Single ended data bus equilibration scheme

1. Field of The Invention

The present invention relates generally to equilibration circuitry for a data bus, and more specifically, to a method and apparatus for equilibration of a single ended data bus.

2. Description of The Related Art

In memory devices, such as dynamic random access memory (DRAM) devices, buses are used as communications pathways linking together various modules of the system. Typically these buses include the data bus, the address bus, and the control bus. The data bus is used to transfer data. System-wide control and timing signals are required to synchronize the operation of the separate and different modules of the memory device, and facilitate their intercommunication activities over the buses. Usually a microprocessor is in control of these buses either directly or through a bus controller and the timing of events on the buses is very important.

Data on the data bus gets latched or stored. During a new data sense time the bus is driven to a known voltage level which facilitates faster data transitions, once the new data arrives. Driving a data bus line to a known voltage level can be done with an equilibration signal used to ramp up the voltage. Typically, a separate equilibration circuit is coupled to the memory device to provide an equilibration signal. This equilibration signal attempts to equalize the voltage levels on complementary pairs of input/output lines in the memory device.

FIG. 1 shows an example of a prior art data bus line system 10 for a memory device having a memory array 20 with array output drivers 21 , 22 , 23 , 24 that supply data signals on respective data bus lines 31 , 32 , 33 , 34 . Each data bus line 31 , 32 , 33 , 34 has a corresponding register 41 , 42 , 43 , 44 and corresponding output pad 51 , 52 , 53 , 54 . A clock register signal CLKREG is used to load data into each register 41 , 42 , 43 , 44 . Each register 41 , 42 , 43 , 44 typically contains a latch and an output driver. The data bus line system 10 also has an additional register 61 and a driver 71 connected to the first data bus line 31 to provide a data feedback to the array 20 .

FIG. 2 is a schematic representation of the prior art system of FIG. 1 showing some of the components of the array 20 . The array 20 contains an array core 25 , a sense amplifier 26 , a latch 27 , and an array driver 21 . A data sense amplifier clock signal CLKDSA is used to operate the sense amplifier 26 .

In a conventional system, such as that depicted in FIGS. 1 and 2, the data bus lines are relatively short for a given memory density. As the memory densities have become larger, however, the data bus lines have become relatively longer. Unfortunately, as data bus lines get longer, the operational speed of the memory device is reduced. Some conventional memory devices also include data bus lines having a pair of complementary output lines (commonly referred to as a dual ended data bus line). The dual ended data bus lines require more die size because of the large number of output lines and for this reason the industry preference is for a single ended data output.

Accordingly, there is a desire and need for a single ended data bus line with improved speed characteristics for use in high density memory devices.

The above mentioned problems are addressed by the present invention which incorporates a unique equilibration circuit for driving a bus line which may be used to output memory array data, address data or command data. In particular, the present invention provides an equilibration circuit that precharges the voltage of the data bus line to allow for faster recognition and latching of output data placed on the data bus line. The equilibration circuit precharges the data bus line to a voltage which is substantially midway of the logic signal voltage levels used to output data on the line.

Other features and advantages of the present invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.

FIG. 1 is a schematic drawing illustrating a portion of a prior art memory device data output path;

FIG. 2 is a schematic drawing illustrating the array component of the memory device portion of FIG. 1;

FIG. 3 is a graphical representation of voltage verses time for signal propagation over short and long data bus lines;

FIG. 4 is a graphical representation of data signals and their associated recognition trip points;

FIG. 5 is a schematic diagram of a data bus line system constructed in accordance with an exemplary embodiment of the invention;

FIG. 6 is a block diagram of a portion of the circuit depicted in FIG. 5;

FIG. 7 is an exemplary clock signal timing diagram in accordance with an exemplary embodiment of the invention;

FIG. 8 is a schematic diagram of a portion of the circuit depicted in FIG. 5;

FIG. 9 is a schematic diagram of a data bus line system constructed in accordance with another exemplary embodiment of the invention; and

FIG. 10 is a block diagram of an exemplary processor system with which the invention may be used.

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments of the invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.

