Patent Publication Number: US-7716619-B2

Title: Design structure for implementing dynamic data path with interlocked keeper and restore devices

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation in Part of U.S. patent application Ser. No. 11/278,169 filed on Mar. 31, 2006, now U.S. Pat. No. 7,307,457. 

   BACKGROUND 
   The present invention relates generally to design structures for dynamic data paths and, more particularly, to a design structure for implementing dynamic data paths with interlocked keeper and restore devices. 
   Dynamic logic is one type of circuit design approach that is used to increase digital circuit speed, as compared to static complementary metal oxide semiconductor (CMOS) logic, for example. A CMOS gate is a fully complementary logic gate using both p-type and n-type devices configured to implement a desired logic function (e.g., a simple inverter). Static CMOS logic gates require large fan-in, which in turn causes large gate input capacitances that slow down the logic circuit. Furthermore, static logic gates use relatively slow p-type metal oxide semiconductor (PMOS) devices to implement a pull-up network, which further increases the capacitance of the gate input and slows rise times. 
   In dynamic logic circuits, a PMOS pull-up network is replaced by a single clocked PMOS transistor. Each clock cycle is divided into two phases, a “precharge” phase and an “evaluate” phase. During the precharge phase, an output node is unconditionally precharged to a high logic state. Then, during the evaluate phase, the output node either remains high or is conditionally discharged to low, depending on the current logic output level. The logic function is implemented by a network of n-type pull down transistors, which are controlled by their respective gate inputs in order to either maintain or discharge the voltage at the output node. 
   A transition period corresponds to the time between when the node is precharged and when the input signal is evaluated. During this time, the node may not be driven by any component but is instead “floating.” As a result, the node is susceptible to leakage paths through the pull down devices, which could result in an erroneous evaluation of the input signal during the subsequent evaluation period. Thus, one existing approach to preventing false evaluations is the use of a “keeper” circuit that utilizes a pull-up transistor to maintain the precharge node at a desired logic level during the transition period. In this approach, the pull-up PMOS transistor of the keeper is coupled between the supply voltage source and the precharged node, with the gate of the transistor being coupled to the inverted node voltage. When the precharge node is precharged high, the inverter provides a logic low signal to render the keeper PMOS transistor conductive and thus maintain the precharge node high even after the precharge phase is complete. 
   Unfortunately, the increased leakage current associated with scaled technologies has forced designers to increase the size of the keeper devices in dynamic circuits to maintain nodes at the precharge state. In addition, the evaluation stacks must be made with longer device lengths or higher voltage thresholds to substantially reduce the leakage. Conversely, if the keeper is too strong, the pull down devices will have greater difficulty pulling down the output node during an evaluation phase. As such, the resulting slower performance of dynamic circuit topologies (as compared to static circuits) would arguably no longer justify the implementation of the dynamic logic. Further, self-resetting techniques typically employed for dynamic data paths are becoming very difficult to control because of the larger device variations and poor control of the evaluation and precharge periods. Accordingly, it would be desirable to be able to provide an improved dynamic data path that maintains acceptable levels of robustness. 
   SUMMARY 
   The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a keeper design structure for dynamic logic, including a first keeper path statically coupled to a dynamic data path, the first keeper path configured to prevent false discharge of the dynamic data path during an evaluation thereof, and a second keeper path selectively coupled to the dynamic data path. The second keeper path is configured to maintain the dynamic data path at a nominal precharge level prior to an evaluation thereof, wherein the second keeper path is decoupled from the dynamic data path during the evaluation. 
   In another embodiment, a memory design structure includes a dynamic evaluation circuit coupled to data outputs of each of a plurality of memory subarrays, and a keeper device having a first keeper path statically coupled to a dynamic data path associated with the dynamic evaluation circuit, the first keeper path configured to prevent false discharge of the dynamic data path during an evaluation thereof. The keeper device also has a second keeper path selectively coupled to the dynamic data path, the second keeper path configured to maintain the dynamic data path at a nominal precharge level prior to an evaluation thereof. The second keeper path is decoupled from the dynamic data path during the evaluation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
       FIG. 1  is a schematic diagram of a computer memory subarray having a dynamic data path configured with conventional restore and keeper circuitry; 
       FIGS. 2(   a ) through  2 ( c ) are waveform timing diagrams illustrating the performance of the dynamic data path of  FIG. 1 , under various process and operating conditions; 
       FIG. 3  is a schematic diagram of a computer memory subarray having a dynamic data path configured with interlocked restore and keeper devices, in accordance with an embodiment of the invention; 
       FIGS. 4(   a ) through  4 ( c ) are waveform timing diagrams illustrating the performance of the inventive dynamic data path of  FIG. 3 , under various process and operating conditions; and 
       FIG. 5  is a flow diagram of a design process used in semiconductor design, manufacturing, and/or test. 
