A NMOS DATA STORAGE CELL AND SHIFT REGISTER

Abstract: An improved NMOS storage cell for use in shift registers is disclosed. Among other components, it contains a pair of inverters (220, 225 and 230, 235) one of them an enabling inverter (230, 235). A pre-charge transistor (215) is placed in parallel with the first inverter (220, 225) to decrease the rise time associated with the transition from a logic low level output to a logic high level output. The result of adding the pre-charge transistor (215) to the circuit is to increase the speed of operation of the storage cell, without the accom­ panying decrease in density with prior are methods, where the components must be enlarged. Another aspect of the present invention which further increases the density of the cell is the elimination of the complement clock line found in many prior art storage cells. The previous combination of a second inverter and a pass transistor connected to a comple­ ment clock line, is replaced by the enabling inverter (230, 235) connected directly to the clock line (210).

Description: This invention relates generally to a method and apparatus for computer memory and more specifically to shift registers using n-channel metal-oxide semiconductor (NMOS) technology. Computers and computer systems require large memory storage capacity, as well as high speed movement of data in the form of binary digits in their operation. A shift register is a component which can shift data in the storage cells (typically, one cell can store one binary digit) sequentially from one group of addresses (locations) to another. NMOS shift registers use n-channel MOS field effect transistors to control the movement of data. One approach in prior art storage cells is to connect two metal-oxide semiconductor field-effect transistors (MOSFETs) to form an inverter. Two inverters are connected in series with input and output (pass) transistors, to control the input and output of data. Once the data is introduced, the back to back pair of inverters retains the signal (alternating in value between a binary 1 and a binary 0 as it passes through each inverter) indefinitely until the output gates transfer the existing signal. The input and output pass transistors are controlled by signals from one clock line. Another approach is to arrange the components similarly to the above, but add a disabling pass transistor between the pair of inverters. The disabling transistor is connected to a line having the complement of the clock line. Some problems are encountered when these approaches are used, inter alia, the former approach has inferior speed qualities, while the latter approach has a lower density than desired. The lower density is generally produced because the size of the transistors comprising the inverters must be increased to obtain added speed. In addition to density decreases due to larger transistors, the latter approach generally has one more input line than is desired. ELEKTRONIK, RT-87, vol. 18, no. 2 Feb. 1969. pages 47-52, describes various MOS data storage cells. In particular, figure 13(b) of the article shows a cell similar to that in Fig. 2, except for the lack of pass transistor 125. IBM TECHNICAL DISCLOSURE BULLETIN, vol, 16, no. 10, March 1974, pages 3261-3262, describes an MOS data storage cell that can be used in a static mode and in a dynamic mode. This cell is controlled by a set of four clocking signals. The inverter drive transistors of the cell are controlled by clock signals which are connected to the sources of these transistors. Increase in speed, which generally lead to decreases in density, ultimately lead to increases in power consumption. Accordingly, a principal advantage of the invention as defined by the appendant claims is a NMOS data storage cell for use in clocked shift registers in such a manner that the overall speed of operation is increased. It is an advantage of the invention that the NMOS data storage cell for use in clocked shift registers has an overall density which is increased. A further advantage of this invention is a decrease in overall power consumption of NMOS data storage cells used in clocked shift register applications. The foregoing and other advantages of the claimed invention are realized by replacing the series combination of an inverter and a series pass transistor by an enabling inverter, which is connected to, and controlled by, the clock signal. The line transporting the complement of the clock signal is eliminated. The foregoing and other advantages and the features of the present invention are apparent from the specification, the drawings and the two taken together. The drawings are: Figure 1 is a schematic diagram of a prior art Random Access Memory (RAM) type storage cell which could be used in clocked shift registers. Figure 2 is a schematic diagram of a prior art storage cell consisting of a static latch, also used in clocked shift registers. Figure 3 is a schematic diagram of the present invention. Figure 1 shows a typical NMOS RAM storage cell. However, it is not an efficient configuration for storage cells used in unidirectional transfer of data, such as for use in a shift register. Figure 2 shows a more efficient configuration for a NMOS storage cell for use in a shift register. Data is entered at input 100. Clock line 105 controls the input gate (a MOSFET pass transistor), 110.. The input signal is passed through gate 110 and then through an inverter comprised of transistors 115 and 120. Transistor 115 is a "pull-up" transistor; it tends to pull the output of gates 115 and 120 toward the high voltage level(+B) when gate 120a is not conducting, essentially functioning as a