 IB, and a processor (not shown in Figure 1) operates on data elements of 32 bits each. A system clock (not shown) provides a CLK signal to the control circuit 12 of the memory device 10. Command signals are provided to the control circuit 12 and decoded by command decode circuitry 14. These signals are well known in the art, and include signals such as CKE (clock enable), CS (chip select), WE (write enable), RAS (row address strobe) and CAS (column address strobe). Distinct combinations of the various command signals constitute distinct processor commands. For example, the combination of CS low, WE high, RAS low and CAS high can represent an ACTIVE command. Examples of other well-known commands include READ, WRITE, NOP and PRECHARGE. Although the processor actually produces distinct command signals that in combination are registered and decoded as commands by the memory device 10, it is convenient to refer to these commands as being issued by the processor. The control circuit 12 sends control signals on control lines (not shown) to other parts of the memory device 10, corresponding to the processor- issued command. These control signals control the timing of access to the memory cells in arrays 1 1A and 1 IB. The memory device 10 is also provided with an address of the memory location to be accessed on a 10-bit wide address bus 15, including a bank address specified by address bit BA and a row or column address specified by address bits A0-A8. The address is input to an address register 16 which provides the address information to the control circuit 12, a row-address mux 17, and a column-address latch and decode circuit 20. In response to one or more control signals provided by the control circuit 12, the row-address mux 17 multiplexes row address information and provides it to one of two row-address latch and decode circuits 18A and 18B corresponding to the memory banks 11 A and 1 IB to be accessed. In response to one or more control signals provided by the control circuit 12, each of the row latch and decode circuits 18A and 18B takes a row address provided by the row- address mux 17 and activates a selected row of memory cells (not shown) in the memory array 11 A and 1 IB by selecting one of several row access lines 22 A and 22B, respectively. In response to one or more control signals provided by the control circuit 12, the column latch and decode circuit 20 takes a column address provided by the address register 16 and selects one of several column access lines 24A and 24B, each of which is coupled to one of the memory arrays I IA and 11B by one of two I/O interface circuits 26A and 26B, respectively. In response to one or more control signals provided by the control circuit 12, each of the I/O interface circuits 26 A and 26B selects the 32 memory cells corresponding to the column location in an activated row. The I/O interface circuits 26A and 26B include sense amplifiers which determine and amplify the logic state of the selected memory cells. The I/O interface circuits 26 A and 26B also include I/O circuits that gate data to 32 data output registers 28 and from a data input register 30, responsive to one or more control signals provided by the control circuit 12. The data registers 28 and 30 are connected to a 32-bit wide data bus 31 at DQ pads DQ0-DQ31 to transfer output data Q0-Q31 to a processor and input data D0-D31 from a processor, responsive to one or more control signals provided by the control circuit 12. The memory device 10 includes a refresh control circuit 32 which, responsive to one or more control signals provided by the control circuit 12, initiates regular and periodic activation of each of the rows of the memory cells in the arrays 11A and 1 IB for purposes of data refresh, as is well known in the art. In response to one or more control signals provided by the control circuit 12, a respective one of the I/O interface circuits A and B senses data stored in the memory cells of the refresh-activated row and rewrites values corresponding to the stored data in each of the memory cells. The memory device 10 is preferably formed on a single semiconductor substrate or die. Several output terminals or pads, electrically coupled to the address lines A0-A9, DQ lines DQ0-DQ31, and other lines, can be formed at the periphery of the die. The die is preferably encapsulated by a protective material to form a packaged chip, and several electrically conductive leads or pins, which are electrically coupled to the pads, extend from the chip (not shown in Figure 1). The memory device 10 also includes a test mode circuit 36 coupled to an internal data output bus 37 connecting the I/O interface circuits 26 A and 26B to the output register 28. The test mode circuit 36 is enabled by the control circuit 12 when the control circuit has received instructions, in the form of a special "address" of test key vector received by the control circuit 12 and indicating that the memory device is to be operated in a test mode. In particular, when the control circuit 12 is instructed to operate the memory device 10 in the speed test mode, the control circuit provides a test-enable signal TEST that enables the test mode circuit 36 to perform testing of the memory device. The test mode circuit 36 provides a comparison signal or test data flag to the data output registers 28. Referring to Figure 2, the left array 11 A is shown diagramatically as having two sub-arrays, sub-array I and sub-array II. The sub-arrays I and II receive incoming data, or outgoing data, through data paths 38 and 39, respectively. The data paths 38 and 39, shown in the block diagram of Figure 2, represent much of the circuitry coupled between the array 11 A and the data output registers 28, including the sense amplifiers, gating and I/O circuits 26A. The data paths 38 and 39 are