Abstract:
Provided is a semiconductor device including a test mode circuit capable of changing the semiconductor device into a test mode with fewer malfunctions and without providing a test terminal. The semiconductor device includes a test circuit configured to compare data of a data input terminal and a data output terminal in synchronization with clock, and control whether or not to change the semiconductor device into a test mode in accordance with a result of the comparison.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-092787 filed on Apr. 25, 2013, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor devices including a test circuit, and more particularly relates to a test circuit to change a semiconductor device to a test mode. 
     2. Background Art 
     It is an effective technique for manufacturing of semiconductor devices to improve efficiency of a shipping test to keep the quality of the devices and reduce its manufacturing cost. As a method to improve the efficiency of the test, some ICs are equipped with a test-mode function that is used only for the test, aside from functions that are used by users. Such a test mode is equipped with unique functions other than functions that users require, such as a function to output an internal node state and a function to rewrite a memory IC by one operation, and so can improve the efficiency of the test. Such test-mode functions require a method to implement the mode in a state that is not available by users, and a configuration for changing has to be devised so as not to let a user change the device into the test mode erroneously. An available method to change the semiconductor device into a test mode includes implementing a test mode function using a test terminal (see Patent document 1, for example). 
     [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-67180 
     SUMMARY OF THE INVENTION 
     A method including such an additional test terminal, however, increases a terminal that is not necessary for a user, which is not suitable for a tendency for smaller-area devices in recent years. 
     Then, in order to solve the above problem, the present invention discloses a method to change a semiconductor device into a test mode that causes few malfunctions without adding a test terminal. 
     A semiconductor device including a test circuit of the present invention has the following configuration. 
     A semiconductor device includes: a plurality of data registers connected in series that temporarily holds instruction input from a data input terminal in synchronization with a clock signal; an instruction decoder that identifies whether data output from the plurality of data registers is a normal instruction or a test instruction, and when the data is a test instruction, outputs a test instruction signal; a comparator that compares instruction data and data of the data output terminal in synchronization with the clock signal, and outputs a detected signal thereof; a latch circuit that designates the detected signal output from the comparator as a set signal; and a logic circuit capable of selecting whether the test instruction signal is to be output or not in accordance with a signal output from the latch circuit. 
     According to the present invention, a semiconductor device including a test circuit is capable of changing its mode into a test mode with fewer malfunctions and without providing a test terminal. This can reduce the number of terminals, and so can reduce the area of a semiconductor device and the footprint thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a configuration of a semiconductor device including a test circuit of the present embodiment. 
         FIG. 2  shows a configuration of a semiconductor device including another exemplary test circuit. 
         FIG. 3  shows a signal waveform indicating a normal operation of the semiconductor device. 
         FIG. 4  shows a signal waveform when the mode is changed into a test mode by a test circuit of the present embodiment. 
         FIG. 5  shows a signal waveform when the mode is changed into a test mode by another exemplary test circuit of the present embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows the configuration of a semiconductor device including a test circuit of the present embodiment. 
     The semiconductor device of the present invention has a circuit including a clock input terminal  101 , a data input terminal  102 , a data output terminal  103 , a register group  105  made up of a plurality of data registers  104 , a comparator  106 , a latch  107 , an instruction decoder  108 , and an output buffer  112 . 
     The register group  105  includes the data registers  104  connected in series, and temporarily holds a value of instruction data SDI of the data input terminal  102  and outputs the same in synchronization with clock SCK that is input to the clock input terminal  101 . The instruction decoder  108  identifies a normal instruction and a test instruction, which are determined beforehand, based on a value of data output from the register group  105 . The comparator  106  compares instruction data SDI of the data input terminal  102  and data SDO of the data output terminal  103  in synchronization with the rising edge of the clock SCK. The latch  107  receives an output signal MIO of the comparator  106  as an input and outputs a test instruction disable signal D_TEST. The data output terminal  103  is provided with the output buffer  112 . 
     The following describes an operation of the semiconductor device including a test circuit of the present embodiment. 
       FIG. 3  shows a signal waveform indicating a normal operation of the semiconductor device. 