resistor. Gate 120 is a "pull-down" transistor; it tends to drive the output voltage 145 of the inverter toward the low voltage level, e.g. ground, when it is conducting. When a high voltage level input is applied to the gate 120a of transistor 120, it conducts, thereby causing the output 145 of the inverter to be at the low voltage level (usually about 0 volts). When a low voltage level input 100 is applied to transistor 120, it is disabled, leaving transistor 115 to affect the output voltage, which results in a high voltage level at output 145 (usually about 5 volts). The input signal 100 is inverted by the transistor pair 120 and 115. The inverted signal is fed back to the gate 120a of transistor 120, i.e. back to the input of the first inverter through another inverter comprised of transistors 135 and 140, which act identically as the transistor pair 115 and 120. However, the signal 145 does not pass from the output of this second inverter unless pass transistor 125 is enabled. Pass transistor 125 is enabled or disabled by a signal 130, which is the complement of the clock signal 105. Therefore, the input signal 100 is stored in the cell of Figure 2 at the output 145b of the second inverter, i.e. at the drain of the transistor 140. The complement of the input signal 100 is stored at the output 145a of the first inverter. The present invention is in improvement over the storage cells diagramed in Figures 1 and 2, and is graphically illustrated in Figure 3. It will thus be described in more detail than the prior-mentioned storage cells. All transistors discussed are of the N-MOSFET type. Turning to Figure 3, data is entered into the storage cell at data input terminal 200, in the form of logic high level and logic low level voltages, and only one voltage level can be stored in the cell at a given time. As in Figure 2, the data is stored at the input of a first inverter made from the transistor pair 220 and 225. The input of the data is controlled by pass transistor 205, which is enabled and disabled by clock 210. The logic levels are typically 5 volts or 0 volts, each level, representing one binary digit (bit) of data. It is characteristic of enhancement mode MOSFETs, such as pass transistor 205 that current flows between the drain and the source when the voltage between the gate and the source is greater than some small positive threshold level -- typically 1 volt. By applying a logic high level pulse (typically 5 volts) to the gate of pass transistor 205 from clock 210, an input signal from input terminal 200 can be passed. Pass transistor 205 remains on as long as the input at its gate from clock 210 is at the logic high level. Pre-charge transistor 215 tends to rapidly drive or maintain the output voltage at output 245 at the logic high level (+B). Transistor 215 is enabled or activated when clock 210 sends a logic high level to its gate, thus causing current to flow from its drain to its source. The drain of pre-charge transistor 215 is connected to +B, a 5 volt power supply, so that a 5 volt signal is passed by transistor 215 when it is enabled. The entered data goes to the gate of transistor 220, which is enabled if the entered data is at the logic high level. Transistor 220, when conducting, pulls the output of transistors 220 and 225 to the logic low level when it is enabled, thus making it a "pull-down" transistor. Transistor 225 serves as a "pull-up" transistor, or more correctly, a "pull-up resistor": it serves to hold the voltage at output 245 at the logic high level when transistor 220 is not conducting. It is a depletion mode type MOSFET, so that it is enabled when the voltage between its source and its gate is greater than some small negative threshold level (e.g., -1 volts). Since the gate and source are tied together, transistor 225 is always enabled, and effectively, the transistor functions as a resistor. The drain of transistor 225 is connected to a 5 volt (+B) power supply, so that a 5 volt signal is at output 245 when transistor 220 is disabled, and a zero or near zero volt level is at output 245 when transistor 220 is enabled. The current flow through MOSFETs is directly proportional to the channel width divided by the channel length. Compared to transistors 225 and 215, this ratio for transistor 220 is greater, thus causing transistor 220 to switch the voltage at output 245 to the logic low level when transistor 220 is enabled. When transistor 220 is disabled, the voltage at output 245 rapidly goes from the zero level to the logic high level (+B), with the aid of pre-charge transistor 215, which causes +B volts to appear at the output 245 shortly after transistor 215 is enabled by clock input 210. If transistor 215 is not present in the circuit, the rise time for +B to appear at output 245 primarily depends on the size of pull-up transistor 225, and is thus longer because of size restrictions due to power consumption. The rise time is the time necessary for the output voltage 245 to rise to +B volts from approximately 0 volts after transistor 220 is disabled. In other words, transistor 215 dramatically decreases the rise time to +B when transistor 220 is disabled. Summarizing the operation of transistors 215, 220 and 225, a low logic level input signal at the gate of transistor 220 is converted to the logic high level at output 245, while a high logic level input is converted to a low logic level signal at output 245. Pre-charge transistor 215 decreases the time necessary to convert a logic low level input signal at transistor 220 to a logic high level signal at output 245. The time required to invert a logic high level input is approximately the same as for the circuit