   

   DETAILED DESCRIPTION 
   Referring initially to  FIG. 1 , there is shown a schematic diagram of a computer memory subarray  100  having a dynamic data path  102  configured with conventional restore and keeper circuitry. In the example illustrated, the subarray includes a subarray control block  104 , a local wordline driver  106 , and an array of memory cells MC (such as SRAM cells for example) each coupled one of a plurality of wordlines WL_ 0  through WL_n, and a pair of true/complement bitlines BLT_ 0 /BLC_ 0  through BLT_n/BLC_n. The memory subarray  100  also includes a mimic path (i.e., mimic wordline MWL, dummy memory cells and mimic bit lines) for generating sense amplifier SET timing signals and restore timing signals for the subarray controls  104  through a variable delay block  108 . The restore timing of the subarray  100  is independent of the restore timing of the wordline addresses (WLADD) and subarray addresses (SUBADD), and is locally generated by the “RESTORE” signal after a sense amplifier  110  is set. The sense amplifier  110  latches data from a selected one of the memory cells MC of the subarray  100 . This sensed data (DSUB) from the subarray  100  is self-resetting and drives a dynamic OR configuration. 
   More specifically, the dynamic OR configuration includes a series of pull down NFET transistors N 0  through Nn, wherein each NFET corresponds to the sensed data of a different subarray (only one subarray being shown in  FIG. 1 ). As data from several subarrays are coupled to the dynamic OR configuration and thus the data path  102 , the series resistance and capacitance of the path is also schematically depicted in  FIG. 1 . In addition, specific locations along the data path are also designated: D 1  NEAR, D 1  MID, D 1  FAR.  FIG. 1  further illustrates a second (or more) data path stage D 2  NEAR, etc. coupled to the inverted output of the first stage prior to the output signal DOUT. 
   In order to precharge and restore the data path  102  to a logic high level, a delay chain  112  is used. The delay chain  112  includes an even numbered configuration of inverter stages, the output of which is coupled to a pull up PFET P 0 . Thus, when the data path  102  is pulled low by one of the pull down NFETs during an evaluation phase, the corresponding low output of the inverter stages will subsequently cause P 0  to restore the data path back to a logic high voltage value once the signal propagates through the delay chain. The number of inverter stages is selected so as to provide sufficient delay for a pull down of the data path  102  to be registered before being restored to V DD . 
   As stated previously, the leakage current (I off ) associated with each of the NFET pull down paths could be sufficient to cause a false evaluation. Thus, a keeper device  114  is also provided to maintain the data path  102  at a sufficiently high level during a transition period. In the conventional keeper configuration of  FIG. 1 , an inverter  116  (having the input thereof coupled to the data path  102 ) drives a single PFET P 1 . However, in order for the keeper device  114  to properly function as intended, at least two key requirements must be met. First, the on current (I on ) of the PFET keeper transistor P 1  with a weak PFET process corner must be greater than the sum of all the NFET I off  paths for a strong NFET process corner. Conversely, the selected (evaluate) NFET must be able to overcome the I on  of the keeper PFET P 0  in a weak-NFET, strong-PFET corner. The challenges of making this topology workable are further illustrated in the waveform diagrams of  FIGS. 2(   a ) through  2 ( c ). 
   The waveform diagrams of  FIGS. 2(   a ) through  2 ( c ) depict a simulation, in 65 nm SOI (silicon on insulator) technology, of various node voltages of an eight-input NFET dynamic OR scheme (i.e., eight subarrays sharing the same data-bus). In particular, there are three cases illustrated: a nominal fabrication/operating condition case in  FIG. 2(   a ), a weak PFET/strong NFET fabrication at elevated operating temperature and higher V DD  voltage in  FIG. 2(   b ); and a “best case” fabrication at a more slightly elevated operating temperature and higher V DD  voltage in  FIG. 2(   c ). The curves depict the various node voltages shown in  FIG. 1 , including DSUB (the data from the subarray), which drives the two stages of the data-bus  102 , D 1  MID and D 2  NEAR. The curve for the voltage at the final output DOUT is also shown. Of particular interest in the simulation curves is the voltage at D 1  MID, which reflects the performance of the dynamic data path during an evaluate condition. 