To facilitate an understanding of the invention it is helpful to illustrate problems caused by relatively long data lines in a memory device. FIG. 3 is a graphical representation illustrating the data signal propagation time difference for a falling edge transition between a short data line and a long data line. Typical for a data bus system, data transitions between two logic levels on a data bus line. Although the data transitions are often idealized as perfectly square wave transitions, in actual practice the rising and falling edges of a data signal take a certain amount of time to occur, due in part to the propagating electrical properties of the propagating transmission line. FIG. 3, for example, illustrates a data signal having an initial value of 2.6 volts which transitions to zero volts. As can be seen from FIG. 3 it takes a longer time for the long data signal to reach the midpoint, 1.3 volts, and the final “zero” volt point. The difference in time it takes for the falling edge to reach the midpoint voltage for a long data line as compared with a short data line is represented by Δt. Thus, it takes longer for a down stream circuit to recognize a transition in a data signal (represented by a crossing of the midpoint voltage) from 2.6 volts to zero volts. The same occurs in a transition from 0 to 2.6 volts.

FIG. 4 is a graphical representation of two data signals. For example, a data bit going toward a “0” logic level is displayed as the data signal which starts at a voltage of 2.6 volts and ramps down to 0 volts. A data bit going toward a “1” logic level is displayed as the data signal which starts at 0 volts and ramps up to 2.6 volts. Referring back to FIGS. 1 and 2 each data latch 41 , 42 , 43 , 44 , will be able to recognize the data on its corresponding data bus line 31 , 32 , 33 , 34 as a logical “1” or a “0” once the voltage reaches a set trip point of the corresponding latch 41 , 42 , 43 , 44 . For example, if 1.4 volts is used as a trip point, then once the data signal rising from 0 volts to 2.6 volts reaches the latch trip point of 1.4 volts the latch will recognize and latch the data as a logical “1” databit. Likewise, if 1.2 volts is set as a trip point, once a data signal falling from 2.6 volts to 0 volts reaches the latch trip point of 1.2 volts, the latch will recognize and latch the data as a logical 0 databit.

Since today's memory and computer systems require larger densities the use of longer data lines associated with the higher densities is unavoidable. However, as evident from FIG. 3, the difference in time At for transitioning data to arrive at a particular point, e.g. the trip point of FIG. 4, becomes more of a problem. The present invention provides a method and apparatus, as will be described in more detail below, for equilibrating a bus line to a predetermined voltage level just prior to receipt of a data pulse. This provides a voltage bias on the bus line approximately at the mid point of the high and low logic level voltage. Therefore, the actual applied logic signal does not need to drive the data bus line all the way from its starting voltage level to the trip point (e.g., from 0 volts to 1.4 volts or from 2.6 volts to 1.2 volts). Instead, the logic signal only needs to drive the bus line from the equilibration voltage (e.g., 1.3 volts) to the trip point, that is to 1.4 volts for a low to high transition or 1.2 volts from a high to low transition in the example described above. As a consequence, the recognition of the data signal as a “1” or “0” data bit can be performed faster as the bus line is already biased to 1.3 volts and the logic signal will only have to make the voltage on the data bus line rise or fall 0.1 volts to reach the trip point. Therefore, the trip point is reached more quickly and this allows for faster logic signal recognition.

FIG. 5 is a block diagram of a data bus system 100 having a memory array 120 and associated output circuit, a data bus line equilibration circuit 110 , a data bus line 131 , an output register 141 and an output pad 151 . The array 120 includes an array core 125 , a sense amplifier 126 , a latch 127 , and an array output driver 121 . Register 141 is formed as a latch and driver circuit.

In one exemplary embodiment, the equilibration circuit 110 may be formed as a separate voltage generator circuit 142 which drives the bus line 131 to a voltage of 1.3 volts immediately before register 141 latches in a next data bit. This structure is shown in FIG. 9, where the clock signal CLKREQ is generated at the same time as the data sense amplifier clock signal CLKDSA. When the sense amplifier 126 senses a data bit the equilibration circuit 110 (voltage generator 142 ) is immediately enabled and line 131 is precharged to 1.3 volts before the sensed logic signal is propagated to register 141 . However, a separate voltage generator circuit 142 may be undesirable in terms of size, and power consumption.