used during both normal and test mode operation of the memory device 10, and therefore represent the standard delays inherent between all of the DQ lines and the arrays. For reasons of simplicity and clarity, only two sub-arrays for the array 11 A, and two DQ lines, DQ5 and DQ7, are shown in Figure 2. The memory device 10, however, has 32 sub-arrays and data paths corresponding to the 32 DQ lines DQ0-DQ31. As explained more fully below, the 32 sub-arrays are divided into four groups of 8 sub-arrays each, where each group outputs 1 bit to form an 8-bit word for each external address applied to the memory arrays I IA and 1 IB. Therefore, for each external address, four data words can be output. The test mode circuitry 36 compares bits of each word together (e.g., compares the even bits together and the odd bits together for each group of 8 sub-arrays), and outputs a test data flag to at least one DQ line for each group (e.g., DQ line DQ6). At least one other DQ line can then directly output the data to the same or an additional DQ pad to allow both the test data flags on line DQ6 and direct data to be output by the memory device 10. As shown in Figure 2, the test mode circuit 36 receives the outputs from the data paths 38 and 39, compares the data from the sub-arrays I and II, and outputs the test flag to a switch, such as 2: 1 multiplexer 40. The multiplexer 40 receives the TEST signal from the control circuit 12, inverts the signal through an inverter 41 and provides both the inverted and non-inverted test signal to a pair of cross-coupled passgates 42 and 43. The first passgate 42 receives the data from the data path 38, while the second passgate 43 receives the test data flag from the test mode circuit 36. If the mcoining TEST signal has a high value, then the test data flag from the test mode circuit 38 is passed by the passgate 43, while if the TEST signal has a low value, the data from the data path 38 is output by the passgate 42. Output circuitry 44 receives the output from the multiplexer 40, and is coupled at its output to the DQ pad DQ7. The output circuitry 44 includes output amplifiers, one of the data output registers 28, etc. The output circuitry 44 primarily includes standard circuitry required to provide data to, and receive data from, the DQ pad DQ7. Therefore, the output circuitry 44 contains standard delays inherent in such circuitry, and which exist along all DQ lines in the memory device 10. Except as explained below, essentially the only delays along the DQ7 path, which differ from other DQ paths, are caused by the multiplexer 40 and test mode circuitry 36. Output circuitry 45, substantially identical to output circuitry 44, is coupled between the DQ pad DQ5 and the data path 39. Importantly, data transferred to and from the sub-array II is transferred to and from standard circuitry between the array and the DQ pad DQ5, i.e., the data path 39 and the output circuitry 45. Therefore, an external testing device can write data to, and read data from, the sub-array II and accurately determine the speed of the memory device 10, since no additional circuitry is provided along the DQ5 line, namely the test mode circuitry 136 and multiplexer 40. Additionally, as noted above, the external testing device can also determine if the test mode circuitry 36 has determined an error by reading the test flag from the DQ7 line. Consequently, the external testing device can determine the speed of the memory device 10 and determine the data read from the memory device from the DQ5 line, while also determining whether the test mode circuitry 36 has detected an error from the DQ7 line. Since the test data flag from the test mode circuitry 36 is provided at the DQ7 line, the test data flag can be detected by the external testing device at a time subsequently to the reading of the data from the DQ5 line. Therefore, while the test mode circuitry 36 and multiplexer 40 provide additional gate delays for the test data flag output on the DQ7 line, such delays are irrelevant to determining the speed the memory device 10, since the speed can be deteπnined from one of the other DQ lines, e.g., DQ5. The test mode circuitry 36 writes the same data values to the memory arrays I IA and 1 IB, compares the bytes of data, and outputs a low value if all of the bits in the bytes match, as explained below. While rare, it is possible that all of the data read from one of the memory arrays in a given byte is erroneous, but that the test mode circuitry nevertheless outputs a low or match signal for the data test flag indicating that the memory device 10 passed the test. However, since the actual data written to the array is also output from the DQ5 pad, the external testing device can compare the data read on the DQ5 pad to the data which it previously wrote to the device to determine if the two match. If they do not match, then the testing device will recognize this error. Referring to Figure 3, the test mode circuit 36 includes four comparison circuits 200, 202, 204, and 206. (Only two of the four comparison circuits, comparison circuits 200 and 204, are shown in Figure 3.) Each of the compare circuits 200-206 includes a NAND gate 208 and a NOR gate 210 that each receive a signal DR representing data read from the memory arrays 1 1 A and 11B. The NAND gate 208 and the NOR gate 210 of the compare circuit 200 receives the data read from the sub-arrays associated with DQ lines DQO, DQ2, DQ4 and DQ6, while the NAND and NOR gates for the compare circuit 202 receives the data read from the sub-arrays associated with DQ lines DQ1, DQ3, DQ5 and DQ7. In other words, the compare circuit 200 compares the even bits of a first 8-bit data word (DQ0-DQ7) to each other, while the compare circuit 202 compares the odd bits from the first data word to each other. The NAND gate 208 