SEMICONDUCTOR MEMORY DEVICE

Abstract: A semiconductor memory device includes an address change detection circuit (TR₀, TR₁...) and a pulse width control circuit (PWC). The pulse width control circuit (PWC) inhibits the passage of write enable signals (WE) having a short pulse width for a predetermined time period after a change of address has occurred. After the predetermined period has elapsed the operation of the pulse width control circuit (PWC) is overridden and thus the write cycle time is reduced.

Description: The present invention relates to a memory device using semiconductors, particularly bipolar type semiconductors, more particularly a semiconductor memory device having a pulse width control circuit for preventing erroneous operation of its write circuit. A semiconductor memory device usually cannot perform a write operation unless the pulse width of the write enable signal is greater than a certain length. One of the exceptions is just after a change of address. At such times, the write operation can be carried out by a write enable signal having a relatively short pulse width. Consequently an erroneous write operation may be caused just after a change of address by noise. To prevent such erroneous write operations, JP-A-53-114651 discloses a device with a pulse width control circuit for inhibiting the passage of write enable signals having a pulse width smaller than a predetermined pulse width. This circuit only permits the passage of a write enable signal having a pulse width greater than a predetermined length and inhibits the passage of signals having less than the predetermined pulse width. This pulse width control circuit prevents erroneous write operations due to noise. However, the circuit requires a certain processing time and a minimum pulse width of the write enable signal to effect the write operation. As a result, the write cycle time is disadvantageously increased. US-A-4337525 describes a semiconductor memory device having an address change detecting means for detecting a change of address of memory cells, for delivering an output signal upon detection of a change of address, said output signal having a first level for a predetermined period of time after the detection of the change of address and a second level for another period of time after the end of the said predetermined period of time, and a write signal generator including a pulse width control circuit responsive to the output signal for inhibiting the passage of write enable signals. According to this invention such a semiconductor memory device is characterised in that the pulse width control circuit includes timing means triggered by the write enable signal; in that when the pulse width control circuit receives the first level of the output signal the pulse width control circuit inhibits the passage of the write enable signal having a pulse width smaller than a predetermined pulse width determined by the timing means and passes a write enable signal having a pulse width greater than said predetermined pulse width; and in that when the pulse width control circuit receives the second level of output signal the pulse width control circuit passes the write enable signal irrespective of its length. With the device in accordance with this invention erroneous write operations due to noise are prevented but the write cycle time is not increased for most of the operating cycle. Particular examples of a device in accordance with the present invention will now be described and contrasted with the prior art with reference to the accompanying drawings; in which:- Figure 1 is a circuit diagram of a prior semiconductor memory device; Figure 2 is a graph illustrating the relationship between a setup time and pulse width of a write enable signal; Figure 3 is a graph illustrating voltage waveforms in a memory cell at a change of address; Figure 4 is a graph of a signal wave-form in a pulse width control circuit shown in Figure 1; Figure 5 is a circuit diagram of a first example of semiconductor memory device in accordance with this invention; Figure 6 is a circuit diagram of a pulse width control circuit for the first example; Figures 7, 8A and 8B, and 9 are graphs illustrating the signal waveforms in the first example; Figure 10 is a circuit diagram of a modification of the pulse width control circuit shown in Figure 5; Figure 11 is a circuit diagram of a second example of the present invention; Figure 12 is a graph illustrating the signal waveforms in the pulse width control circuit shown in Figure 11; and, Figure 13 is a graph showing the relationship between a setup time and pulse width of a write enable signal in accordance with the present invention. For a better understanding of the preferred embodiments, the problems in the prior art will first be explained with reference to Figures 1 to 4. Figure 1 shows a semiconductor memory device of the prior art. In Figure 1, MC₀₀ through MC₁₁ are memory cells of an emitter-coupled logic (ECL) type, WL₀ and WL₁ are word lines, Tr₂ through Tr₅ are bitline selection transistors, Tr₆ through Tr₉ are read/write control transistors, SA is a sense amplifier, WCC is a write control circuit, PWC is a pulse-width control circuit, and I H and I O are current sources. Each of memory cells MC₀₀ through MC₁₁ includes two multi emitter transistors Q1 and Q2, as typically shown by MC₀₀. In a write operation on the memory cell MC₀₀, the word driver WD₀ and bit line selection transistors Tr₂ and Tr₃ are used to address the memory cell MC₀₀. A write enable signal WE is then supplied to the write control circuit. One of the control signals D₀ and D₁ become "0" and the other "1" in accordance with the write data D(in). Consequently the write operation of the memory cell MC₀₀ is effected. In this write operation, the write enable signal is applied after a setup time t(SA) from the change of address. The write enable signal must have a pulse width greater than a certain magnitude. This certain magnitude which is necessary for the write operation varies in accordance with the setup time t(SA). Figure 2 shows the relationship between the setup time t(SA) of the write enable signal and the minimum necessary pulse width. In Figure 2, the ordinate represents the minimum necessary pulse width PW, and the abscissa represents the setup time t(SA). As clear from Figure 2, a write operation is possible just after the change of address even with short pulse width. As the setup time t(SA) becomes longer, however, the write operation cannot be effected unless the pulse width is greater than a certain magnitude. This reason is described below. Assume that the transistor Q1 of the memory cell MC₀₀ is in the on state and that the transistor Q2 is in the off state. Figure 3 thus shows the variations of the collector voltage of the these transistors Q1 and Q2 before and after the change of address. In Figure 3, (1) represents the collector voltage of the transistors Q1, (2) the collector voltage of the transistors Q2, and (3) the voltage of the word line WL₀. When the word line voltage (3) rises up due to the address selection, the collector voltage (2) rises up gradually from H₁ to H₂. On the other hand, the collector voltage (1) of the transistor Q1 rises up rapidly from L₁ in an "overdamping" state and then stabilizes at L₂. As clear from Figure 3, just after the change of address, the voltage difference between the collector voltages (1) and (2) becomes very small. Therefore, a write enable signal having a short pulse width can readily cause the on-off states of the transistors Q1 and Q2 to invert and, thus, a write operation to be effected. Since the write operation can be effected by means of a write enable signal having a short pulse width just after the change of address, it can also be effected by noise, resulting in an erroneous write operation. To solve this problem, as mentioned before there has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 53-114651 a pulse width control circuit which inhibits passage of write enable signals with pulse widths smaller than a certain magnitude, permitting passage of only signals with pulse widths more than that. Such a pulse width control circuit PWC is shown at the centre bottom of Figure 1. The circuit includes a delay element D, an AND gate G10, and a pulse width extension circuit PWE which extends the pulse width of the output of the AND gate G10 to a certain magnitude. In this pulse width control circuit PWC, the output of the AND gate G10 appears only when input signals are simultaneously applied on its two input terminals. Accordingly, when the pulse width of the write enable signal WE is smaller than the delay time of the delay element D, the passage of the write enable signal is inhibited by the AND gate G10. Figure 4 shows signal waveforms at individual points of the pulse width control circuit PWC of Figure 1. In Figure 4, (1) represents the state of change of the address, (2a) a write enable signal having a short pulse width, (2b) a write enable signal having a long pulse width, (3) an output signal of the delay element D, (4) an output signal of the AND gate G10, and (5) an output signal of the pulse width extension circuit PWE as an internal write enable signal WE(INT) whose pulse width is extended to the pulse width of original write enable signal WE. As clear from Figure 4, when the pulse width of the write enable signal WE is short (Figure 4 - 2a), no output signal appears at the output of the AND gate G10, therefore, no internal write enable signal WE(INT) appears. When the pulse width is long (Figure 4 - 2b), however, an internal write enable signal WE(INT) is delivered from the AND gate G10, whereby the write operation is effected. The above-described circuit PWC may be used to prevent erroneous write operations caused by noise just after the change of address. However, the circuit requires the delay time caused at the delay element D in addition to a minimum pulse duration. This results in an increased write cycle time. A preferred embodiment of the present invention will now be explained with reference to Figure 5 to Figure 9. Figure 5 illustrates a semiconductor memory device of an embodiment of the present invention. In Figure 5, MC₀₀, MC₀₁, MC₁₀, and MC₁₁ are memory cells of the ECL type, WL₀ and WL₁ are word lines, and BL₀₀, BL₀₁, BL₁₀, and BL₁₁ are bit lines. The word lines WL₀ and WL₁ are connected to one of the emitters of multi emitter type transistors TR₀ and Tr₁, respectively. WD₀ and WD₁ are word drivers which receive input signals from an address decoder (not shown in Figure 5). The bit lines BL₀₀, BL₀₁, BL₁₀, and BL₁₁ are connected to current sources via the bit line selection transistors Tr₂, Tr₃, Tr₄, and Tr₅, respectively. Furthermore, the bit lines BL₀₀, BL₀₁, BL₁₀, and BL₁₁ are connected to a sense amplifier SA via the transistors TR₆, TR₇, Tr₈, and Tr₉, respectively. The bases of the transistors Tr₆ through Tr₉ are connected to a write control circuit WCC. These transistors Tr₆ through Tr₉ are driven by the control signals D₀ and D₁ delivered from the write control circuit WCC. The write control circuit WCC generates the control signals D₀ and D₁, one being the logical level "1" and the other "0", in accordance with write