Dual Strobed Negative Pumped Wordlines for Dynamic Random Access Memories

The present invention relates to dynamic random access memories, and more particularly, to a circuit and a method for driving a potential of an inactive wordline to a negative potential.

The invention is a method and a circuit which efficiently drives a potential of an inactive wordline to a negative potential. When the wordline is deactivated during reset its potential is first driven toward a reference potential in response to a first control signal and is then driven toward a negative potential in response to a second control signal.

In one embodiment the wordline is first coupled to a variable potential node held at a ground reference potential and then is further coupled to a supply node held at a negative potential (for example Vbb). Thus the potential of the wordline is driven toward the ground reference potential and then toward the negative potential. The wordline is simultaneously decoupled from the variable potential node and coupled to the supply node.

FIG. 1 is a block schematic of a computer system of the invention.

FIG. 2 is a block schematic of decode circuitry and driver circuitry of the invention.

FIG. 3 is a more detailed schematic of the decode circuitry and the driver circuitry shown in FIG. 2.

FIG. 4 is a timing diagram descriptive of a method of the invention when implemented with respect to the circuit of FIG. 3.

FIG. 5 is a simplification of the driver circuitry shown in FIG. 3.

The following patents of Loren McLaury's are incorporated by reference: U.S. Pat. No. 5,410,508, entitled PUMPED WORDLINES and filed on May 14, 1993, and U.S. Pat. No. 5,245,578 entitled A DRAM WITH A TWO STAGE VOLTAGE PULL-DOWN SENSE AMPLIFIER and filed on Aug. 12. 1992.

The invention is a computer system 1, as shown in FIG. 1, which stores memory dam. The computer system 1 has a microprocessor 2 which receives an address and a read or write command. The microprocessor 2 routes the address and command to a memory chip 3 via a memory controller 4 and routes data to and from the memory chip 3. The memory chip 3 may be a dynamic random access memory device. In any case the memory chip 3 comprises the circuit and the method of the invention which drives a potential of a selected wordline to a select potential and which resets the select wordline by driving the potential of the wordline to a non-select potential. The non-select potential is driven to the wordline being reset by first driving the potential of the wordline toward a ground reference potential and by secondly driving the potential of the wordline toward a negative potential. During reset an active wordline is deactivated. Typically the circuit of the invention responds to a first control signal to couple the wordline to a first node driven toward the ground reference potential and responds to a second control signal to couple the wordline to a second node connected to the negative potential which is less than the ground reference potential or less than the lowest potential to which the digit lines are driven.

FIG. 2, described below using a specific example, is a block schematic of a portion of the circuit of the invention. A primary decode circuit 5 determines a select group 6 of wordlines 7 with a first potential on its output node 10A and determines non-select groups 8 of wordlines 15 with a second potential on its output nodes 10B-D. (Although the number of wordlines in the select and non-select groups 6 and 8 is equal to four, in the example, it is possible for the number of wordlines in a group of wordlines to vary.) One of the wordlines in the select group 6 of wordlines 7 is then selected as the select wordline 7A by the secondary decode circuit 20. When wordline 7A is selected it is driven to a high potential which actuates the gates of access transistors 21A&A' which are N-channel enhancement or depletion type transistors. When actuated the access transistors 21A&A' couple the memory storage cells 22A&A' to the digit lines 23A&A', respectively. Memory storage cells 22B&B'-D&D' remain isolated from the digit lines 23A&A'.

For each wordline there is a driver circuit 24(A-D) which drives the select and non-select wordlines to appropriate potentials. Each driver circuit 24A-D is comprised, respectively, of a first driver circuit 25A-D and a second driver circuit 30A-D. Potentials of first nodes 32A-D of first driver circuits 25A-D are driven toward first supply potentials, and second nodes 33A-D of first driver circuits 25A-D are connected to a supply node 45 which is connectable to a negative supply potential. The first supply potentials vary between a ground reference potential and a positive potential. Therefore the first nodes 32A-D can be thought of as variable potential nodes whose potentials are controlled by secondary decode circuit 20. The second driver circuits 30A-D are controlled by the secondary decode circuit at control inputs 34A-D, respectively. The second driver circuits 30A-D are interposed between the wordlines 7A-D and supply node 45.

The secondary decode circuit 20 has a first set of four output nodes 35A-D. Each one of the output nodes 35A-D is connected to a corresponding one of the first nodes 32A-D of the first driver circuits 25A-D. The secondary decode circuit 20 also has a second set of four output nodes 40A*-D*. Although the potentials of the second set of output nodes 40A*-D* are complements of the potentials on the first set of output nodes 35A-D, respectively, a transition to the complement on nodes 40A*-D* may be delayed from a corresponding transition on nodes 35A-D. Each one of the second set of output nodes 40A*-D* is connected to a corresponding one of the control inputs 34A-D, respectively, of the second driver circuits 30A-D.