and NOR gate 210 of the compare circuit 204 receives the data read from the sub-arrays associated with DQ lines DQ16, DQ18, DQ20, DQ22 and DQ6, while the NAND and NOR gates for the compare circuit 206 receives the data read from the sub-arrays associated with DQ lines DQ 17, DQ19, DQ21, DQ23 and DQ7. In other words, the compare circuit 204 compares the even bits of a third data word DQ 16-DQ23, while the compare circuit 206 compares the odd bits of this data word. While such comparison is divided by even and odd bits, other divisions of the words or DQ lines could be employed. The compare circuits 204 and 206 also compare 1 bit from the first data word (i.e., DQ6 and DQ7) to bits of the third data word to reduce area on the die, as explained below. The outputs of the NAND and NOR gates 208 and 210 are input to an exclusive OR gate 212 in each of the compare circuits 200-206. The NAND and NOR gates 208 and 210 both output a high value if all of their inputs are low, and alternatively both output a low value if all of their inputs are high. In response thereto, the exclusive OR gates 212 output a low value only if both of their inputs are either high or low. Therefore, if any of the inputs to the NAND or NOR gates are not equal to the remaining inputs, then the exclusive OR gates 212 will not output a low value. As is known, exclusive OR gates require a greater amount of surface area on the die than NAND or NOR gates. Typically, prior on-chip testing circuitry for semiconductor memory devices employed an additional comparison circuit that compared two compared data words. For example, such prior systems employed another comparison circuit, having at least one more exclusive OR gate, that would compare the outputs from the exclusive OR gates 212 to thereby compare the first data word with a third data word. Since during address compression testing, a single bit is written to all DQ lines DQ0-DQ31, the two compared words should match. The present invention, however, avoids the need for such an additional comparison by instead comparing 1 bit from the first byte to the entire third byte. Therefore, to reduce the number of exclusive OR gates, and thereby realize increased surface area on the die, the compare circuits 204 and 206 compare not only the even and odd bits of the third data word to each other, but also a bit from the first data word, e.g., DQ6 and DQ7, respectively. Therefore, the NAND and NOR gates 208 and 210 of the compare circuit 204 receive not only the even bits of the third byte, but also the last even bit from the first byte, i.e., DQ6. Similarly, the compare circuit 206 compares the even bits of the third byte, DQ 17, DQ 19, DQ21 and DQ23, to the most significant odd bit of the first byte, DQ7. If all of the bits written to DQ lines DQ16-DQ23 in the third data word, and the bits DQ6 and DQ7 written to the first data word all have the same value, then the NAND and NOR gates 208 and 210 of the compare circuits 204 and 206 all output low values to the exclusive OR gates 212. The additional comparison required in the prior art test circuits is avoided because all of the bits in the first word and all of the bits in the third word must be all O's or all l's for the exclusive OR gate 212 to output a low value. In theory, the exclusive OR gate 212 could output a low value if all of the bits in the first word were all logic O's and all of the bits in the third word were all logic l's (or vice versa). However, by comparing one bit from the first word to the bits of the third word, the exclusive OR gate 212 can output a low value only if the bits of the first and third words are all identical. As a result, this comparison effectively compares the third data word to the first data word. While not as robust as comparing each bit in the actual data words, the present invention provides a good substitute and reduces die area. If the data read signals DR output on the DQ lines DQ0-DQ7 and DQ16-DQ23 all match, then the compare circuits 200 and 202 output compare even and compare odd byte 0 match signals CEBO and COBO, and the compare circuits 204 and 206 output compare even and compare odd byte 2 signals CEB2 and COB2, all respectively. Output circuits 220 and 222, each having a 2-input NOR gate 224, receive therein the match signals CEBO and CEB2, and COBO and COB2, respectively. If either of the match signals has a high value, then the NOR gate 224 outputs a low signal which is inverted by an inverter 226 and output as compare even and compare odd signals CMPE and CMPO from the output circuits 220 and 222, respectively. The NOR gate 224 in the output circuits 220 and 222 can have a disable input 228 that receives a probe signal PROBE from the control circuit 12. If the control circuit 12 outputs a high PROBE signal to the disable input 228 of the NOR gate 224, then the output circuits 220, 222 are disabled, and do not output the compare signals CMPE and CMPO. The disable input 228 allows the control circuit 12 to effectively disable the test mode circuit 36 when the memory device 10 is being tested in die form by probes from an external testing device coupled to the die. When so tested, external comparison circuitry overrides the test mode circuitry 36. The passgates 43 in the multiplexing circuit 40 receive the compare signals CMPE and CMPO from the output circuits 220 and 222, respectively. As shown in more detail in Figure 3, the passgates 43 of Figure 2 preferably consists of a first passgate 43 A that receives the compare signal CMPE, and a second passgate 43B that receives the compare signal CMPO. A NAND gate 230 in the multiplexing circuit 40 receives the test enable signal TEST and a clock data output register signal CLKDOR*. When the memory device 10 enters compression mode testing, the control circuit 12 outputs a high value for