data D(in). A pulse width control circuit PWC supplies this write control circuit WCC with an internal write enable signal WE (INT). The pulse width control circuit PWC operates so as to allow the passage of a write enable signal WE applied externally only when the signal WE has pulse width greater than a predetermined magnitude. Furthermore, this circuit PWC itself selects whether to enable or inhibit such an operation depending upon the address change. At the bottom of Figure 5 is shown the pulse width control circuit PWC, including, for example, an inverter G1, NOR gate G2, and OR gate G3, which operates in the manner of a negative logic. In this circuit PWC, the write enable signal WE is directly applied to one input terminal of the OR gate G3 and is applied to the other input terminal of the OR gate G3 after being delayed by the inverter G1 and the NOR gate G2. The other input terminal of the NOR gate G2 is connected to the other emitter of the multi emitter transistors TR₀ and Tr₁. Therefore, when a change of memory address is effected, it receives a signal AD of a logical level "1" (Low level) for a predetermined period from the time of the change. Consequently, the NOR gate G2 is opened for the predetermined period after the change, whereby pulse width control of the write enable signal WE is effected. That is, when the pulse width of the write enable signal WE is smaller than the delay time caused at the inverter G1 and the NOR gate G2, the passage of the write enable signal WE is inhibited by the OR gate G3. Therefore, no internal write enable signal WE (INT) appears. On the contrary, when the pulse width of the write enable signal WE is greater than the delay time, an internal write enable signal WE (INT) having a pulse width corresponding to the differential value is applied to the write control circuit WCC. After the predetermined period passes from the change, the signal AD becomes the logical level "0", (High level) whereupon the NOR gate G2 is closed and its output is kept at the logical level "1". Accordingly, the write enable signal WE is directly delivered as the internal write enable signal WE (INT). Figure 6 shows an embodiment of the pulse width control circuit PWC in Figure 5, constituted in the form of an ECL. This circuit corresponds exactly to that shown in Figure 5, therefore, operates in the manner of a negative logic. In Figure 6, IN₁ is an input terminal for the write enable signal WE , IN₂ is an input terminal for the signal AD, OUT is an output terminal where the internal write enable signal WE (INT) appears, and V R1 is a reference voltage. The mode of operation of the above-described embodiment will now be described. As shown in Figure 7, when the address of the memory is changed, the signal AD becomes the "L" level for a certain period due to the time lag between the rise and fall of the outputs of the transistors TR₀ and Tr₁. When the signal AD is the "L" level, as shown in Figure 8A, pulse width control is effected. That is, when the write enable signal WE is applied to the input terminal IN₁, a signal P delayed with respect to the signal WE is generated. As a result, a logical product signal of this delayed signal P and the write enable signal WE is delivered from the output terminal OUT as the internal write enable signal WE (INT). It is also possible to extend the pulse width of this logical product signal by integration and adopt it as the internal write enable signal WE (INT). After a certain time passes from the change of the address, the signal AD becomes the "H" level. Therefore, as shown in Figure 8B, no pulse width control is effected. That is, when the signal AD becomes the "H" level, the transistor Tr₁₀ in Figure 6 turns on, so the signal P is always the "L" level. Consequently, the input write enable signal WE is directly delivered as the internal write enable signal WE (INT). When the internal write enable signal obtained as described above is applied to the write control circuit WCC, the control signals D₀ and D₁ having inverse logics are formed in accordance with the write data D(in). These control signals D₀ and D₁ are applied to the bases of the transistors Tr₆ through Tr₉. By this, the data D(in) is written into the addressed memory cell. Although a preferred embodiment has been described heretofore, various modifications and alterations are possible within the scope of the present invention. For example, the pulse width control circuit PWC is not limited to the constitution described above. Figure 10 shows another embodiment of the pulse width control circuit PWC. This embodiment is somewhat more simplified compared with circuit of Figure 6, but is also constituted in the form of an ECL, operating in the manner of a negative logic. IN₁, IN₂, and OUT are terminals the same as those shown in Figure 6, and VR₁ and VR₂ are reference voltages. In the circuit of Figure 10, when the signal AD becomes the "H" level, the transistors TR₁₁ turns on and the delayed signal P is kept at the "L" level. As a result, the write enable signal WE is directly output as the internal write enable signal WE (INT). Although the above-described examples referred to ECL's, transistor-transistor logic (TTL) is of course also possible. Furthermore, the pulse width control circuit PWC may be constituted by a positive logic circuit instead of a negative logic circuit described above. Figure 11 shows such a circuit PWC, including an inverter G4, a NAND gate G5, and an AND gate G6. Figures 12A and 12B show a write enable signal WE, an internal write enable signal WE(INT), and a delayed signal P in the circuit PWC of Figure 