Semiconductor memory device having an address transition detection circuit

Abstract: A signal transition detection circuit comprises a decoder circuit (1) for decoding a plurality of signals, delay circuit means (2) having a rise time period and a fall time period which are different from each other, and a logic circuit (3). The logic circuit (3) performs a logic operation upon the outputs of the delay circuit means (2) to generate a pulse signal (ATD) for indicating at least one transition of the signals.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device having an address transition detection (ATD) circuit. 2. Description of the Related Art In a semiconductor memory device such as a mask ROM, a static RAM, a dynamic RAM, and the like, when an address transition occurs, a reset signal is generated to reset the internal circuit. For this purpose, there has been used an address transition detection circuit, i.e., a signal transition detection circuit. One such circuit, which corresponds to the preamble of claim 1, is described in patent document EP-A-0 096 421. In the prior art signal transition circuit which will be later explained, the occupied area is relatively large, which is disadvantageous from the viewpoint of the degree of integration. SUMMARY OF THE INVENTION An object of the present invention is, therefore, to provide a signal transition detection circuit which is small in size. According to the present invention, the signal transition detection circuit meets the above objects by virtue of a semiconductor memory device that employs a signal transition detection circuit as defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more clearly understood from the description as set forth below with reference to the accompanying drawings, wherein: Fig. 1 is a circuit diagram illustrating a first prior art signal transition detection circuit; Figs. 2A through 2H are timing diagrams showing the operation of the circuit of Fig. 1; Fig. 3 is a circuit diagram illustrating a second prior art signal transition detection circuit; Figs. 4A, 4B, and 4C are timing diagrams showing the operation of the circuit of Fig. 3; Fig. 5 is a circuit diagram illustrating an embodiment of the signal transition detection circuit according to the present invention; Fig. 6 is a detailed circuit diagram of the circuit of Fig. 5; Figs. 7 and 8 are circuit diagrams of the delay circuit of Fig. 6; Figs. 9A through 9G are timing diagrams showing the operation of the circuit of Fig. 6; and Figs. 10 and 11 are circuit diagrams illustrating semiconductor memory devices to which the signal transition detection circuit is applied. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before the description of the present invention, prior art signal transition detection circuits will be explained with reference to Figs. 1 through 4. In Fig. 1, which illustrates a prior art signal transition detection circuit (see: Fig. 4 of Unexamined Japanese Patent Publication No. 61-151894), a delay circuit 11 and an AND circuit 12 are provided for detecting a rise in an address signal A i , and an inverter 13, a delay circuit 14, and an AND circuit 15 are provided for detecting a fall in the signal A i . Reference numeral 16 designates an OR circuit, and 17 a NAND circuit. The operation of the circuit of Fig. 1 will be explained with reference to Fig. 2. When the address signal A i is changed as shown in Fig. 2A, a delay signal DA i which is an output of the delay circuit 11 is obtained as shown in Fig. 2B. Therefore, the output S₁ of the AND circuit 12 is changed as shown in Fig. 2C. Simultaneously, since the address signal A i is changed as shown in Fig. 2D, a delay signal DA i , which is an output of the delay circuit 14, is obtained as shown in Fig. 2E. Therefore, the output S₂ of the AND circuit 15 is changed as shown in Fig. 2F. As a result, the output S₃ of the OR circuit 16 is changed as shown in Fig. 2G, and therefore, an ATD pulse, which is output from the NAND circuit 17, is changed as shown in Fig. 2H. In Fig. 3, which illustrates another prior art signal transition detection circuit (see Unexamined Japanese Patent Publication No. 61-170989), there are provided a delay circuit 31, an exclusive OR circuit 32, and an OR circuit 33, and therefore, the exclusive OR circuit 32 detects both the rise and the fall in the signal A i as shown in Figs. 4A, 4B, and 4C. In the above-mentioned prior art signal transition detection circuits, however, two series circuits (Fig. 1) for the detection of the rise and the fall are required, or an exclusive OR circuit (Fig. 3) having a large number of elements is required, thereby increasing the occupied area, which is disadvantageous from the viewpoint of integration. In Fig. 5, which illustrates an embodiment of the present invention, reference 1 designates a decoder circuit for decoding n address signals A₀( A ₀), A₁( A ₁), ..., A n-1 ( A n-1 ) to generate 2 n decoding signals. For example, in a row address decoder for a semiconductor memory device, such decoding signals select one word line from 2 n word lines. Connected to the decoder circuit 1 are delay circuit means 2 having rise and fall characteristics which are different from each other. For example, if the high and low levels of a decoding signal of the decoder circuit 1 