   Referring first to  FIG. 2(   a ), the simulation therein illustrates the slope change of the voltage at node D 1  MID as the evaluate device attempts to overcome the pull up action keeper device. As is shown, there is only a slight “kink” in the degradation of the D 1  MID node from logic high to logic low. Thus, under “nominal” device processing results and operating conditions (e.g., 85° C., 1.0 volts), the performance of the dynamic data path with conventional keeper and precharge circuitry is, for the most part, acceptable. However, as shown in  FIG. 2(   b ), the results are quite different under non-ideal device characteristics and operating conditions. First, it will be noted that as a result of the keeper PFET being relatively weak with respect to the leakage of the NFETs, the voltage value maintained at D 1  MID (about 1.1 volts) is slightly less than the nominal value of V DD  (1.2 volts). This becomes more pronounced at the second stage of the data path, as the precharge voltage at D 2  NEAR is further decreased to about 0.9 volts. In addition, due the elevated operating temperature of the device, a more pronounced kink is seen in the discharging D 1  MID node, which result in a slightly slower performance than in the nominal case of  FIG. 2(   a ). 
   In  FIG. 2(   c ), the keeper strength is now relatively high, and so the precharge levels of D 1  MID and D 2  NEAR are better than in  FIG. 2(   b ), although still not quite at the nominal 1.2 volt value of V DD . However, because the keeper device is strong in this instance, the time taken for D 1  MID to discharge is visibly longer. This reflects the difficulty of the dynamic NFETs in overcoming the pull of keeper device. It will thus be realized from  FIGS. 2(   a ) through  2 ( c ) that the conventional keeper scheme is not robust at scaled technologies, due to the effect that variations in device dimensions or operating conditions have on performance. 
   Therefore, in accordance with an embodiment of the invention,  FIG. 3  is a schematic diagram of a computer memory subarray  300  having a dynamic data path  302  configured with interlocked restore and keeper devices. Moreover, the present keeper scheme employs a hybrid keeper topology that utilizes a first (weak) non-gated keeper path and a second (strong) gated keeper path. The strong, gated keeper is disabled prior to the arrival of data using a subarray interlock such that the evaluate pull down path no longer has to overcome a strong keeper device, thereby achieving a performance gain. In addition, the restore operation of the data path in the present embodiment is implemented from the restore signal of the local subarray. This eliminates self-resetting timing variations across the data path, and is particularly helpful in maintaining well controlled data pulse widths across several cascoded data path stages. 
   As more specifically shown in  FIG. 3 , the memory array portion of the subarray  300  is substantially the same as shown in  FIG. 1 . Thus, similar elements are designated using the same reference numerals for purposes of clarity. As will first be noted, the conventional inverter/single PFET keeper device of  FIG. 1  is now replaced with a dual path keeper device  304 . Although the keeper device  304  is shown repeated at various locations along the dynamic data path  302  (e.g., at D 1  NEAR, D 1  MID, D 1  FAR) to provide improved precharge timing and levels across the wire, only one instance thereof is described. A first keeper path includes inverter  306  and weak PFET P 2 . As this path is not gated (i.e., not selectively enabled), PFET P 2  will still prevent a false evaluation during an evaluation stage. Because the strength of P 2  is relatively weak with respect to the leakage of the evaluate NFETs, it will not cause performance issues by strongly opposing a pull down of the output node during an evaluation stage. 
   Moreover, the problem associated with weak keeper PFETs (i.e., the inability to maintain the node at the proper precharge level following a restore operation and prior to an evaluate operation) is overcome, since a second (strong) keeper path is also enabled during this time. The second keeper path includes inverter  306  and PFET P 3  in series with gated PFET P 4 . When enabled, the resulting pull up strength of the combination of P 3  and P 4  is greater than the pull up strength of P 2 , and may be (for example) on the order of about 10 times as strong. 
   As further illustrated in  FIG. 3 , the gated PFET P 4  of the keeper device  304  is triggered by the output of a static OR gate  308  that receives the SET signals sent to the sense amplifiers in each of the subarrays utilizing the data path  302 . Thus, the strong keeper path is interlocked through and deselected with the SET signals provided to the respective sense amplifiers. Because the arrival of the SET signal precedes the arrival of data on a DSUB data line, the pull down NFETs of the dynamic evaluate device are not opposed by the strong, gated keeper path. In other words, the dynamic evaluate device need only overcome the weak keeper PFET P 2 . Again, the weak keeper PFET P 2  prevents an unselected data path from significantly discharging during the brief period that a SET signal disables the strong keeper path. In addition, a cross-coupled PFET pair may also be used to prevent discharge of unselected data path as shown in the dashed portion  310  of  FIG. 3 , which would utilize a complementary data path topology. Another advantage of the interlocking of the strong keeper path with the SET signal is that by preventing the disabling of the strong keeper device until the SET signal is generated, the data path is prevented from being prematurely evaluated. 