11, Figure 12A showing the case just after the change of address where pulse width control is effected and Figure 12B showing the case after a certain time elapses from the change of address where pulse width control is not effected. Furthermore, in the above description, the signal AD taken out from the emitters of the transistors TR₁ and TR₁ was directly supplied to the pulse width control circuit PWC. In another modification, however, the signal may be supplied via the circuit PWE of Figure 11, which extends the duration of the signal AD. By this circuit, as shown in Figure 7, a signal AD' with an arbitrarily defined duration can be obtained. Further, the above description has been made with reference to four memory cells. In practice, however, a large number of memory cells are arranged in the form of a matrix. As described heretofore, according to the present invention, pulse width control of the write enable signal is possible only in a predetermined period just after the change of address. As a result, problems such as the pulse width of the write enable signal becoming too large or the write cycle time becoming too long are eliminated. As shown by PW₁ in Figure 13, in the prior art, the write operation is enabled even by a write enable signal having a short pulse width just after the change of address. Consequently, erroneous operations readily occur. If pulse width control is always effected so as to prevent such erroneous operations, the continuous delay as shown by PW₂ in Figure 13 increases the pulse width of the write enable signal, i.e., increases the write cycle time. On the other hand, according to the present invention as shown by PW₃ in Figure 13, these problems are eliminated since pulse width control is effected only in a predetermined period from the change of address which is very first portion of the each address cycle. Therefore, the delay time due to the pulse width control circuit function does not affect the whole cycle time. During this pulse width control, a write operation cannot be effected by a write enable signal having a short pulse width. Therefore, no erroneous write operation due to noise occurs.

Claims: A semiconductor memory device having an address change detecting means (TR₀, TR₁, ...TR n ) for detecting a change of address of memory cells, for delivering an output signal (SIG(AD)) upon the detection of a change of address, said output signal having a first level for a predetermined period of time after the detection of the change of address and a second level for another period of time after the end of the said predetermined period of time, and a write signal generator including a pulse width control circuit (PWC) responsive to the output signal (SIG(AD)) for inhibiting the passage of write enable signals ( WE ); characterised in that the pulse width control circuit (PWC) includes timing means (G1,G3) triggered by the write enable signal ( WE ); in that when the pulse width control circuit (PWC) receives the first level of the output signal (SIG(AD)), the pulse width control circuit (PWC) inhibits the passage of the write enable signal ( WE ) having a pulse width smaller than a predetermined pulse width determined by the timing means (G1,G3) and passes a write enable signal ( WE ) having a pulse width greater than said predetermined pulse width; and in that when the pulse width control circuit (PWC) receives the second level of the output signal (SIG(AD)), the pulse width control circuit (PWC) passes the write enable signal ( WE ) irrespective of its length. A semiconductor memory device according to claim 1, wherein the address change detecting means comprises two-emitter transistors (Tr₀, Tr₁,... Tr n ) whose collectors are connected to word lines (WL₀, WL₁ ...) and whose bases are connected to word drivers (WD₀, WD₁...) one of each of the two emitters being connected to current sources (I H ), and the other of each of the two emitters being commonly connected, the output (SIG(AD)) of the detecting means being taken out from their common junction. A semiconductor memory device according to claim 1, or 2 wherein the pulse width control circuit (PWC) comprises: a delay element (G1) for delaying the write enable signal (WE); a first gate circuit (G2) for controlling the passage of the delayed signal from the delay element (G1) in accordance with the output signal of the address change detecting means (Tr₀, Tr₁, ... Tr n ); and a second gate circuit (G3) for controlling the passage of the write enable signal in accordance with the output signal of the first gate circuit (G2). A semiconductor memory device according to any preceding claim, wherein the pulse width control circuit (PWC) is constituted by a negative logic circuit. A semiconductor memory device according to claim 4, wherein the delay element is an inverter (G1), the first gate circuit is a NOR gate (G2), and the second gate circuit is an OR gate (G3). A semiconductor memory device according to claim 1, 2 or 3, wherein the pulse width control circuit (PWC) is constituted by a positive logic circuit (Figure 11). A semiconductor memory device according to claim 6, wherein the delay element is an inverter (G4), and the first gate circuit is a NAND gate (G5), and the second gate circuit is an AND gate (G6). A semiconductor memory device according to any one of the preceding claims, wherein said address change detecting means further comprises a pulse width extension circuit (PWE) for extending the output signal (SIG(AD)).

Inventors: TOYODA, KAZUHIRO
Applicants: FUJITSU LIMITED
Application year: 1985
Main classification: G11C7
IPC codes: G11C8, G11C11, G11C7