Abstract:
An improved semiconductor integrated circuit having a circuit for monitoring an internal signal which is not taken out from a chip is disclosed. The monitoring circuit includes a transistor having a control electrode to which the internal signal is applied, and a voltage switching element connected in series with the transistor between two external terminals. The voltage switching element is rendered conductive when a voltage of a predetermined value or more is impressed between the two external terminals.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to integrated circuits and more particularly to integrated circuit memory devices having an automatic refreshing function. 
     Integrated circuits have been widely used in various fields, and there are many types of integrated circuits. 
     In order to check functions of integrated circuits, it has been practiced that relationships between inputs - outputs are observed. This technique is effective for integrated circuits having combination logic circuits such as AND, NOR gates since an input to the integrated circuit and its relevant output therefrom can be obtained through a small lapse of time or almost at the same time. However, for integrated circuits having sequential logic circuits such as shift registers, and memory circuits, an output is obtained after a predetermined time has elapsed from the application of an input. Therefore, it has been difficult to check the function of such integrated circuits by observing input-output characteristics. This problem is very typical in an integrated circuit memory (IC memory) having an automatic or internal refreshing function in which a refresh operation for memory cells is internally performed repeatedly without receiving any refresh address information and refresh timing signals. The above type IC memory has a refresh timer circuit and an auto-refresh timing signal generator. When a refresh control signal applied to the refresh timer circuit from the outside is kept at an active level. the refresh timer circuit produces an internal refresh command signal intermittently and the auto-refresh timing signal generator produces a refresh timing signal for allowing an internal refresh operation in response to the internal command signal. In this memory, as long as the refresh control signal is kept active, refresh of memory cells is sequentially conducted without necessity of control from the outside. Heretofore, a long period of time has been required to evaluate a data-holding function of memory cells in such memory because a holding time of memory cells is generally 10 seconds or more and generally two to three times the holding time is employed in evaluation for the sake of safety. In this point of view, if it is possible to observe the internal signal for controlling the refresh operation directly, evaluation of memories would be conducted effectively. However, the number of pins of such memories has been limited to a certain number, e.g. 16, according to the commercial specification or standard, and therefore there has been no allowance to provide an additional pin for observing the internal signal. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide an improved integrated circuit with the function of monitoring an internal signal. 
     It is another object of the present invention to provide a semiconductor memory device of the self-refreshing type with an improved function of monitoring self refresh. 
     The integrated circuit according to the invention is of the type having a semiconductor chip, first and second functional circuits formed on the chip, the first functional circuit generating an internal signal in response to at least one input signal, the second functional circuit receiving the internal signal and generating an output to be driven outside the chip, and is featured in that a detection circuit for detecting the internal signal is provided on the chip. The detection circuit includes a transistor adapted to receive the internal signal at a control electrode, and a voltage switching element which assumes a conductive stage when a voltage of a predetermined value or more is impressed thereacross. The transistor and the voltage switching elements are connected in series between two of the external terminals. 
     By applying a voltage of the predetermined value or more between the above two terminals, one voltage switching element is rendered conductive, so that a current corresponding to the internal signal flows through the series connection of the transistor and the voltage switching element. Thus, the state of the internal signal can be detected by observing the current flowing through the series connection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing a first embodiment of the invention; 
     FIG. 2 is a diagram showing the operation of the embodiment of FIG. 1; 
     FIG. 3 is a schematic circuit diagram showing a second embodiment of the invention; and 
     FIG. 4 is a schematic circuit diagram showing a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a dynamic memory to which the present invention is applied is described. 
     The memory device formed on a semiconductor chip 1 includes a memory cell matrix 101 of dynamic type memory cells arranged in rows and columns, a circuit block 102 of sense amplifiers and data I/O gates. The memory is a known multi-addressing type in which row address signals and column address signals are taken in via a set of address input terminals Ai to An in response to active levels of a row address strobe RAS and a column address strobe CAS, respectively. A RAS input buffer control logic circuit 114 receives RAS and drives a RAS clock generator 115 which generates a timing signal φ C  for enabling a row address inverter buffer 118, a timing signal φ D  for enabling a row decoder 116 to select one of the rows of the matrix 101 and a timing signal φ R  for enabling sense amplifiers to refresh contents of the memory cells on the selected row. A CAS clock generator 108 receives CAS and the timing signal φ R  and generates a timing signal φ C  for enabling a column address inverter buffer 104, a column decoder 103, data I/O gates, a write clock generator 107, a data input buffer 106 and a data output buffer 105, in a known predetermined sequence. Although the signal φ C  is representatively illustrated as a single timing signal, a plurality of sequential signals are employed for the above blocks 104, 103, 107, 106 and 105 in practical case. 