the test enable signal TEST to enable the NAND gate 230. Additionally, during compression mode testing, the control circuit 12 outputs the CLKDOR* signal, which is a timing signal. As a result, when high values for the CLKDOR* and TEST signals are input to the NAND gate 230, the passgates 43A and 43B close to output the compare signals CMPE and CMPO as data test flags DT6 and DT7, respectively. When the memory device 10 is not in the compression test mode, then the control circuit 12 outputs a low value for the TEST signal, which causes the NAND gate 230 to always output a high value, which in turn causes the passgates 43A and 43B to open and not output the compare signals CMPE and CMPO. A second NAND gate 234 receives the inverted TEST signal and the CLKDOR* signal, and outputs a data read select signal DRSEL*. The NAND gate 234 is disabled to output a low value for the DRSEL* signal when the TEST signal is high. When the TEST signal is low, the NAND gate 234 is enabled so that it passes the clock data output register signal CLKDOR* in inverted form as a low data read select signal DRSEL*. The data read select signal DRSEL* is then inverted by an inverter 236 and the inverted and non- inverted DRSEL* signals are used to switch the passgate circuits 42A and 42B, respectively, of the multiplexer 40. The passgate circuits 42 A and 42B receive and pass the data signals read from the DQ lines DQ6 and DQ7, respectively, under normal operation of the device 10, when not in test mode and the TEST signal is low. The output of the passgate circuits 42A and 42B are input to data output registers 28A and 28B, respectively, where each data output register has a slave latch 242 and a master latch 240 formed by two inverters 244, 246 connected in a loop when a passgate 248 is closed by CLKDOR* being low. The slave latch 242 is of known construction. Under normal operation, when the TEST signal is low, then the low data read select signal DRSEL* closes the passgates 42 to allow the data read signals DR from the DQ lines to be input to the master latch 240 in the data output latch 28. During compression test mode, however, the TEST signal is high, which causes the data read select signal DRSEL* to open the passgates 42A and 42B, thereby allowing the data test flags DT6 and DT7 to be input to the master latch 240 from the passgates 43A and 43B. Therefore, the master latches 240 in the data output registers 28A and 28B have a selectable input—either the data read signals from the DQ lines are input therein if the passgates 42 are closed, or the data test flags are input therein if the passgates 43 are closed. As noted above, 32 data output registers 28 are employed, one for each of the 32 DQ lines DQ0-DQ31. Each of the data output registers has a master latch and a slave latch, similar to the output latches 28A and 28B of Figure 3. Four of the data output registers 28, however, have switchable inputs, two of which are shown in Figure 3 as data output registers 28A and 28B that receive either the data read from DQ lines DQ6 and DQ7, or the data test flags DT6 and DT7, respectively. The remaining two data output registers that have switchable inputs are coupled to DQ lines DQ23 and DQ24, as discussed below. During test mode operation, when the CLKDOR* signal is high, the passgates 43 A and 43 B are closed to allow the test flag signal DT6 and DT7 to be input to the data input A of the slave latch 242, through the first inverter 244 of the master latch 240. Since the TEST signal is high, the inverter 41 always inputs a low value to the NAND gate 234, thereby providing a constant high value for the data read select signal DRSEL*, which opens the passgate 42 and prohibits the data read signals DR from being output to the master latch 240. Thereafter, when the CLKDOR* signal is low, the passgate 248 opens, and the passgates 43A and 43B similarly open. As a result, the test data flags DT6 and DT7 are latched in the master latch 240, as it loops through the first and second inverters 244 and 246. The clock data output register signal CLKDOR* is input to the inverted clock input LAT* of the slave latch 242 (and simultaneously inverted by an inverter and input to the non-inverted clock input LAT). Therefore, when the CLKDOR* signal goes low, the data in the master latch 240 is clocked into the slave latch 242. As a result, the master latch 240 holds its output value for a clock cycle of the CLKDOR* signal, and inputs this value to the data input A of the slave latch 242, until the CLKDOR* signal transitions again. The slave latch 242 similarly holds its output value for a clock cycle of the CLKDOR* signal. As a result, additional output circuitry 252, such as output drivers and gates, can sense and amplify the data output from the slave latch 242 to the corresponding DQ pad (e.g., pads DQ6 and DQ7). The operation of the master and slave latches 240, 242 are identical for the data read signal DR input, when the passgates 43 A and 43 B are open and the passgate 42 is closed. In summary, the data output latch 28 has a typical master-slave latch configuration, except that it has a switchable input due to the passgates 42A, 42B, 43A and 43B. To summarize the operation of the test node circuitry 36, multiplexer 40 and output circuitry 44 in Figure 3, the compare circuits 200 and 202 compare the even and odd bits in the first data word DQ0-DQ7, respectively, while the compare circuits 204 and 206 compare the even and odd bits for the third data word DQ 16-DQ23, respectively. The compare circuits 200 and 202 output compare even and compare odd byte 0 signals CEBO and COBO, while the compare circuits 204 and 206 output the compare even and compare odd byte 2 signals CEB2 and COB2 to output circuits 220 and 222, respectively. The output circuits 220 and 222 output compare even