are a selection level and a non-selection level, respectively, the rise time period of delay circuit means 2 is made larger than the fall time period thereof. A logic circuit 3 performs a logic operation upon the outputs of delay circuit means 2 to generate an ATD signal for indicating at least one transition of the signals A₀( A ₀); A₁( A ₁), ..., and A n-1 ( A n-1 ) In the above-mentioned configuration, when any one of the address signals A₀ , A₁ , ..., A n-1 is changed, all of the outputs (decoding signals) of delay circuit means 2 are in a non-selected state for a predetermined time period. The logic circuit 3 detects a change of the address signals by detecting such a non-selected state. In Fig. 6, which is a detailed circuit diagram of the circuit of Fig. 5, the decoder circuit 1 is comprised of 2 n NOR circuits 1₀ , 1₁ , ..., thereby selecting one word line from 2 n word lines. That is, if n = 2, the address signals are defined by A₀ and A₁ , and the NOR circuits are defined by 1₀ , 1₁ , 1₂ , and 1₃ In this case, if (A₀ , A₁) = (0, 0), the connections for the NOR circuits are provided so that the output of the NOR circuit l0 is made high. If (A₀ , A₁) = (1, 0), the connections for the NOR circuits are provided so that the output of the NOR circuit 1₁ is made high. If (A₀ , A₁) = (0, 1), the connections for the NOR circuits are provided so that the output of the NOR circuit 1₂ is made high. If (A₀ , A₁) = (1, 1), the connections for the NOR circuits are provided so that the output of the NOR circuit 1₂ is made high. Delay circuit means 2 comprise delay circuits 2₀ , 2₁ , ... connected to the NOR circuits 1₀ , 1₁ , ..., and the outputs thereof serve as decoding signals and are supplied to the logic circuit 3. In the delay circuits 2₀ , 2₁ , ..., as explained above, the rise time period T r is different from the fall time period T f For example, if the rise time period is longer than the fall time period, the delay circuit 2₀ can be comprised of a transistor Q₁ as a capacitor; transistors Q₂ and Q₃ as an inverter; transistors Q₄ and Q₅ as an inverter; and a transistor Q₆ as a capacitor; as illustrated in Fig. 7. Note that the conductance g m of the P-channel transistor Q₂ is larger than that of the N-channel transistor Q₃ , and the gate capacity of the P-channel transistor Q₂ is smaller than that of the N-channel transistor Q₃. Contrary to this, the conductance g m of the P-channel transistor Q₄ is larger than that of the N-channel transistor Q₅ , and the gate capacity of the P-channel transistor Q₄ is smaller than that of the N-channel transistor Q₅. On the other hand, the delay circuit 2₀ is incorporated into the NOR circuit 1₀ by reducing the size of the NOR circuit 1₀. That is, as illustrated in Fig. 8, the NOR circuit 1₀ is comprised of two P-channel transistors Q₁₁ and Q₁₂ , two N-channel transistors Q₁₃ and Q₁₄ , and a transistor Q₁₅ as a capacitor Q₁₅ In this case, the conductance g m of the P-channel transistors Q₁₁ and Q₁₂ is smaller than that of the N-channel transistors Q₁₃ and Q₁₄ , and the gate capacity of the transistors Q₁₁ and Q₁₂ is larger than that of the N-channel transistors Q₁₃ and Q₁₄. Note that, if the fall time period T f is longer than the rise time period T r , the above-mentioned circuits are reversed. The operation of the circuit of Fig. 6 will be explained with reference to Figs. 9A through 9G. Note that n = 2, and the rise time period T r of the delay circuits 2₀ , 2₁ , 2₂ , and 2₃ is longer than the fall time period T f thereof. When the address signals A₀ and A₁ are changed as illustrated in Figs. 9A and 9B, the decoding signal X₀ is in a selected state when (A₀ , A₁) = (0, 0) (see Fig. 9C); the decode signal X₁ is in a selected state when (A₀ , A₁) = (1, 0) (see Fig. 9D); the decode signal X₂ is in a selected state when (A₀ , A₁) = (0, 1) (see Fig. 9E); and the decode signal X₃ is in a selected state when (A₀ , A₁) = (1, 1) (see Fig. 9F). In this case, the transition time period T r of each of the decode signals X₀ , X₁ , X₂ , and X₃ from a non-selected state to a selected state is shorter than the transition time period T f from a selected state to a non-selected state. Therefore, when at least one of the address signals A₀ and A₁ is changed, a non-selection period (T r - T f ) of all the decoding signals X₀ , X₁ , X₂ , and X₃ always occurs. As a result, the logic circuit 3 detects this period (T r - T f ) and generates an ATD pulse signal as shown in Fig. 9G. In Fig. 10, the signal transition detection circuit according to the present invention is applied to the column portion of a mask ROM. That is, reference 101 designates a memory cell matrix including memory cells; 102 an address buffer; 103 a column address decoder for selecting one bit line from bit lines BL₀ , BL₁, ... , and BL₁₅; 104 a row decoder for selecting one word line from the word lines (not shown); 105 a logic circuit corresponding to the logic circuit 3 of Fig. 5; 106 a bit line charge-up circuit for charging up data buses DB₀ to DB₃; 107 a sense amplifier; and 108 a data buffer for outputting data D out . In Fig. 10, the bit lines BL₀ to BL₁₅ are divided into four groups. That is, the first group includes the bit lines BL₀ to BL₃; the second group includes the bit lines BL₄ to BL₇; the third group includes the bit lines BL₈ to