In the example the primary decode circuit 5 determines that the selected wordline is one of wordlines 7A-D and that the non selected wordlines are 15. In the example, wordline 7A is selected to be activated by the secondary decode circuit 20. Therefore, secondary decode circuit 20 also determines that wordlines 7B-D are non-selected. The secondary decode circuit 20 supplies the positive potential at nodes 34B-D. The positive potential activates a transistor within the second driver circuits 30B-D. The non select wordlines 7B-D remain at a negative potential and are coupled to supply node 45 through the activated transistors.

Thus if one of the wordlines in a group of wordlines is addressed for activation, the group 6 of wordlines containing the addressed wordline is selected by the primary decode circuit 5 which generates a first potential at the appropriate output node, in this example output node 10A. Since the remaining groups 8 of wordlines 15 are not selected the primary decode circuit 5 generates a second potential at nodes 10B-D. The first potential at node 10A activates circuitry within the first driver circuits 25A-D to couple the wordlines 7A-D to the first nodes 32A-D, respectively. The secondary decode circuit 20 determines which potential is driven to the first nodes 32A-D. If the wordline is selected, in the example, 7A, the secondary decode circuit 20 provides the positive potential at the first node, 32A in the example. The potential of the wordline 7A is then driven toward the positive potential. However, if the wordline is not selected the secondary decode circuit 20 supplies the ground reference potential at the first nodes 34B-D, in the example. In this example, the complement of the ground reference potential, a high potential, is supplied by the secondary decode circuit 20 to output nodes 40B*-D* and thus control inputs 34B-D. The high potential on control inputs 34B-D actuates transistors of secondary driver circuits 30B-D, and the non-selected wordlines 7B-D are coupled to the supply node 45, and the potentials of the non-selected wordlines are driven toward the negative potential at supply node 45 through the transistor actuated within each driver circuit 30B-D, respectively. The potential at the control input 34A of secondary driver circuit 30A is low which deactivates the transistor of second driver 30A.

In the case of the non-selected groups 8 of wordlines the output potential on output nodes 10B-D couples the non-selected wordlines 15 to supply node 45 to maintain the negative potential existing on the non-selected wordlines.

During reset of the selected wordline to a non-select potential the secondary decode circuit 20 takes the potential of node 35A, and thus the potential of first node 32A, to the ground reference potential; nodes 35B-D are already at the ground reference potential. The first potential, which remains on the primary decode circuit's 5 output node 10A, causes the first driver circuit 25A to couple the wordline 7A to the ground reference potential at first node 32A. Following a time delay, the primary decode circuit 5 causes the potential at node 10A to switch logic states to the second potential, and the secondary decode circuit simultaneously supplies a positive potential to node 40A*. In response to the second potential now on 10A the first driver circuit 25A decouples the wordline 7A from node 32A and couples the wordline 7A to second node 33A, thereby driving the potential of wordline 7A toward the negative potential at supply node 45 and providing parallel coupling of wordlines 7B-D to the supply node 45. Of course the positive potential at node 34A activates the transistor of the second driver circuit 30A, thereby providing parallel coupling of wordline 7A to the negative potential at supply node 45. Now wordline 7A is reset.

Often a level translator 50 is interposed between each of the output nodes 10A-D of the primary decode circuit 5 and the first driver circuits 25A-D for each of the groups of wordlines 6 and 8. The level translator 50 in each of the groups of wordlines 6 and 8 drives the driver circuits 25A-D with potentials at a appropriate levels. Level translators are well known to those in the art.

For clarification it should be noted that in a precharge or standby mode secondary decode circuit 20 supplies a high potential on all of the complementary output nodes (such as 40A*-D*). This high potential on the output nodes actuates all of the second driver circuits on the memory chip to ensure the deactivation of all of the wordlines by driving the potential of all of the wordlines negative. In addition all of first driver circuits of the memory chip couple the wordlines to the negative supply node 45 in precharge.

Circuitry of a portion of the secondary decode circuit 20 and the first driver circuit 25 A is shown in FIG. 3. In order to better understand the circuitry and method of the invention the reader is encouraged to refer to and study the timing diagram shown in FIG. 4 as the circuitry and method are explained with respect to FIG. 3.