of Figure 2. Thus, the result of passing data through transistors 220 and 225 is to invert it at a higher speed than in prior art data storage cells, such as diagramed in Figure 2. At the same time that clock 210 activates pass transistor 205, it inactivates pass transistor 230, which is part of a second inverter comprised of pull-down transistor 230 and pull-up transistor 235. The second inverter acts almost identically as the first inverter comprised of transistors 220 and 225. However, the transistor pair 230 and 235 act as an inverter only when the clock input 210 is at a logic low level. During the clock time, the gate to source bias of transistor 230 is incorrect for the transistor pair to operate as an inverter. Clock input 210 is connected to the source of transistor 230, resulting in disablement whenever the input from clock 210 is at a logic high level. The threshold voltage for transistor 230 is never reached during this cycle of the clock, since the voltage at the gate of transistor 230 can at best equal the logic high level. In the preferred embodiment transistor 230 has a high impedance (the ratio of the channel width to the channel length is small) to isolate the gate of transistor 220 from the clock line 210, such that loading effects are minimized when multiple storage cells are driven in parallel from a common clock driver, as in a register array. Between the high levels of the clock 210, transistors 230 and 235 act as an inverter, inverting the signal from output node 245. The input to the gate transistor 220 between clock pulses is held at the level of the initial input at transistor gate 220. Therefore, the data is "latched" into the storage cell at the gate of transistor 220 until new data of a different logic level is entered. Only one clock input line, rather than a clock line and a clock complement line taught by the prior art, needs to be routed through each cell, thus permitting a higher packing density for the present storage cell because an inverter transistor pair is not needed to generate the clock complement. Pre-charge transistor 215 may be very small and still serve its speed increasing function, so that no appreciable density penalty results. The prior art teaches increasing the size of the transistors serving the inverting functions of 220, 225, 230 and 235 of the storage cell to increase the operating speed, resulting in a noticeable density penalty. The present invention can achieve higher speeds without the use of larger transistors, thus resulting in a higher packing density for the storage cell. Another advantage of the present invention is a lower power consumption than typical prior art NMOS storage cells, such as shown in Figure 2. Since pull-up transistors 225 and 235 function only to retain a logic high level voltage under static conditions, the ratio of channel width to channel length for these transistors is very small, and power consumption is minimized. Additionally, because transistors 220 and 215 are both enhancement type MOSFETs, changes due to process parameter (or manufacturing) variations will not affect transistor drive ratios and, hence, output voltage characteristics. Modifications and variations of the foregoing will occur to those skilled in the art, and as such, are encompassed within the scope of this invention. For example, the second inverter comprised of transistors 230 and 235 of Figure 3 can be substituted for the second inverter comprised of transistors 135 and 140, and pass transistor 125 in Figure 2.

Claims: A NMOS data storage cell comprising clocking means input (210) for inputting a timing signal, data input means (200) for inputting data to the data storage cell, a first pass transistor (205) connected to the clocking means input (210) to allow the input of data from the data input means (200) when the signal of the clocking means input (210) is at a logic high level and to prevent the input of data when the signal of the clocking means input (210) is at a logic low level, a first inverter (220, 225) connected to the first pass transistor (205) and to the output (245) of the data storage cell, for inverting input data received at the input of the inverter (220, 225) from the first pass transistor (205), and a second inverter (230, 235) including a transistor pair connected at its input to the output (245) of the data storage cell and at its output to the input of the first inverter (220, 225), for inverting data received at its input from the output (245) of the data storage cell, characterized in that the second inverter's (230, 235) drive transistor (230) has the source connected to the clocking means input (210), so that the second inverter (230, 235) is enabled to pass data when the signal of the clocking means input (210) is at a logic low level, and is disabled to prevent the passage of data when the signal of the clocking means input (210) is at a logic high level. A NMOS data storage cell according to claim 1, characterized in that a pre-charge transistor (215) is connected at its gate to the clocking means input, at another of its leads to the output of the data storage cell, and at the third of its leads to the logic high level voltage, for pre-charging the output (245) of the storage cell to the logic high level voltage when the timing signal of the clocking means input (210) is a logic high level voltage. A shift register for unidirectional propagation of data comprising a plurality of NMOS data storage cells of claims 1 or 2.

Inventors: BESSOLO, JEFFREY M., WOLF, MICHAEL A.
Applicants: WANG LABORATORIES INC.
Application year: 1986
Main classification: G11C19
IPC codes: G11C11, H01L27, H01L21, G11C19, H03K3