Single-chip memory system having a decoder for pulse word line method

Abstract: To avoid selecting multiple word lines, a memory system including a pulse word line method capability includes a decoder for activating one word line when a clock signal has an active level and for forcibly inactivating all word lines when the clock signal has an inactive level.

Description: The present invention generally relates to a memory system having a "pulse word line method" capability, and more particularly to a memory system including a decoder for activating a word line according to the pulse word method. Memory systems are needed to access data faster. Therefore, memory systems, typically having a cache and a main memory array comprising dynamic random access memories (DRAMs), are used. The cache comprises static random access memories (SRAMs) because an access speed of an SRAM is faster than that of a DRAM. However, an SRAM is more expensive than a DRAM, and therefore a main memory array typically comprises DRAMs, and an SRAM is used only for the cache for assisting the main memory array. However, power consumption of the memory systems should be reduced as much as possible. Therefore, a pulse word method is used for accessing the SRAM. Figure 1 shows a conventional memory system including a single-chip memory device 50 and an external central processing unit (CPU) 51. The memory device includes an array of SRAM cells 52 and an array of DRAM cells (not shown in Figure 1). The array 52 has a plurality of word lines, a plurality of bit lines and a plurality of SRAM cells. The CPU 51 processes instructions by using data from the memory device. The memory device 50 also includes input terminals (unreferenced) for receiving a row address strobe (RAS) signal, a column address strobe (CAS) signal, a write enable (WE) signal, a clock (CLK) signal and address signals having row address signals and column address signals. A row decoder 53 activates one word line of the word lines in response to row address signals. The row decoder is activated when the CPU changes the RAS signal to an active high level (e.g., "1") from an inactive low level (e.g., "0"). A column decoder 55 activates bit lines in response to column address signals. The column decoder is activated when the CPU changes the CAS signal to an active high level (e.g., "1") from an inactive low level (e.g.,"0"). The memory device 50 also includes I/O terminals (unreferenced) for inputting or outputting data. An I/O buffer 56 transfers data from the I/O terminals to a sense amplifier 54 when the CPU changes the WE signal to an active high level (e.g., "1") from an inactive low level (e.g., "0"), and transfers data from the sense amplifier 54 to the I/O terminals when the CPU changes the WE signal to an inactive level from an active level. The sense amplifier 54 amplifies data from the SRAM cell. A delay circuit 57 receives the CLK signal and outputs a delayed clock 1 (CLK1) signal. A delay circuit 58 also receives the CLK signal and outputs a delayed pulse (PULSE) signal. Figure 2 illustrates the row decoder 53 in detail, which includes an address signal receiving circuit 1, a plurality of register circuits 2a and a word line activating circuit 3a. In this example, the array of SRAM cells 52 has four word lines WL0 - WL3 and the row decoder 53 receives four row address signals A0 - A3 for easy understanding. Exemplary relationships between the levels of the address signals A0 - A3, outputs of register circuits X0 - X3, and the word lines WL0 - WL3 to be selected are as shown in the following Table 1. Address Signals Register Circuit Outputs Selected Word lines A0 A1 A2 A3 X0 X1 X2 X3 H H H H H L H L WL0 L H L H L H L H WL3 L H H H L H H L WL1 H H L H H L L H WL2 The address signal receiving circuit 1 includes AND circuit D1 for receiving the address signals A0 and A1, AND circuit D2 for receiving the address signal A1 and an inverted address signal A0, AND circuit D3 for receiving the address signals A2 and A3, and AND circuit D4 for receiving the address signal A3 and an inverted address signal A2. Each register circuit 2a includes a P-type metal oxide semiconductor (MOS) transistor TR1 having a source-drain path between node N1 and node N2, an inverter I1 for inverting a signal of node N2 and outputting the inverted signal to node N3, an inverter I2 for inverting a signal of node N3, an N-type MOS transistor TR2 having a source-drain path between node N2 and an output of the inverter I2, an N-type MOS transistor TR3 having a source-drain path between node N3 and node N4, an inverter I3 for inverting a signal of node N4 and outputting the inverted signal to node N5, an inverter I4 for inverting a signal of node N5, and a P-type MOS transistor TR4 having a source-drain