It can be seen that the secondary decode circuit 20 responds to a row address strobe (RAS) signal and an address signal RA78*, in addition to a gap phase signal which is generated internally in response to RAS and an enable row phase (ENROWPHASE) signal which is also generated internally in response to the RAS. It can also be seen that the signals PHI and PH1* are complementary signals at nodes 35A and 40A*, respectively, although there is a longer time delay from the PHI transitioning low to the PHI* transitioning high than exists when PHI* transitions low and PHI transitions high. In the later case the result is the elimination of crossing current. Efficiency increases with the elimination of crossing current. PHI swings between a positive VCC and a ground reference potential, and PHI* swings between Vcc or Vccp and a negative potential less than the ground reference potential, since these are the potentials supplied to inverters 55 and 60, respectively. Inverters 55 and 60 may include internal level translation circuitry, not shown in the Figures, but well known to those skilled in the art.

The level translator 50 responds to the potential on node 10A to generated a level translated signal TDI*, which controls the first driver circuit 25A. When TDI* is low it actuates PMOS transistor 65 and when it is high it actuates NMOS transistor 70. Typically, NMOS transistor 70 can be much smaller than the NMOS transistor in second driver circuit 30A. During selection of the wordline 7A, PHI is at a positive potential and the PMOS transistor 65 is actuated coupling the wordline 7A to first node 32A through PMOS transistor 65 to pull the potential of wordline 7A toward the positive potential of PHI. During reset PHI transitions to the ground reference potential in response to RAS; TDI* is still low; and wordline 7A is coupled to first node 32A through PMOS transistor 65 and the potential of the wordline is pulled toward the ground reference potential of PHI. As the potential of the wordline approaches the ground reference potential the source to drain potential ideally decreases to a value at which transistor 65 ceases to conduct current. When TDI* transitions high in response to RAS* transitioning high and in response to a tree decode enable (TREEDECEN) signal transitioning low, the PMOS transistor 65 is deactuated and the NMOS transistor 70 actuates to couple the wordline 7A to supply node 45 through NMOS transistor 70. In addition PHI* goes high simultaneously actuating second driver circuit 30A thereby coupling the wordline 7A to supply node 45 through the actuated NMOS transistor of second driver circuit 30A. Thus the potential of wordline 7A is pulled from the value at point 115 shown on the timing diagram in FIG. 4 toward the negative potential on supply node 45. There is zero crossing current since PMOS transistor deactivates simultaneously to the activation of the NMOS transistor 70 and the NMOS transistor of second driver circuit 30A.

FIG. 5 is a simplified logic circuit of the invention. Although the logic circuit has been shown as a NOR gate 100 it is obvious to those skilled in the art that a Negative AND gate would also work. In addition other logic gates can be used if the control signals differ from those used in this embodiment. Which ever logic circuit is used, one concept of the invention remains the same: A selected wordline at a first potential is reset to a second potential by first driving a potential of the wordline to a third potential having a value lying between values of the first and second potentials. In this embodiment the wordline is driven to the first and the third potentials in response to a first control signal and is driven to the second potential in response to a second control signal.

It can been seen by studying the timing diagram of FIG. 4 that when RAS* returns high RAS internal is delayed, see point 110 on the timing diagram, to allow sufficient time for a write back to occur. Thus a selected wordline can be reset after a time delay. The time delay is dependent on ENROWPHASE which is generated internally in response to RAS. ENROWPHASE controls a generation of interal signal GPH* which in mm controls the generation of PHI*. During reset the potential of the wordline is first coupled toward the ground reference potential, see point 115, in response to the PHI which is responsive to RAS. After a time delay, the potential of the wordline is further coupled toward Vneg, see point 125. Typically, the time delay is at least equal to the time required for the potential of the wordline to be driven to a value at which current ceases to flow in transistor 65 of FIG. 3. However even if the time delay is shorter and current still flows through transistor 65 transistor 65 is deactivated by TDI* when NMOS transistor 70 is activated by TDI*. Thus the circuit an method of the invention eliminate contention between the ground reference potential and Vneg.

It can be seen that the invention provides an efficient circuit and method for driving a potential of a wordline to a reset or a non select potential. Using the circuit and method of the invention it is no longer necessary to pump Vcc to a pumped potential although the invention does not preclude the use of a pumping circuit. By using depletion access devices in the array a complete write back of a "1" can be accomplished with the wordline at Vcc. The circuit of the invention is very efficient since there is no power loss due to crossing current when taking the potential of an activated wordline to a deactivated potential. In related art applications this crossing current typically involves a pumped supply potential (Vccp). It is well known that pumped supply potentials consume more power than non-pumped supply potentials. Thus power is conserved and efficiency is increased by using the present invention where a pumped potential is not required. There is also a possible better correlation of a Row Hammer Test Mode when the Vccp pump is not used. There is also minimal leakage from memory cells via the access devices in the circuit of the invention.