Abstract:
A semiconductor device includes a first terminal which receives a signal within a predetermined potential range in a first operation mode, and receives a potential higher above the predetermined potential range in a second operation mode, a high potential detection circuit which is connected to the first terminal, and detects the high potential to generate a high potential detection signal, a second terminal which receives a command signal, a latch circuit which latches the command signal supplied to the second terminal in response to the high potential detection signal, and a third terminal which resets the latch circuit in response to a signal within the predetermined potential range supplied from an exterior of the device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to semiconductor devices equipped with a test function, and particularly relates to a semiconductor device which receives a high potential signal at an external pin thereof that triggers a test mode. 
     2. Description of the Related Art 
     Manufactures of semiconductor devices need to check manufactured semiconductor devices before shipping them out in order to insure proper operations. To this end, semiconductor devices are provided with a special function for test purposes. In order to prevent users having purchased semiconductor devices from using a test mode, however, the detail of the test mode is generally not provided to the users. Further, the setting of a test mode is specially designed so as not to let users from accidentally engaging in the test mode. 
     For example, a test mode is selected by applying high potential signals to a plurality of external pins where such high potential signals are normally not used. Alternatively, a test mode is engaged by entering a test command. 
     Semiconductor devices of today have highly complex functions, and the number of test modes has been on the increase. The number of external pins to which high potential can be applied is limited, and, also, there is a limit to the number of test modes that can be represented by the combination of these high potential signals. Because of this, there are cases more often than not in which command inputs are used to select a test mode. In order to prevent users from accidentally engaging in a test mode, however, it is desirable to require the application of a high potential to a particular node as a prerequisite for entering into a test node even when command inputs are used. 
       FIG. 1  is a block diagram of a related-art control circuitry for controlling test modes. 
     In this example, a high potential VHH is applied to a R/B terminal  11 . With this high potential, a command is entered through I/O( 0 )-I/O(n) terminals  14  while a /WE terminal  12  is kept at the LOW level. This selects a desired test mode. 
     The I/O terminals  14  shown as I/O( 0 )-I/O(n) are input/output pins for exchanging data with an exterior of the device, and are connected to an input/output buffer  25 . Output signals IN( 0 )-IN(n) of this input/output buffer are supplied to a test command decoder  31 . When data is to be entered from the exterior, the I/O terminals  14  need to be set in a signal-input state. Setting of the state of the I/O terminals  14  is made by controlling a /OE terminal  13  that is used to indicate an output-enable state. In detail, the /OE terminal  13  is set to LOW to place the I/O terminals  14  in the signal input state. The /OE terminal  13  is connected to an input buffer  24 , which produces an output signal OE that is supplied to the input/output buffer  25  associated with the I/O terminals  14 , thereby controlling the setting of states of the I/O terminals  14 . 
     The /WE terminal  12  is a control pin that controls a command input. A command specified at the I/O terminals  14  is received during the period of /WE being LOW, and is latched at the time /WE changes to HIGH. The /WE terminal  12  is connected to an input buffer  23 , which produces an output signal WEB, which is supplied to the test command decoder  31 . 
     The R/B terminal  11  is an output pin for outputting a ready/busy signal indicative of whether the device is operating or not. The R/B terminal  11  outputs LOW during an operation, and outputs HIGH during a standby state. The LOW level is 0 V, and the HIGH level is equal to VCC that is a power supply potential of the device. The R/B terminal  11  is connected to a high potential detection circuit  22  in addition to an output buffer  21 . When the high potential VHH is applied to the R/B terminal  11 , the high potential detection circuit  22  produces an output signal RBH that is HIGH. This output signal RBH is supplied to the test command decoder  31 . 
     The test command decoder  31  receives the signals RBH, WEB, and IN( 0 )-IN(n). The signal RBH sets a latch circuit in a latch-ready condition where the latch circuit is provided in the test command decoder  31 . The signals IN( 0 )-IN(n) are stored in the latch circuit, and indicate a test mode through a particular combination thereof. The signal WEB opens a signal path through which the signals IN( 0 )-IN(n) are supplied to the latch circuit. 
       FIG. 2  is a timing chart showing the timing at which a high potential is applied and a test mode is set. 
     With reference to FIG.  1  and  FIG. 2 , the high potential VHH is applied to the R/B terminal  11 , thereby turning the signal RBH to HIGH that is supplied to the test command decoder  31 . In response, the latch circuit inside the test command decoder  31  is placed in a condition to be ready to latch. Further, the /OE terminal  13  is changed to LOW, and, at the same time, command signals are supplied to the I/O terminals  14 , thereby supplying signals IN( 0 )-IN(n) indicative of a particular command to the test command decoder  31 . While this is done, the /WE terminal  12  is set to LOW, thereby turning the signal WEB to HIGH that is supplied to the test command decoder  31 . In response, a signal path through which the signals IN( 0 )-IN(n) are supplied to the latch circuit is opened in the test command decoder  31 , resulting in the signals IN( 0 )-IN(n) being latched by the latch circuit. 