     In order to conduct internal refresh operations which are performed without receiving any address signals, RAS and CAS, the memory is provided with a refresh control timer 109, an auto refreshing timing generator 112, a refresh address counter 113 and a multiplexer 117. The refresh timer 109 receives a refresh control signal RFSH, and generates control signals R 1  to R 3  when RFSH is kept at a low level, thereby to introduce an internal refresh mode to the memory. In this instance, the control signal R 1  controls the multiplexer 117 so that the multiplexer connects parallel outputs RC of the address counter 113 to the row decoder 116 in place of the output of the row address inverter buffer 118. The control signal R 2  enables the auto refresh timing generator 112 so that the generator 112 produces a refresh timing signal φ Ra  at the beginning of each refresh cycle which is applied to the RAS input buffer control logic circuit 114 and a refresh address incrementing signal φ Rb  which is applied to the refresh address counter 113 thereby to increment the content of the counter 113. In response to the signal φ Ra , the circuit 114 controls the RAS  clock generator 115 so that the generator 115 generates the timing signals φ C , φ D  and φ R . Thus, the row decoder 116 selects one of the rows of the matrix 101 based on the content of the refresh address counter 113 and the sense amplifiers 102 are enabled in response to the timing signal φ R  thereby to refresh the memory cells on the selected row. Thereafter, the address content of the address counter 113 is incremented in response to the signal φ Rb . The above sequence of operations is repeated as long as the RFSH   is kept at a low level. The further detailed feature of this memory is disclosed in U.S. Pat. No. 4,334,295 issued to Nagami. 
     In this memory, it is important to detect the cycle time of internal refresh operation in order to evaluate data holding function of the memory. 
     In this point of view, a refresh detecting circuit 10 is provided on the semiconductor chip 1 according to the present invention. The refresh detection circuit 10 includes a field effect transistor Q12 having a gate receiving the signal φ R  and a source coupled to a power voltage terminal Vcc and a field effect transistor Q11 having a gat and a drain commonly connected to a write control terminal WE which assumes a high level during a read cycle and a refresh cycle and a low level during a write cycle, and a source connected to a drain of the transistor Q12. 
     Referring to FIG. 2, the internal refresh operations of the memory of FIG. 1 are explained. 
     RFSH   is turned to a low (GND) level from a high level at a time point t 1 , and the refresh timer 109 generates the signals R 1  and R 2  at a time point t 2  so that an internal refresh mode is introduced in the memory. In response to this, the timing generator 112 generates the signal φ Ra  at a time point t 3  so that the RAS clock generator 115 generates the signals φ D  and φ R  so that the output RC of the refresh address counter 113 drives the row decoder 116 so as to select one row of the matrix and contents of memory cells on the selected row are refreshed by the sense amplifiers. Meanwhile, the terminal WE is raised to the voltage (Vcc+α) which is larger than the sum of the voltage VCC and a threshold voltage of the transistor Q11, from t 2 . In this instance, the memory does not operate in a write cycle. A write clock generator 107 recognizes the above potential at the terminal WE as an inactive level which does not indicate a write operation, and the above high voltage at WE does not cause any conflict between the operation of the generator 107 and the internal refresh operation. Accordingly, the transistor Q11 assumes a conductive state so that a detection current Idet flows from the terminal WE to the voltage terminal in response to the activation of the signal φ R  substantially at t 3 . After the refresh conducted by the signal φ R , the timing generator 112 generates the signal φ Rb  to increment the content of the refresh address counter 113 for preparation of a subsequent refresh cycle, at a time point t 4 . After a refresh cycle period TF has elapsed from t 3 , the timing generator 112 generates the signal φ Ra  and the clock generator 115 generates the signals φ D  and φ R , so that memory cells on the different rows are refreshed. As long as the RFSH is maintained at a low level, the above refresh cycle is repeatedly carried out with a refresh cycle TF which is solely determined by the timing generator 112 and cannot be controlled from the outside. As illustrated, the above refresh cycle TF can be observed by the current Idet flowing through the terminal WE according to the invention. 
     In this embodiment, in place of applying the signal φ R  to the gate of the transistor Q12, the signal φ D  or φ Ra  may be applied to the gate of the transistor Q12. Also, instead of the terminal WE, the gate and drain of the transistor Q11 may be connected to another terminal which is not used in the refresh cycle. 
     FIG. 3 shows another embodiment of the refresh detecting circuit 10. In this embodiment, a field effect transistor Q13 and a boot capacitor C21 are further provided to the circuit 10 of FIG. 1. Namely, a drain-source path of the transistor Q13 is coupled between the power voltage terminal Vcc and the gate of the transistor Q12 with a gate coupled to Vcc. The internal signal φ R  is applied to the gate of the transistor Q12 via the capacitor C21. When the internal signal φ R  is at a low level, the gate of Q22 is at a potential of (Vcc-Vth) in which Vth is at the threshold voltage of the transistor Q12. Therefore, Q12 assumes a non-conductive state. When the internal signal φ R  is turned to a high level, the gate of Q12 is raised above (Vcc+Vth) by a known boot-strap effect, the transistor Q12 assumes a conductive state. Therefore, when the voltage of (Vcc+α) is applied to the terminal WE, a detection current Idet flows from the terminal WE. 
     FIG. 4 shows a third embodiment of the invention. In this embodiment, diode-connected field effect transistors Q21 to Q23 are inserted in series between the source of the transistor Q11 and the drain of the transistor Q12. Furthermore, in place of the terminals WE and VCC, two address input terminals Ai and Aj are connected to the transistors Q11 and Q12. In this embodiment, by applying a voltage more than the sum of threshold voltage of the transistors Q11, Q21 to Q23 and Q12 between the terminals Ai and Aj, a detection of the internal signal φ R  is performed. 
     As described above, the detection of an internal signal is conducted effectively according to the invention. It should be noted that the present invention is not limited to the above embodiment. For example, the present invention is applicable to integrated circuits other than memories.