FIG. 6 illustrates another exemplary embodiment of the equilibration circuit 110 . The equilibration circuit 110 contains a switch 111 , a pair of reversely connected inverters 114 , 116 and an output driver 117 . A feedback line 118 extends from the output of driver 117 back to the input switch 111 . Further an equilibration clock signal CLKDREQ is used to simultaneously disable the switch 111 and enable the driver 117 and vice versa.

In operation, as seen in FIGS. 5 and 6, the sense amplifier clock signal CLKDSA powers the sense amplifier 126 , which senses a data bit from the array core 125 and provides the bit to the latch 127 . The bit is subsequently sent down the data bus line 131 through the array driver 121 .

The equilibration clock signal CLKDREQ is generated at the same time as the data sense amplifier clock signal CLKDSA. Therefore, when the sense amplifier 126 senses a data bit, the equilibration clock signal CLKDREQ disables the switch 111 and enables driver 117 . The previous data signal on line 131 passes through the switch 111 before switch 111 is disabled by CLKDREQ and is stored by the latch created by inverters 114 , 116 . This occurs when the signal CLKDREQ is not present. Then, when the signal CLKDREQ appears, the driver 117 will take the signal stored in latch 114 , 116 and begin, on the rising edge of CLKDREQ, driving bus line 131 from the signal in the latch toward its opposite logic state. The duration of CLKDREQ is selected such that driver 117 is then disabled when the output voltage of driver 117 is approximately mid-way of the logic levels for a “1” and “0” logic signals which is 1.3 volts in the example given.

For example, assuming a data voltage signal of 2.6 volts representing a logic “1” entered the equilibration circuit 110 through switch 111 and is stored in latch 114 , 116 , the entered logic “1” data signal is inverted by the first inverter 114 to a logic “0” data signal. The logic “0” data signal is then sent to both the second inverter 116 and the driver 117 . The signal sent to the second inverter 116 is inverted to a logic “1” and holds the latch formed by the two inverters 114 , 116 in a stable state. The logic “0” signal at the input to driver 117 causes driver 117 to drop the logic “1” output voltage on line 131 towards a “0” output voltage under control of the CLKDREQ signal. Driver 117 stops trying to drive the signal on line 131 to a “0” level when the CLKRDEQ signal disappears and a voltage of 1.3 volts is achieved. The timing for enabling the driver 117 to produce a voltage of 1.3 volts on line 131 is controlled by varying the pulse width of the equilibration clock signal CLKDREQ. That is, CLKDREQ will disable driver 117 at a time when the signal on line 131 is approximately 1.3 volts. The 1.3 volts is then used to bias the bus line 131 for the next arriving logic signal. FIG. 7 shows a bus line 131 which is initially at logic “1” (e.g. 2.6 volts) and is driven toward the “0” logic level output by device 117 stopping at 1.3 volts under control of signal CLKREQ. FIG. 7 also shows the operation of the FIG. 6 circuit when the initial logic signal on line 131 is at a logic “0” level in which driver 117 drives the voltage on line 131 toward a logic level “1” , stopping at approximately 1.3 volts, again under control of the CLKDREQ signal.

The bias voltage produced by device 117 on bus line 131 cannot pass through switch 111 which is disabled when the signal CLKDREQ is present. However, after the signal CLKDREQ disappears, a subsequent data signal on line 131 from driver 121 (FIG. 5) passes through switch 111 and is latched in latch 114 , 116 for the next cycle of operation of driver 117 .

The register 141 (FIG. 9) is set with predetermined trip points, typically 1.2 volts as a low trip point and 1.4 volts as a high trip point for a logic signal that transitions between 0 and 2.6 volts. As the data bit signal increases or decreases from the biased voltage of 1.3 volts it will reach the trip points in a shorter time than it would take for the signal to rise or fall from the typical starting point of 0 or 2.6 volts, which allows the register 141 to more quickly determine if the data bit signal represents 20 a logical “1” or “0” . The register 141 will correspondingly latch and drive the data bit on line 131 to the proper location on the pad 151 .