path between node N4 and an output of the inverter I4. Each gate of MOS transistors TR1, TR2, TR3 and TR4 receives clock signal CLK1. The word line activating circuit 3a includes AND circuit WD1 for receiving the signals X0, X2 and the PULSE signal, AND circuit WD2 for receiving the signals X1, X2 and the PULSE signal, AND circuit WD3 for receiving the signals X0, X3 and the PULSE signal, and AND circuit WD4 for receiving the signals X1, X3 and the PULSE signal. Each of AND circuits WD1 - WD4 activates a corresponding word line when all signals received by the AND circuit have an active level (e.g., "1"), and inactivates a corresponding word line when at least one signal of the signals received by the AND circuit has an inactive level (e.g., "0"). Figure 3 is a timing chart for explaining a normal operation of the circuit of Figure 2. At timing T0, a bus cycle CYCLE2 starts, and a CLK1 signal changes to an active level (e.g., "1") from an inactive level (e.g., "0") in synchronism with the CYCLE2. In this example, each of the signals X0 and X2 has an inactive level (e.g., "0"), and each of the signals X1 and X3 has an active level (e.g., "1") at a bus cycle CYCLE1. Thereafter, for example, each of the signals X0 and X2 becomes active, and each of the signals X1 and X3 becomes inactive at a bus cycle CYCLE2. At this time, although each of register circuits 2a has the same construction, the timing of the signals X0 - X3 are different from each other. The reasons for the timing difference are that the area where the circuits 2a are positioned is different from each other on the single-chip memory system 50, and that each of the circuits 2a has a different parasitic resistor and a different parasitic capacitor from each other. However, for an easy explanation, the signals X0 and X1 have the same timing T1, and the signals X2 and X3 have the same timing T2 as shown in Figure 3. In Figure 3, the PULSE signal is changed to an active level (e.g., "1") from an inactive level (e.g., "0") at the timing T3 after the timings T1 and T2. Therefore, only one word line WL0 among word lines WL0 - WL3 is changed to an active level (e.g., "1") from an inactive level (e.g., "0"). Moreover, all word lines WL0 - WL3 are changed to an inactive level from an active level when the PULSE signal is changed to an inactive level from an active level. This is a so-called "pulse word line method". However, the timings T1 and T2 may be changed (e.g., opposite) depending on the circuit design because they depend on a parasitic resistance and a parasitic capacitance. Therefore, it is possible for timing to occur as shown in Figure 4. In Figure 4, the timing T2 occurs after the timing T3. Therefore, two word lines WL0 and WL2 are selected erroneously during a bus cycle CYCLE2. This is a so-called "multiple word line selection". In such a case, either the position of each apparatus on the chip 50 must be redesigned, or the delay time of the delay circuit 58 must be changed. Thus, problems arise since redesigning the chip 50 is costly. Further, increasing the delay time of the PULSE signal by changing the delay time of the delay circuit slows the access time since the time to activate a word line increases (e.g., shifts later). EP 0 533 096 A2 discloses a memory system according to the preamble of claim 1. In this system two inverters are employed which are directly cross-connected to form a flip-flop. With this structure it takes a relatively long time to change the output of such a flip-flop in response to a newly decoded address signal. In view of the foregoing problems of the conventional memory system, it is therefore an object of the present invention to provide an improved memory system having a decoder for activating a word line according to the pulse word line method. It is another object of the present invention to provide an improved decoder for activating a word line according to the pulse word line method. These objects are achieved by a memory system as claimed in claim 1; the dependent claims are related to further developments of the invention. With the unique and unobvious structure of the present invention, the decoded signals outputted to the activating circuit are terminated in response to an inactive level of the clock signal. Therefore, selecting an improper word line is prevented and the problems of the above-mentioned conventional memory system are prevented. The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention with reference to the drawings, in which: Figure 1 is a diagram showing a conventional memory system; Figure 2 is a circuit diagram showing a row decoder 53 in the memory system of Figure 1; Figure 3 is a timing chart showing normal timings of signals in the decoder 53 of Figure 2; Figure 4 is a timing