     The combination of the latched signals IN( 0 )-IN(n) in the latch circuit determines a test mode that is selected from a plurality of test modes. If five input/output terminals are used, for example, 32 different combinations can be specified in principle. Since the particular combination of the signals IN( 0 )-IN(n) that is comprised of all LOW inputs generates latch outputs that are the same as those of a normal and routine mode other than a test mode, the 31 remaining combinations are used to represent test modes. 
     In the configuration as described above that determines a test mode by use of both a command input and a high potential input, a change from one test mode to another test mode requires resetting of a current test mode. In order to do this, all the latches provided in the test command decoder  31  need to be reset to LOW before a next operation starts. 
     To this end, the R/B terminal  11  to which a high potential has been being applied is returned to the normal voltage VCC as shown in  FIG. 2 , thereby setting the signal RBH to LOW. This makes the latch circuit unable to latch, thereby resetting it. Thereafter, the high potential VHH is applied to the R/B terminal  11 , and a test command is entered, thereby carrying out an operation that switches from the normal mode to a test mode. 
     In general, changes of signal potentials can be made in an order of nanoseconds if the signal potentials are around normal potentials. In order to avoid malfunction and/or device destruction caused by overshooting, however, changes of signal potentials are made by taking time in an order of milliseconds if the signal potentials are at high potentials. Because of this, it takes time to switch test modes, resulting in a lengthy test time. 
     Accordingly, there is a need for a semiconductor device which can complete the switching of test modes in a short period of time. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a semiconductor device that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art. 
     Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a semiconductor device particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a semiconductor device according to the present invention includes a first terminal which receives a signal within a predetermined potential range in a first operation mode, and receives a potential higher above the predetermined potential range in a second operation mode, a high potential detection circuit which is connected to the first terminal, and detects the high potential to generate a high potential detection signal, a second terminal which receives a command signal, a latch circuit which latches the command signal supplied to the second terminal in response to the high potential detection signal, and a third terminal which resets the latch circuit in response to a signal within the predetermined potential range supplied from an exterior of the device. 
     In the semiconductor device described above, a function is provided that resets the latch circuit when the signal within the predetermined potential range is supplied from the exterior to the third terminal. This function makes it possible to reset a test mode without manipulating the high potential signal, thereby achieving a reduction in the time required for switching of test modes. Changes of signal levels at high potentials require an operation time of an order of milliseconds whereas changes of signal levels around normal potentials require an operation time of an order of nanoseconds. A significant reduction in the time required for switching of test modes can thus be achieved. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a related-art control circuitry for controlling test modes; 
         FIG. 2  is a timing chart showing the timing at which a high potential is applied and a test mode is set in the configuration of  FIG. 1 ; 
         FIG. 3  is a block diagram of a semiconductor device according to the present invention; 
         FIG. 4  is a block diagram showing a surrounding configuration around a test command decoder; 
         FIG. 5  is a timing chart showing the timing at which a high potential is applied and a test mode is set in the configuration of  FIG. 4 ; 
         FIG. 6  is a circuit diagram showing a circuit configuration of the test command decoder; and 
         FIG. 7  is a block diagram showing a variation of a surrounding configuration around the test command decoder. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
       FIG. 3  is a block diagram of a semiconductor device according to the present invention.  FIG. 3  shows a nonvolatile semiconductor memory device as an example of a semiconductor device. However, the present invention is applicable to semiconductor devices in general as long as they require a test operation and the setting of a test mode, and is not limited to application to a nonvolatile semiconductor memory device. 
     The nonvolatile semiconductor memory device of  FIG. 3  includes a R/B (ready/busy) terminal  11 , a /WE (write-enable) terminal  12 , an /OE (output-enable) terminal  13 , I/O (input/output) terminals  14 , a /RST (reset) terminal  15 , a CS (chip-enable) terminal  16 , address terminals  17 , an output buffer  21 , a high potential detection circuit  22 , an input buffer  23 , an input buffer  24 , an input/output circuit  25 , an input buffer  26 , an input buffer  27 , an address buffer  28 , a test command decoder  31 A, a control circuit  32 , an erase circuit  33 , a write circuit  34 , a read circuit  35 , an X decoder  36 , a Y decoder  37 , and a memory cell array  38 . 
     In the read operation, a /RST terminal  15 , a CS terminal  16 , and a /WE terminal  12  are set to HIGH, HIGH, and HIGH, respectively. In response, the control circuit  32  controls the read circuit  35  and so on, thereby performing a read operation. At this time, the /OE terminal  13  is set to LOW. 