BL₁₁; and the fourth group includes the bit lines BL₁₂ to BL₁₅. One bit line within each group is selected by switching gates G₀ through G₁₅ , and one of the groups is selected by switching gates G₁₆ through G₁₉. The switching gates are controlled by the column decoder 103. In Fig. 10, the column decoder 103 incorporates a delay function having a rise time period and a fall time period which are different from each other. That is, the column decoder 103 has a similar configuration to that of the decoder circuit of Fig. 8. Therefore, if at least one transition occurs in the column address signals (not shown), the logic circuit 105 generates an ATD pulse signal and transmits that signal to the data bus charge-up circuit 106. As a result, only after one transition of the column address signals, does the data bus charge-up circuit 106 charge the data bus DB₀ to DB₃ for a predetermined time period corresponding to the time period (T r - T f ) of Fig. 9. In Fig. 10, after a column selection is performed upon the memory cell matrix 101 by the column decoder 103, the row decoder 104 performs a row selection operation upon the memory cell matrix 101. That is, after the data buses DB₀ to DB₃ are charged, a row selection operation is carried out so that the potential at the data buses DB₀ to DB₃ , i.e., the potential at the input of the sense amplifier 107, is determined in accordance with data read out of the memory cell matrix 101. In Fig. 11, the signal transition detection circuit according to the present invention is applied to the row portion of a mask ROM. In Fig. 11, the elements which are the same as those of Fig. 10 are given the same references. In this case, the row decoder 104 incorporates a delay function having a rise time period and a fall time period which are different from each other. Further, the logic circuit 105 comprises a wired OR circuit formed by a plurality of enhancement-type transistors each having a common drain, a gate connected to one of the word lines WL₀ , WL₁ , ..., which are also connected to the row decoder 104, and a source connected to the ground. Therefore, when at least one transition occurs in the row address signals (not shown), all the transistors of the logic circuit 105 are turned OFF due to the difference between the rise time period T r and the fall time period T f of the row decoder 104, thereby generating an ATD pulse signal. This ATD pulse signal is used for resetting the sense amplifier 107. In Figs. 10 and 11, the increase by the addition of the signal transition detection circuit according to the present invention is very small. Especially, in Fig. 11, this increase corresponds to only one bit line. As explained above, in the signal transition detection circuit according to the present invention, since the decoder circuit used for other purposes is commonly used in the signal transition detection circuit, the area occupied by the detection circuit is substantially reduced, thus improving the degree of integration thereof.

Claims: A semiconductor memory device comprising: a memory cell matrix (101); decoder means (102, 103, 104), connected to said memory cell matrix, for decoding a plurality of address signals (A0, A1, ..., An-1) to generate decoded signals (X0, X1, ...) to selection lines; delay circuits (20, 21, ...), one delay circuit being provided for delaying a respective output of said decoder means, each of said delay circuits having a rise delay time period (Tr) and a fall delay time period (Tf) which are different from each other; and an address transition detection circuit (105), connected to said delay cicuits to generate an address transition detection signal; and a circuit (106), connected to said address transition detection circuit and to said memory cell matrix, for resetting a charge amplification circuit of the memory device by said address transition signal; characterized in that : respective outputs of the delay circuits (20, 21, ...) are fed to corresponding selection lines in the memory cell matrix as selection signals for selecting respective memory cells; said address transition detection circuit is constituted by a logic circuit (3) having a plurality of inputs, each connected to a corresponding output of said delay circuits (20, 21, ...), whereby said difference in the rise and fall time periods of said delay circuits (20, 21, ...) causes a non-selected state to appear at all the inputs to said logic circuit for a time equal to said difference, said logic circuit being responsive to said non-selected state at all of its inputs to thereby indicate a transition in response to at least one of the address signals changing state. A device as set forth in claim 1, wherein each of said delay circuits comprises : a capacitor (Q₁) connected to said decoder circuit; two serially-connected inverters (Q₂ - Q₅) connected to said capacitor, each of said inverters having a rise time period and a fall time period which are different from each other; and a capacitor (Q₆) connected to an output of said serially-connected inverters. A device as set forth in claim 1, wherein said delay circuits are incorporated into said decoder circuit.

Inventors: KASA YASUSHI, KASA, YASUSHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1987
Main classification: H03K5
IPC codes: G11C11, H03K5, G11C8