INTEGRATED CIRCUIT WITH FUNCTION OF MONITORING AN INTERNAL SIGNAL

Abstract: An improved semiconductor integrated cicuit 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 automatic refreshing function. Integrated circuits have been widely used in various fields, and types of integrated circuits are very large in number. In order to check functions of integrated circuits, it has been practised that relationship between inputs - outputs are observed. This technique is effective for integrated circuit 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 elapse 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 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 of the holding time is employed in evaluation in order to take the safety side. In this point of view, if it is possible to observe the internal signal for controlling 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 additonal 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 function of monitoring an internal signal. It is another object of the present invention to provide a semiconductor memory device of self-refreshing type with an improved function of monitoring self refresh. The integrated circuit according to the invention is defined in claim 1. 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 operation of the embodiment of Fig. 1; Fig. 3 is a schematic circuit diagram showing second embodiment of the invention; and Fig. 4 is a schematic circuit diagram showing third embodiment. 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 known multi-addressing type in which row address signals and column address signals are taken in via set of address input terminals A₁ to A n 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  L for enabling a row address inverter buffer 118, a timing signal  D for enabling a row decoder 116 to select one of rows of the matrix 101 and a timing signal  D 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  R and generates a timing signal  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  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 practice 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 R₁ to R₃ when RFSH is kept at a low level, thereby to introduce an internal refresh mode to the memory. In this instance, the control signal R₁ 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 R₂ enables the auto refresh timing generator 112 so that the generator 112 produces a refresh timing signal  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  Rb which is applied to the refresh address counter 113 thereby to increment the content of the counter 113. In response to the signal  Ra , the circuit 114 controls the RAS clock generator 115 so that the generator 115 generates the timing signals  L ,  D and  R . Thus, the row decoder 116 selects one of 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  R thereby to refresh the memory cells on the selected row. Thereafter, the address content of the address counter 113 in response to the signal  Rb . The above sequence of operations are repeated as long as the RFSH is kept at a low level. The further detailed feature of this memory is disclosed in United States Patent Specification 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  R and a source coupled to a power voltage terminal V CC and a field effect transistor Q₁₁ having a gate 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 is explained. RFSH is turned to a low (GND) level from a high level at a time point t₁, and the refresh timer 109 generates the signals R₁ and R₂ at a time point t₂ so that an internal refresh mode is introduced in the memory. In response to this, the timing generator 112 generates the signal  Ra at a time point t₃ so that the RAS clock generator 115 generates the signals  D and  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. While, the terminal WE is raised to the voltage (V CC + α) which is larger than the sum of the voltage V CC and a threshold voltage of the transistor Q₁₁, from t₂. In this instance, the memory does not operate in a write cycle, there is affection on a write clock generator 107. 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  R substantially at t₃. After the refresh conducted by the signal  R , the timing generator 112 generates the signal  Rb to increment the content of the refresh address counter 112 for preparation of a subsequent refresh cycle, at a time point t₄. After a refresh cycle period T F has elapsed from t₃, the timing generator 112 generates the signal  RA and the clock generator 115 generates the signals  D and  R , so that memory cells on the different row are refreshed. As long as the RFSH is maintained at a low level, the above refresh cycle is repeatedly with a refresh cycle T F which is solely determined by the timing generator 112 and cannot be controlled from the outside. As illustrated, the above refresh cycle T F 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  R to the gate of the transistor Q₁₂, the signal  D or  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 other 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 V CC and the gate of the transistor Q₁₂ with a gate coupled to V CC . The internal signal  R is applied to the gate of the transistor Q₁₂ via the capacitor C₂₁. When the internal signal  R is at a low level, the gate of Q₁₂ is at a potential of (V CC - V TH ) in which V TH is the threshold voltage of the transistor Q₁₃. Therefore, Q₁₂ assumes a non-conductive state. When the internal signal  R is turned to a high level, the gate of Q₁₂ is raised above (V CC + V TH ) by a known boot-strap effect, the transistor Q₁₂ assumes a conductive state. Therefore, when the voltage of (V CC +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 V CC , two address input terminals A i and A j are connected to the transistors Q₁₁ and Q₁₂. In this embodiment, by applying a voltage more than the sum of threshold voltage of the transistor Q₁₁, Q₂₁ to Q₂₃ and Q₁₂ between the terminals A i and A j , a detection of the internal signal  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 applicable to integrated circuits other than memories.

Claims: A semiconductor integrated circuit operable with a power voltage (Vcc) and a ground voltage (GND) and comprising a semiconductor chip (1), first (109, 112, 114, 115), second (102, 105) and third (106) circuit means formed on the semiconductor chip, a plurality of external terminals including a power voltage terminal receiving said power voltage (Vcc), a first input terminal receiving a first signal ( RESH ), a second input terminal receiving a second signal ( WE ) and an output terminal (DOUT), and fourth circuit means (107) responsive to said second signal ( WE ) for controlling said second and third circuit means, said first circuit means receiving the first signal ( RESH ) and generating an internal signal ( R ), said second circuit means receiving said internal signal and having an output end (DATA OUT) coupled to said output terminal, said fourth circuit means enabling said second circuit means and disenabling said third circuit means when a level of said second signal ( WE ) is close to said power voltage and enabling said third circuit means and disenabling said second circuit means when the level of said second signal is close to said ground voltage, characterized in that said semiconductor integrated circuit further comprises a detection circuit (10) for detecting said internal signal, said detection circuit being coupled between said power voltage terminal (Vcc) and said second input terminal ( WE ) and including a first transistor (Q12) having a control electrode, means for applying said internal signal to said control electrode, voltage switching means (Q11, Q21-Q23), said voltage switching means assuming a conductive state when a voltage of a predetermined potential or more is applied thereto, and means for connecting said first transistor and said voltage switching means in series between the power voltage terminal and said second input terminal thereby to allow said voltage switching means to assume conductive state when said second input terminal is supplied with a voltage which is higher than said power voltage by said predetermined potential or more. The integrated circuit according to claim 1, char acterized in that said integrated circuit includes a memory circuit and said first circuit means includes a refresh control circuit for performing refresh operation of said memory. The integrated circuit according to claim 1, characterized in that said applying means includes a capacitor (C21) having a first end receiving said internal signal and a second end coupled to the control electrode of said first transistor.

Inventors: WATANABE, HIROSHI
Applicants: NEC CORPORATION
Application year: 1985
Main classification: G11C7
IPC codes: G11C29, G11C11, G11C7