and compare odd signals CMPE and CMPO to passgates 43 A and 43 B in the multiplexer 40. When in test mode, the TEST and CLKDOR* signals are high, which close the passgates 43 A and 43B, allowing the compare even and compare odd signals CMPE and CMPO to be input to the master latches 240 in data output registers 28A and 28B, as data test flags DT6 and DT7, all respectively. The slave latches 242 in the data output registers 28A and 28B clock out the data test flags DT6 and DT7 to DQ pads DQ6 and DQ7, at a rate based on the clock data output register signal CLKDOR*. If any of the even bits in the first or third data words do not match, then the compare circuits 200 and 204 and output circuit 220 provide a high value for the compare even signal CMPE, which is passed by the passgate 43 A as the data test flag DT6, and output on the DQ6 pad as a test data flag. Similarly, if any of the odd bits in the first or third data words do not match, then the compare circuits 202 and 206 and output circuit 222 provide a high value for the compare odd signal CMPO, which is passed by the passgate 43 B as the data test signal DT7, and output on the DQ7 pad as a test data flag. The present invention is generally described above for providing test data flags for the first and third data words stored in the left memory array 11 A. However, the present invention is equally applicable to the odd memory array for comparing the second and fourth data words DQ8-DQ15 and DQ24- DQ31 for the right memory array 1 IB. Therefore, while not shown, the test mode circuit 36 also includes four additional compare circuits, similar to compare circuits 200-206, that receive the data read from the right memory array 11B for the second data word on DQ lines DQ8-DQ15 and the fourth data word on DQ lines DQ24-DQ31. The second pair of compare circuits compare not only the even and odd bits of the second data word DQ8-DQ15, but also the least significant even and odd bits of the fourth data word, i.e., DQ24 and DQ25. Similarly, the test mode circuit 36 includes two additional output circuits, similar to output circuits 220 and 222, that similarly output compare signals CMPE and CMPO to multiplexing circuits 40 and data output registers 28. The test data flags for the even and odd bits for the second and fourth data words are output to DQ22 and DQ23. Referring to Figure 4, an alternative embodiment of the memory device 10 is shown as a memory device 200. The alternative embodiment is similar to the previously described memory device 10, and all elements are similarly numbered when of similar construction. Only the significant differences in the construction or operation are discussed in detail. Importantly, an error latch 202 receives and latches the compare even and compare odd signals CMPE and CMPO from the test mode circuitry 36. As a result, an external testing device can test the speed of the device from the DQ5 line, during one clock cycle, and then at some time thereafter, read the value stored in the error latch 202 from the DQ7 line. For example, during a first clock cycle, the external testing device can determine the speed of, and data from, a sub-array on the DQ7 line. Due to delays inherent in the test mode circuitry 36, error latch 202, and multiplexer 40, the testing device can then read the data latched in the error latch 202 at one or more clock cycles thereafter (e.g., two cycles thereafter). If a test data flag has been latched in the error latch 202, then the testing device can determine which memory cells in the memory array are defective by determining the addresses applied to the sub-array two clock cycles previously. To provide a second output for the error latch 202, a multiplexer 204 is coupled between the output of the error latch and any pad 206 of the device, such as an address or additional power supply pad. Therefore, when the device 200 is in its compression test mode, a low value for the TEST signal is applied to the multiplexer 204 to cause the error latch 202 to be coupled to the additional pad 206. As a result, the multiplexer 204 frees up a DQ pad which otherwise would be used to output the test data flag. While not shown, appropriate output drivers may be required between the multiplexer 204 and the additional pad 206 so as to provide an appropriately amplified output signal for the test data flag. Such output drivers, however, need not be as large as those typically employed with standard DQ pads. The present invention allows the memory device 10 to be tested when in its package chip form, to thereby be expediently tested. Figure 5 shows a testing station 50 for testing a plurality of packaged chips 60, each containing one or more of the memory devices 10 according to the present invention. The testing station includes a test board 52 releasably holding each of the chips 60 and providing electrical connections between chip leads 62 and external test circuitry 54. Automated pick and place machinery 56 can automatically manipulate and insert the chips 60 into the test board 52. Each of the input and output terminals of the memory device 10, such as the clock and command signal inputs, address inputs A0-A8, and data bus in/outputs DQ0-DQ31, are electrically coupled to some or all of the chip leads 62. The test circuitry 54 may include microprocessor or microcontroller circuitry, a system clock, and memory for perforating testing routines and storing the results thereof. The above-described testing of memory devices 10 can be performed by the testing station 50 during various testing processes, such as during the burn-in process, when the chips 60 are subject to elevated temperatures and operating voltages. The testing can be performed at different clock speeds to identify and sort the memory devices 10 according to their respective speed grades. Figure 