chart showing abnormal timings of signals in the decoder 53 of Figure 2; Figure 5 is a diagram showing a memory system of a first embodiment according to the present invention; Figure 6 is a circuit diagram showing a row decoder 62 of a first embodiment according to the present invention; Figure 7 is a timing chart showing timings of signals in the decoder 62 of Figure 6; and Figure 8 is a diagram showing a memory system of a second embodiment according to the present invention. Referring now to the drawings, and more particularly to Figure 5, a single-chip semiconductor memory system 60 has a row decoder 62 according to a first embodiment of the present invention. The same parts in Figure 5 as those in Figure 1 are numbered with the same reference numerals as in Figure 1 and for brevity explanations of these parts are omitted from the following description. As shown in Figure 5, a single-chip semiconductor memory system 60 has no delay circuit 58 as in Figure 1, and therefore row decoder 62 does not receive the PULSE signal in Figure 1. Figure 6 illustrates the row decoder 62 in detail, which includes an address signal receiving circuit 1, a plurality of register circuits 2 and a word line activating circuit 3. While the array of SRAM cells 52 is shown with four word lines, the array typically has more than four word lines. However, for brevity and ease of understanding in the exemplary configuration, the array of SRAM cells 52 has four word lines WL0 - WL3 and the row decoder 53 receives four row address signals A0 - A3. Further, the relationships between the levels of the address signals A0 - A3, outputs of register circuits X0 - X3, and the word lines WL0 - WL3 to be selected are as shown in the above Table 1. The address signal receiving circuit 1 includes AND circuit D1 for receiving the address signals A0 and A1, AND circuit D2 for receiving the address signal A1 and an inverted address signal A0, AND circuit D3 for receiving the address signals A2 and A3, and AND circuit D4 for receiving the address signal A3 and an inverted address signal A2. Each register circuit 2 includes a P-type MOS transistor TR1 having a source-drain path between node N1 and node N2, an inverter I1 for inverting a signal of node N2 and outputting the inverted signal to node N3, an inverter I2 for inverting a signal of node N3, an N-type MOS transistor TR2 having a source-drain path between node N2 and an output of the inverter I2, an N-type MOS transistor TR3 having a source-drain path between node N3 and node N4, an inverter I3 for inverting a signal of node N4 and outputting the inverted signal to node N5, and a P-type MOS transistor P1 having a source-drain path between node N4 and a reference voltage Vcc. The reference voltage Vcc has a fixed level (e.g., "1" or in a different configuration the reference voltage could be ground). Each gate of MOS transistors TR1, TR2, TR3 and P1 receives a delayed clock signal CLK1. The word line activating circuit 3 includes AND circuit WD11 for receiving the signals X0 and X2, AND circuit WD21 for receiving the signals X1 and X2, AND circuit WD31 for receiving the signals X0 and X3, and AND circuit WD41 for receiving the signals X1 and X3. Each of AND circuits WD11 - WD41 activates a corresponding word line when all signals received by the AND circuit have an active level (e.g., "1"), and inactivates a corresponding word line when at least one signal of the signals received by the AND circuit has an inactive level (e.g., "0"). Figure 7 is a timing chart for explaining an operation of the circuit of Figure 6. At timing T0, a bus cycle CYCLE2 starts, and CLK1 signal changes to an active level (e.g., "1") from an inactive level (e.g., "0") in synchronism with the CYCLE2. Thereafter, for example, each of the signals X0 and X2 becomes active (e.g., "1"), and each of the signals X1 and X3 becomes inactive (e.g., "0") at a bus cycle CYCLE2. At this time, P-type MOS transistors P1 are "OFF" because the CLK1 signal has an active level. Moreover, although each of register circuits 2 has the same construction, the timing of the signals X0 and X2 are different from each other. The reasons for the timing difference are that the area where the circuits 2 are positioned is different from each other on the single-chip memory system 60, and that each of the circuits 2 has a different parasitic resistance and a different parasitic capacitance from each other, as mentioned above. The word line activating circuit 3 activates only one word line WL0 among word lines WL0 - WL3 after the timing T2 because both signals X0 and X2 have an active level. Thereafter, the CLK1 signal changes to an inactive level from an active level at the timing T3. Therefore, the signals X0 and X2 become an inactive level because the