Referring again to FIG. 7, a system clock signal CLK, the equilibration clock signal CLKDREQ and data sense amplifier clock signal CLKDSA are illustrated. The equilibration clock signal CLKDREQ is used to disable/enable the switch 111 and correspondingly enable/disable circuit driver 117 . The equilibration clock signal CLKDREQ is timed to operate in conjunction with a rising edge of the data sense amplifier clock signal CLKDSA. Therefore, the driver 117 is only enabled or powered when the sense amplifier 126 senses a new data bit or signal.

FIG. 8 is a circuit diagram for an exemplary circuit that implements the FIG. 6 equilibration circuit 110 . The circuit contains two transmission gates 112 , 113 , three inverters 114 , 115 , 116 , a NAND gate 202 , a NOR gate 204 , and a CMOS driver formed by a p-type transistor 206 and an n-type transistor 208 . The two transmission gates 112 , 113 form the switch 111 illustrated in FIG. 6 . The NAND gate 202 , NOR gate 204 and transistors 206 , 208 form the driver 117 illustrated in FIG. 6 . The equilibration clock signal CLKDREQ is used to enable/disable the two transmission gates 112 , 113 and to disable/enable the driver 117 . Data from the array 120 (FIG. 5) enters the equilibration circuit 110 at node 211 .

In operation, the logic signal from the array core 125 is latched by the latch 127 , see FIG. 5, sent down the bus line 131 and enters the equilibration circuit 110 at node 211 . The logic signal is transmitted by the feedback line 118 to the first transmission gate 112 . The first transmission gate 112 is disabled by the equilibration clock signal CLKDREQ. At the same time, equilibration clock signal CLKDREQ enables the second transmission gate 113 , NAND gate 202 and NOR gate 204 . Once a logic signal on line 131 passes through the first transmission gate 112 , the signal is inverted by the first inverter 114 . The inverted signal then propagates to a both the second inverter 116 and the NAND 202 and NOR 204 gates.

When the first transmission gate 112 is enabled and opened, the second transmission gate 113 is closed. When the equilibration clock signal CLKDREQ is off, the first transmission gate 112 is open and the second transmission gate 113 is closed, thereby forming a storage latch with the first and second inverters 114 , 116 , that stores the voltage of the last data signal.

The NAND 202 and NOR 204 gates, in conjunction with the p-type transistor 206 and n-type transistor 208 , form the driver 117 depicted in FIG. 6 . The NAND 202 and NOR 204 gates are controlled by the logic level of the equilibration clock signal CLKDREQ. The NAND 202 gate is enabled by the equilibration clock signal CLKDREQ. The third inverter 115 is used to invert the equilibration clock signal CLKDREQ. This inverted signal is used to enable the NOR gate 204 and the transmission gates 112 , 113 .

Based upon the voltage of the data signal received and the voltage of the clock signal received, the NAND 202 and NOR 204 gates will enable the transistors 206 , 208 . The transistors 206 and 208 will then act as a driver to drive the biased voltage signal (e.g., 1.3 volts in the example given)the data bus line 131 through node 211 .

Although the invention has been described in the environment of output data lines of memory device, the invention can be used in any digital device which has data lines for transporting data signals from one location to another.

The invention may be used in a processor system. FIG. 10 is a block diagram of a processor system 300 constructed in accordance with one of the embodiments of the present invention. The processor system 300 may be a computer system, a process control system or any other system employing a processor and associated memory. The system 300 includes a central processing unit (CPU) 302 , e.g., a microprocessor, that communicates with a RAM 312 and an I/O device 308 over a bus 320 . It must be noted that the bus 320 may be a series of buses and bridges commonly used in a processor system, but for convenience purposes only, the bus 320 has been illustrated as a single bus. A second I/O device 310 is illustrated, but is not necessary to practice the invention. The processor system 300 also includes read-only memory (ROM) 314 and may include peripheral devices such as a floppy disk drive 304 and a compact disk (CD) ROM drive 306 that also communicates with the CPU 302 over the bus 320 as is well known in the art. The invention as described above with reference to FIGS. 5-9 may be used in any device connected to a data line of the system 300 .

While the invention has been described and illustrated with respect to exemplary embodiments, it should be apparent that many modifications and substitutions may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to considered as limited by the forgoing description, but is only limited by the scope of the appended claims.