6 is a block diagram of a computer system 70 that uses a memory device 10 according to the present invention. The computer system 70 includes computer circuitry 72 for performing such computer functions as executing software to perform desired calculations and tasks. Circuitry 72 typically contains a processor (not shown) and the memory device 10 as shown. One or more input devices 74, such as a keyboard or a pointing device, is coupled to the computer circuitry 72 via a bus 75 and allows an operator (not shown) to manually input data thereto. One or more output devices 76 is coupled to the computer circuitry 72 via the bus 75 to provide to the operator data generated by the circuitry. Examples of output devices 76 include a printer and a video display unit. One or more data storage devices 78 is coupled to the computer circuitry 72 via the bus 75 to store data on or retrieve data from external storage media (not shown). Examples of storage devices 78 and associated storage media include drives that accept hard and floppy disks, magnetic tape recorders, and compact-disc read only memory (CD-ROM) readers. It will be appreciated that, although embodiments of the invention have been described above for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, while the present invention has been described as being employed in an SDRAM device, the invention is equally applicable to other semiconductor memory devices. Furthermore, while an exemplary testing routine has been described above, various other testing routines can be employed under the present invention, such as check sum testing, cyclic redundancy checks, etc. Furthermore, the test flag and data can be applied to a single output terminal, rather than two separate terminals. Moreover, the present invention is applicable to other semiconductor devices requiring rapid testing of such devices. Accordingly, the invention is not limited except as by the appended claims.

Claims: Claims 1. A semiconductor memory device comprising: an array of memory cells operable to store data therein; a plurality of output terminals; a control circuit coupled to the array of memory cells and being operable to transfer data from at least one memory cell to one of the output terminals during a read operation; a test circuit coupled to the array of memory cells, the test circuit, in response to a test mode signal, testing data written to at least one memory cell of the array during the read operation and outputting a test signal based on the testing to one of the output terminals; and a data path coupling at least one of the memory cells being tested by the test circuit to one of the output terminals during the read operation. 2. The semiconductor memory device of claim 1 wherein the array of memory cells includes at least first and second sets of sub-arrays, each set having eight sub-arrays corresponding to a data word; and wherein the test circuit includes first and second compare circuits, the first compare circuit comparing bits in a first data word written to the first set of sub- arrays to each other and outputting a pass value for the test signal if all of the bits in the first data word match, and the second compare circuit comparing bits in a second data word written to the second set of sub-arrays to each other and to at least one bit from the first data word and outputting a pass value for the test signal if all of the bits of the second data word and the one bit from the first data word match. 3. The semiconductor memory device of claim 1 wherein the array of memory cells includes first and second sub-arrays, and wherein the test circuit tests data written to the first sub-array by comparing data written to the first sub-array to data written to the second sub-array during testing of the semiconductor memory device, and wherein the test circuit outputs a pass value for the test signal if the data written to the first sub-array corresponds to the data written to the second sub-array. 4. The semiconductor memory device of claim 1, further comprising a latch coupled between the test circuit and one of the output terminals, the latch storing the test signal and allowing the test signal to be read from the latch through the one output tera inal. 5. The semiconductor memory device of claim 1, further comprising a data output register having an input switch, a master latch and a slave latch, and wherein the array of memory cells includes first and second sub-arrays, the input switch being coupled to the first sub-array and electrically disconnecting the data output register from the first sub-array in response to the test mode signal; the master latch being coupled to the passgate and selectively receiving the test signal in response to the predetermined signal or the data stored in the first sub-array in the absence of the predetermined signal, and outputting the test signal or stored data in response thereto, respectively; and the slave latch being coupled between the master latch and at least one output terminal, wherein the slave latch provides the test signal or the stored data to the one output terminal. 6. The semiconductor memory device of claim 1 wherein the plurality of output terminals includes first and second output terminals, wherein a test circuit is coupled to the first output terminal to provide the test signal thereto, and wherein the second output terminal is coupled to the data path, and wherein the semiconductor memory device further comprises an additional output terminal and a passgate coupled between the test circuit and the additional output terminal, the passgate providing the test signal to the additional output terminal in response to the test mode signal. 