     Address signals supplied from an exterior to the address terminals  17  are provided to the X decoder  36  and the Y decoder  37  via the address buffer  28 . The X decoder  36  decodes the address signals supplied thereto, resulting in data of a specified X address being read from the memory cell array  38 . The Y decoder  37  decodes the address signals supplied thereto, and selects data of a memory cell corresponding to a specified Y address among the retrieved data of the specified X address, followed by supplying the selected data to the read circuit  35 . The read circuit  35  compares the read data with data of a reference memory cell, so as to determined whether the read data is 0 or 1. The determination made is output to an exterior from the I/O terminals  14  via the input/output circuit  25 . 
     In the write operation, the /RST terminal  15  and the CS terminal  16  are both set to HIGH. With this setting being intact, a LOW pulse is supplied to the /WE terminal  12 , and, at the same time, a write command is supplied to the I/O terminals  14 . In response, the write circuit  34  and relating circuits operate under the control of the control circuit  32  to perform a write operation. At this time, the /OE terminal  13  is set to HIGH. 
     Address signals supplied from an exterior to the address terminals  17  are provided to the X decoder  36  and the Y decoder  37  via the address buffer  28 . The X decoder  36  and the Y decoder  37  decode the address signals supplied thereto, thereby selecting a memory cell of a specified X address and a specified Y address in the memory cell array  38 . The control circuit  32  controls the write circuit  34  to generate a bias that is necessary for the write operation. This bias is applied to the selected memory cell via the X decoder  36  and the Y decoder  37 , so that the write operation with respect to the selected memory cell is carried out. During the write operation, the R/B terminal  11  outputs a LOW level, indicating that the chip is in the operating state. 
     In the erase operation, both the /RST terminal  15  and the CS terminal  16  are set to HIGH. With this setting being intact, a LOW pulse is applied to the /WE terminal  12 , and, at the same time, an erase command is supplied to the I/O terminals  14 . In response, the erase circuit  33  and relating circuits operate under the control of the control circuit  32  to perform an erase operation. At this time, the /OE terminal  13  is set to HIGH. 
     Address signals supplied from an exterior to the address terminals  17  are provided to the X decoder  36  and the Y decoder  37  via the address buffer  28 . The X decoder  36  and the Y decoder  37  decode the address signals supplied thereto, thereby selecting a memory cell that is to be erased in the memory cell array  38 . The control circuit  32  controls the erase circuit  33  to generate a bias that is necessary for the erase operation. This bias is applied to the selected memory cell via the X decoder  36  and the Y decoder  37 , so that the erase operation with respect to the selected memory cell is carried out. During the erase operation, the R/B terminal  11  outputs a LOW level, indicating that the chip is in the operating state. 
     If it is desired to suspend the write or erase operation in the middle of operation, a LOW level is applied to the /RST terminal  15 . In response to the LOW input, the control circuit  32  suspends the operation of the erase circuit  33  or the write circuit  34 . 
     The nonvolatile semiconductor memory device of  FIG. 3  is provided with the test command decoder  31 A, which decodes a command input into the I/O terminals  14  so as to set the nonvolatile semiconductor memory device to a desired test mode. According to the test mode specified by the test command decoder  31 A, the control circuit  32  carries out a corresponding test operation. 
       FIG. 4  is a block diagram showing a surrounding configuration around the test command decoder  31 A. 
     The I/O terminals  14  are coupled to the input/output circuit  25 . The output signals IN( 0 )-IN(n) are supplied to the test command decoder  31 A. Status settings of the I/O terminals  14  are controlled by the /OE terminal  13  that is used to indicate an output-enable state. In detail, the /OE terminal  13  is set to LOW if the I/O terminals  14  are to be placed in the signal-inputting state. 
     The /WE terminal  12  is a control pin that controls a command input. A command specified at the I/O terminals  14  is received during the period of /WE being LOW, and is latched at the time /WE changes to HIGH. The /WE terminal  12  is connected to the input buffer  23 , which produces an output signal WEB, which is supplied to the test command decoder  31 A. 
     The R/B terminal  11  that outputs a ready/busy signal is connected to the high potential detection circuit  22  in addition to the output buffer  21 . When a high potential VHH is applied to the R/B terminal  11 , the high potential detection circuit  22  produces an output signal RBH that is HIGH. This output signal RBH is supplied to the test command decoder  31 A. 
     The /RST terminal  15  is a pin that receives a reset signal, and is connected to the input buffer  26 . A signal RSTB output from the input buffer  26  is supplied to the test command decoder  31 A. 