   As also discussed above, the conventional restore circuit  112  of  FIG. 1  is replaced by pull up PFETs P 5  that are interlocked with the RESTORE signals of the subarrays. In this manner, any mistrack in data bus pulse widths with respect to subarray restore timings is eliminated. In the embodiment of  FIG. 3 , the RESTORE signals of the subarrays are each fed into a static NOR gate  312 , the output of which is connected to the gate(s) of the restore PFET(s) P 5 . As is the case with the dual path keeper device  304 , the restore device may also be repeated along various locations of the data path  302  to improve precharge performance and timing. 
   Finally,  FIGS. 4(   a ) through  4 ( c ) are waveform timing diagrams illustrating the improved performance of the inventive dynamic data path of  FIG. 3 , under various process and operating conditions. In  FIG. 4(   a ), the waveforms of the various nodes under nominal process and operating conditions demonstrate a performance improvement of about 16% with respect to the conventional keeper device. In other words, the registration of the output signal on DOUT is quicker with the present keeper topology. It will also be noted that the decaying voltage on the data path (node D 1  MID) is much smoother than for the conventional device of  FIG. 1 . 
   As illustrated in  FIG. 4(   b ), the performance improvement is even more significant (about a 44% timing improvement) over a conventional device having a weak keeper PFET. In addition, where the NFETs of the present configuration are formed relatively strong with respect to the weak keeper path, they do not have the same adverse impact on the precharged voltage level as in the case of  FIG. 2(   b ), due to the activated strong keeper path prior to SET activation. In  FIG. 4(   c ), the decay of the node voltage at D 1  MID signal is shown for both the present embodiment of  FIG. 3  and the conventional device of  FIG. 1  (old) to illustrate the improvements in performance and signal integrity. Whereas D 1  MID (old) exhibits a significant kink, the waveform D 1  MID demonstrates a smooth, rapid decay. 
     FIG. 5  shows a block diagram of an example design flow  900 . Design flow  900  may vary depending on the type of IC being designed. For example, a design flow  900  for building an application specific IC (ASIC) may differ from a design flow  900  for designing a standard component. Design structure  920  is preferably an input to a design process  910  and may come from an IP provider, a core developer, or other design company or may be generated by the operator of the design flow, or from other sources. Design structure  920  comprises inventive circuit of which  FIG. 3  is an embodiment in the form of schematics or HDL, a hardware-description language (e.g., Verilog, VHDL, C, etc.). Design structure  920  may be contained on one or more machine readable medium. For example, design structure  920  may be a text file or a graphical representation of circuit  100 . Design process  910  preferably synthesizes (or translates) circuit  100  into a netlist  980 , where netlist  980  is, for example, a list of wires, transistors, logic gates, control circuits, I/O, models, etc. that describes the connections to other elements and circuits in an integrated circuit design and recorded on at least one of machine readable medium. This may be an iterative process in which netlist  980  is resynthesized one or more times depending on design specifications and parameters for the circuit. 
   Design process  910  may include using a variety of inputs; for example, inputs from library elements  930  which may house a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a given manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.), design specifications  940 , characterization data  950 , verification data  960 , design rules  970 , and test data files  985  (which may include test patterns and other testing information). Design process  910  may further include, for example, standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. One of ordinary skill in the art of integrated circuit design can appreciate the extent of possible electronic design automation tools and applications used in design process  910  without deviating from the scope and spirit of the invention. The design structure of the invention is not limited to any specific design flow. 
   Ultimately, design process  910  preferably translates the inventive circuitry of this invention, along with the rest of the integrated circuit design (if applicable), into a final design structure  990  (e.g., information stored in a GDS storage medium). Final design structure  990  may comprise information such as, for example, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, test data, data for routing through the manufacturing line, and any other data required by a semiconductor manufacturer to produce the keeper circuit provided by this invention. Final design structure  990  may then precede to a stage  995  where, for example, final design structure  990 : proceeds to tape-out, is released to manufacturing, is sent to another design house or is sent back to the customer. 
   While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.