P-type MOS transistors are "ON" after the timing T3, and then the word line WL0 changes to ah inactive level by the word line activating circuit 3 before an end of a bus cycle CYCLE2. These signal levels correspond to the pulse word line method. In the first embodiment of the present invention, all signals X0 - X3 become an inactive level forcibly before an end of a bus cycle CYCLE2, and then the word line activating circuit changes all word lines WL0 - WL3 to an inactive level before the end of the bus cycle CYCLE2. Therefore, the "multiple word line selection" does not occur at a next bus cycle of the bus cycle CYCLE2, as in the conventional system. Moreover, in the first embodiment, the single-chip memory system can be made smaller than the single-chip memory system 50 as shown in Figure 1 because the single-chip memory system 60 according to the invention has no delay circuit 58 as shown in Figure 1. Further, word line activating circuit 3 does not receive (or require) the signal PULSE as shown in Figure 2. Additionally, register circuits 2 do not have an inverter I4 as shown in the conventional decoder 53 of Figure 2. In Figure 5, the connection relationship between an external CPU 51 and the chip 60 is shown. As mentioned above, the chip 60 is operated by external CPU 51 because the chip 60 has no on-board CPU. However, the memory system may be modified to contain an on-board CPU 71, as shown in Figure 8. In this case, all structure in the chip 60 (except for the terminals and delay circuit 57 which is made unnecessary) and CPU 71 are contained in a single semiconductor chip 70. As mentioned above, since the chip 70 of Figure 8 contains CPU 71, the delay circuit 57 for synchronizing the internal clock signal CLK1 on the chip 60 with a bus cycle is unnecessary. It is noted that, with suitable modification, the row decoder 62 could receive a clock signal directly outputted by CPU 51 of Figure 5. In such a case, the delay circuit 57 on the chip 60 also would become unnecessary. As mentioned above, the exemplary array of SRAM cells 52 has four word lines, and therefore row decoder 62 has four register circuits 2. However, as known by one of ordinary skill in the art taking the present specification as a whole, the number of word lines and address signals, and thus the number of the above-mentioned structures, can be changed readily depending on the design of the memory system. Further, the logic (e.g., levels) of the signals shown in the timing chart of Figure 7 is exemplary and may be suitably changed (e.g., reversed) depending on the designer's requirements and constraints. While the invention has been described in terms of several preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the scope of the appended claims.

Claims: A memory system comprising: a memory cell array including a plurality of word lines, a terminal receiving a clock signal (CLK1), a decoder (1) receiving an address signal and generating a decoded signal, a register (2) capturing said decoded signal in response to a first level of the clock signal and outputting the captured decoded signal at an output end (N5) thereof in response to a second level of the clock signal, said register invalidating the level at the output end during the first level of the clock signal, and a word line activating circuit (3) activating one of the word lines in response to the output of the register, wherein said register includes a first transistor (TR1) having a source-drain path connected between a first node (N1) and a second node (N2), a first inverter (I1) connected between said second node and a third node (N3), and a second inverter (I2) characterized in that , said register further includes a second transistor (TR2) connected in series between said second node and said third node in parallel to said first inverter to form a flip-flop circuit during a conductive state of said second transistor, a third transistor (TR3) having a source-drain path connected between said third node and a fourth node (N4), and a fourth transistor (P1) having a source-drain path connected between said fourth node and a reference voltage (Vcc), said first, second, third and fourth transistors receiving said clock signal at the gates thereof. The memory system as claimed in claim 1, characterized in that said register further comprises a third inverter (I3) inserted between said fourth node and said output end. The memory system as claimed in claim 2, characterized in that each of said first and fourth transistors is of a P-type and each of said second and third transistors is of an N-type.

Inventors: KATO YOSHIYUKI, KATO, YOSHIYUKI
Applicants: NEC ELECTRONICS CORP, NEC ELECTRONICS CORPORATION
Application year: 1997
Main classification: G11C8
IPC codes: G11C11, G11C8