     As described above, the test command decoder  31 A receives the signals RSTB, RBH, WEB, and IN( 0 )-IN(n). The signal RBH sets a latch circuit in a latch-ready condition where the latch circuit is provided in the test command decoder  31 A. The signals IN( 0 )-IN(n) are stored in the latch circuit, and indicate a test mode through a particular combination thereof. The signal WEB opens a signal path through which the signals IN( 0 )-IN(n) are supplied to the latch circuit. Further, the signal RSTB serves to reset the latch circuit of the test command decoder  31 A. 
       FIG. 5  is a timing chart showing the timing at which a high potential is applied and a test mode is set.  FIG. 6  is a circuit diagram showing a circuit configuration of the test command decoder  31 A. 
     The test command decoder  31 A includes NMOS transistors  51 - 0  through  51 -n, an inverter  52 , a buffer  53 , inverters  54 - 0  through  54 -n, inverters  55 - 0  through  55 -n, NAND circuits  56 - 0  through  56 -n, and NOR circuits  57 - 1  through  57 -m. The NAND circuit  56 -i and the inverter  54 -i (i=1, 2, . . . , n) together form a latch circuit  41 -i (i=1, 2, . . . , n) by receiving the output of the other as an input each other. 
     With the signal RSTB being HIGH in response to HIGH at the /RST terminal  15 , the high potential VHH is applied to the R/B terminal  11  to change the signal RBH to HIGH. In response, the NAND circuit  56 -i operates as an inverter with respect to the input signal IN(i), so that the latch circuit  41 -i is placed in a latch-ready condition. The /OE terminal  13  is then turned to LOW, and command signals are supplied to the I/O terminals  14 , with LOW being applied to the /WE terminal  12 . In response, the NMOS transistor  51 -i situated at the input of the latch circuit  41 -i becomes conductive, so that the data at the I/O terminal  14  is latched by the latch circuit  41 -i. 
     The /WE terminal  12  is thereafter returned to HIGH. The NMOS transistor  51 -i becomes nonconductive in response, but the latched data remains to be held by the latch circuit  41 -i. Outputs of the latch circuits  41 - 0  through  41 -n are decoded by the NOR circuits  57 - 1  through  57 -m, which output decoded signals T 1  through Tm indicative of a selected test mode. The decoded signals T 1  through Tm are supplied to the control circuit  32  shown in FIG.  3 . 
     The combination of the latched signals IN( 0 )-IN(n) in the latch circuit determines a test mode that is selected from a plurality of test modes. If five input/output terminals are used, for example, 32 different combinations can be specified in principle. Since the particular combination of the signals I/O( 0 )-I/ 0 ( 4 ) that is comprised of all LOW inputs generates latch outputs that are the same as those of a normal and routine mode other than a test mode, the 31 remaining combinations are used to represent test modes. 
     In the present invention, the signal RSTB associated with the /RST terminal  15  is supplied to the test command decoder  31 A. The /RST terminal  15  is set to LOW during a test mode, thereby resetting the test mode. The /RST terminal  15  is normally kept at the HIGH level, but is changed to LOW when there is a need to reset the test mode in the middle of the test mode. In response, the RSTB signal changes to LOW, which forces the output of the NAND circuit  56 -i to be HIGH in the latch circuit  41 -i. As a result, the latch is reset. Since all the latch circuits are reset to LOW, no test mode is now selected, thereby the current test mode being reset. The /RST terminal  15  is thereafter returned to HIGH, followed by a test command being entered into the I/O terminals  14 , which initiates a change to a next test mode. 
     As shown in  FIG. 5 , the /RST terminal  15  to which the high potential VHH is applied is kept at the high potential level during the switching of test modes as described above. 
     In the present invention as described above, test modes can be switched without changing the high potential that is applied to a predetermined terminal of the semiconductor device during a test mode. This makes the time required for switching of test modes shorter. 
     When data is written in the nonvolatile semiconductor memory device during a test mode, for example, the device is set to the write mode to apply the write bias to memory cells, and, then, is set to the write verify mode in order to check whether the writing of data is sufficiently done. If the verify operation finds that the writing of data is insufficient, the write mode is again engaged to write data, which is followed by another write verify mode for performing a verify operation. Such write operation and write verify operation are repeated until sufficient data writing is achieved. If the entire memory cell array needs to be written, this operation needs to be repeated a large number of times. The same argument applies in the case of erase operations. In the semiconductor memory device of the present invention, the time required for switching of modes is shortened compared with the time required in the related-art semiconductor memory devices, thereby significantly reducing the test time. 
     The above embodiment is but an example provided for the purpose of explaining the present invention, and is not intended to be limiting in any sense. For example, although the R/B terminal was used as a terminal to receive a high potential, a CS terminal can be alternatively used for the same purpose as shown in FIG.  7 . The /RST terminal for reset instruction and the /WE terminal for write instruction used in the above embodiment do not have to be the terminals as described, but can be any terminal pins as long as they are not already assigned in the test mode. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 2001-320908 filed on Oct. 18, 2001, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.