     Clock SCK is input to the clock input terminal  101 . Instruction data SDI as a normal instruction in synchronization with the clock SCK is input to the data input terminal  102 . The instruction data SDI is then output from the register group  105 , and is determined as a normal instruction by the instruction decoder  108 . Then, the semiconductor device determines its operation in accordance with the normal instruction. 
     In the normal operation, the data input terminal  102  and the data output terminal  103  are connected independently with each other or via a resistor, and so when the semiconductor device receives an instruction, values of the data input terminal and the data output terminal agree, or repeat disagreement and agreement. The comparator  106  compares, in synchronization with rising edge of the clock SCK, instruction data SDI of the data input terminal  102  and data SDO of the data output terminal  103 , and when the data agree, the comparator  106  sets the output signal MIO at a high level, for example. The latch  107  is set by the output signal MIO to output a test instruction disable signal D_TEST to a logic circuit  110  so as not to output a test instruction signal  111 . 
     As stated above, in the normal operation, instruction data SDI as a normal instruction that is input to the data input terminal  102  is decoded by the instruction decoder  108 , and is output as a normal instruction signal  109 . Then, since the logic circuit  110  does not output a test instruction signal  111 , the semiconductor device keeps the normal operation. 
       FIG. 4  shows a signal waveform when the mode is changed into a test mode by a test circuit of the present embodiment. 
     To change a semiconductor device into a test mode, instruction data SDI as a test instruction that is in synchronization with clock SCK is input to the data input terminal  102 , and data SDO that disagrees with the test instruction at the time of rising of the clock SCK is input to the data output terminal  103 . The comparator  106  compares data of the data input terminal and the data output terminal, and the disagreement state is always kept, and so no test instruction disable signal D_TEST is output. Then, the instruction decoder  108  determines the instruction data SDI as a test instruction, and then outputs a test instruction signal  111  from the logic circuit  110 , thus changing the semiconductor device into a test mode. 
     Alternatively, the comparator  106  of the test circuit of the present embodiment may be configured to compare instruction data SDI and data SDO, and when the data disagree, to set the output signal MIO at a high level, for example. The logic for other signals also is not limited especially to  FIG. 3  and  FIG. 4 , as long as their functions are implemented. 
       FIG. 2  shows the configuration of a semiconductor device including another exemplary test circuit. 
     The test circuit of  FIG. 2  includes an additional second comparator  206  and a latch  207  instead of the latch  107 . 
     The second comparator  206  compares instruction data SDI of the data input terminal  102  and data SDO of the data output terminal  103  at the time of falling of the clock SCK, and when the data disagree, the second comparator  206  sets an output signal MIO 2  at a high level. The latch  207  receives an output signal MIO 1  of the comparator  106  and the output signal MIO 2  of the second comparator  206  as an input, and if one of them is at a high level, the latch  207  outputs a test instruction disable signal D_TEST. 
       FIG. 5  shows a signal waveform when the mode is changed into a test mode by another exemplary test circuit of the present embodiment. 
     To change a semiconductor device into a test mode, a test instruction that is synchronization with clock SCK is input to the data input terminal  102 , and data SDO that disagrees with the test instruction at the time of rising of the clock SCK and that agrees with the test instruction at the time of falling of the clock SCK is input to the data output terminal  103 . The comparator  106  compares data of the data input terminal and the data output terminal, and since the disagreement state is always kept, no test instruction disable signal D_TEST is output. The comparator  206  compares data of the data input terminal and the data output terminal, and since the agreement state is always kept, no test instruction disable signal D_TEST is output. Therefore, the instruction decoder  108  determines the instruction data SDI as a test instruction, and then outputs a test instruction signal  111  from the logic circuit  110 , thus changing the semiconductor device into a test mode. 
     The semiconductor device including a test circuit of the present embodiment further can reduce malfunctions. 
     As stated above, according to the semiconductor device including a test circuit of the present embodiment, any new terminal is not required to change the semiconductor device into a test mode, and a method to change it into a test mode can achieve a semiconductor device that is reliable and with low possibility of malfunctions. 
     Conditions to set the output signal MIO 1  and the output signal MIO 2  by the comparator  106  and second comparator  206  of the test circuit of the present embodiment or the logic for other signals are not limited especially to  FIG. 5  as long as their functions are implemented.