7. The semiconductor memory device of claim 1, further comprising a 2: 1 multiplexing circuit coupled between the test circuit and one of the output terminals, wherein the test circuit includes at least one exclusive OR gate, and the wherein the array of memory cells are synchronous dynamic random access memory cells. 8. The semiconductor memory device of claim 1, further comprising a switch coupled between the test circuit and one of the output terminals to selectively permit the test circuit to provide the test signal to the one output terminal. 9. The semiconductor memory device of claim 1 wherein the plurality of output terminals includes first and second terminals, the first terminal being coupled to the test circuit to allow the test circuit to output the test signal thereto, and the second terminal being coupled to the data path. 10. The semiconductor memory device of claim 1, further comprising a switch coupled to the test circuit, data path, and a single output terminal, the switch selectively coupling the test circuit and the data path to the single output terminal. 11. A circuit device comprising: a plurality of circuit cells operable to store data therein, the plurality of circuit cells including first and second sub-arrays of circuit cells; a plurality of output terminals, including first and second output terminals coupled to the first and second sub-arrays, respectively; a control circuit coupled to the first and second sub-arrays and being operable to transfer data from the first and second sub-arrays to the first and second output terminals, respectively; a test circuit coupled to the control circuit and the first and second sub- arrays, the test circuit, in response to a predetermined signal, testing data written to the first sub-array and outputting a test signal based on the testing; and a switch coupled to the test circuit and one of the plurality of output terminals, the switch receiving the predetermined signal from the control circuit and coupling the test circuit to the one output teπninal in response thereto. 12. The circuit device of claim 11 wherein the plurality of circuit cells includes at least first and second sets of sub-arrays, each set having eight sub- arrays corresponding to a data word; and wherein the test circuit includes first and second compare circuits, the first compare circuit comparing bits in a first data word written to the first set of sub- arrays to each other and outputting a pass value for the test signal if all of the bits in the first data word match, and the second compare circuit comparing bits in a second data word written to the second set of sub-arrays to each other and to at least one bit from the first data word and outputting a pass value for the test signal if all of the bits of the second data word and the one bit from the first data word match. 13. The circuit device of claim 11 wherein the test circuit tests data written to the first sub-array by comparing data written to the first sub-array to data written to the second sub-array during testing of the circuit device, and wherein the test circuit outputs a pass value for the test signal if the data written to the first sub- array corresponds to the data written to the second sub-array. 14. The circuit device of claim 11, further comprising a latch coupled between the test circuit and the first output terminal, the latch storing the test signal and allowing the test signal to be read from the latch through the first output terrninal. 15. The circuit device of claim 11, further comprising a data output register having an input switch, a master latch and a slave latch, the input switch being coupled to the first sub-array and electrically disconnecting the data output register from the first sub-array in response to the predetermined signal; the master latch being coupled to the passgate and selectively receiving the test signal in response to the predetermined signal or the data stored in the first sub-array in the absence of the predeteimined signal, and outputting the test signal or stored data in response thereto, respectively; and the slave latch being coupled between the master latch and the first output terrninal, wherein the slave latch provides the test signal or the stored data to the first output terminal. 16. The circuit device of claim 11, further comprising an additional output terminal and a passgate coupled between the test circuit and the additional output teirninal, the passgate providing the test signal to the additional output terminal in response to the predetermined signal. 17. The circuit device of claim 11 wherein the switch is a 2: 1 multiplexing circuit, the test circuit includes at least one exclusive OR gate, and the wherein the plurality of circuit cells are synchronous dynamic random access circuit cells. 18. A semiconductor memory device comprising: a plurality of memory cells operable to store data therein, the plurality of memory cells including first and second sets of sub-arrays of memory cells, each set having eight sub-arrays corresponding to a data word; a control circuit coupled to the plurality of memory cells and being operable to write data to, and read data from, the plurality of memory cells; and a test circuit having first and second compare circuits, the first compare circuit comparing bits in a first data word written to the first set of sub-arrays to each other and outputting a pass value for a test signal if all of the bits in the first data word match, and the second compare circuit comparing bits in a second data word written to the second set of sub-arrays to each other and to at least one bit from the first data word and outputting a pass value for the test signal if all of the bits of the second data word and the one bit from the first data word match. 19. A computer system comprising: an input device; an output device; an address bus; a data bus; a set of control lines; a memory controller coupled to the address and data buses and to the set of control lines; a processor coupled to the input and output devices, to the address and data buses and to the set of control lines; and a memory device coupled to the memory controller through the address and data buses and the set of control lines, the memory device comprising: an array of memory cells operable to store data therein; a plurality of output terminals; a control circuit coupled to the array of memory cells and being operable to transfer data from at least one memory cell to one of the output terminals during a read operation; a test circuit coupled to the control circuit and array of memory cells, the test circuit, in response to a test mode signal, testing data written to at least one memory cell of the array during the read operation and outputting a test signal based on the testing to one of the output terminals; and a data path coupling at least one of the memory cells being tested by the test circuit to one of the output terminals during the read operation. 20. The computer system of claim 19, further comprising a latch coupled between the test circuit and the one output terrninal, the latch storing the test signal and allowing the test signal to be read from the latch through the one output terrninal. 21. The computer system of claim 19, further comprising: a data output register having an input switch, a master latch and a slave latch, wherein the array of memory cells includes first and second sub-arrays, the input switch being coupled to the first sub-array and electrically disconnecting the data output register from the first sub-array in response to the test mode signal; the master latch being coupled to the passgate and selectively receiving the test signal in response to the test mode signal or the data stored in the first sub- array in the absence of the test mode signal, and outputting the test signal or stored data in response thereto, respectively; and the slave latch being coupled to the master latch and the one output terrninal, wherein the slave latch provides the test signal or the stored data to the first output terminal. 22. A semiconductor device testing apparatus comprising: a die having a semiconductor circuit formed thereon, the semiconductor circuit having a plurality of memory cells operable to store data therein; a plurality of electrically conductive leads coupled to the semiconductor circuit on the die, the leads including address leads for receiving address signals , and wherein the plurality of memory cells are addressable by electrically conductive row and column lines based on an address word applied to the address leads; a control circuit coupled to the memory cells and being operable to transfer data from the memory cells to one of the first and second leads; a test circuit coupled to the control circuit and the memory cells, the test circuit testing data written to at least one memory cell and outputting a test signal based on the testing to one of the leads; a data path coupling at least one of the memory cells being tested by the test circuit to one of the leads during the read operation; an encapsulating material that encapsulates the die and at least a portion of the plurality of leads to form a packaged chip; testing circuitry for applying the address signals; and a socket for releasably holding the packaged chip and electrically coupling the plurality of leads with the testing circuitry. 23. The semiconductor device testing apparatus of claim 22, further comprising a pick and place apparatus for automatically inserting the packaged chip into the socket. 24. A method of testing a semiconductor memory device having an array of memory cells and at least one output terminal, the method comprising the steps of: entering into a test mode; writing data to at least one memory cell of the array; testing the data written to the memory cell; providing a test signal indicating the results of the testing; selectively providing the test signal to one of the output terminals when the memory device is in the test mode; and reading the data from the memory cell and providing the data to one of the output terminals. 25. The method of claim 24, further comprising the steps of: reading the data from one output terminal; and reading the test signal from the one output terminal at a time after the step of reading the data from the one output terminal. 26. The method of claim 24 wherein the memory device includes first and second sub-arrays of memory cells, and includes first and second output terminals respectively coupled thereto, wherein the step of selectively providing the test signal provides the test signal to the first output terminal, and wherein the step of reading the data provides the data to the second output terminal. 27. The method of claim 24, further comprising the steps of: reading the data from a first output terrninal; and reading the test signal from a second output teiminal at a time after the step of reading the data from the first output teirninal. 28. A method of testing a semiconductor memory device having first and second sets of memory cells, the method comprising the steps of: writing first and second sets of data to the first and second sets of memory cells; reading the first set of data from the first set of memory cells and comparing the first set of data with itself; reading the second set of data from the second set of memory cells and comparing the second set of data to itself and to at least one bit from the first set of data; and generating a test signal based on the reading steps.

Inventors: FULLER PAUL M, WRIGHT JEFFREY P, ZHENG HUA, FULLER, PAUL, M., WRIGHT, JEFFREY, P., ZHENG, HUA
Applicants: MICRON TECHNOLOGY INC, MICRON TECHNOLOGY, INC.
Application year: 1998
Main classification: G11C29
IPC codes: G01R31, G11C29