Abstract:
A dual-rail static logic gate with a self cut-off mechanism is disclosed. In an embodiment, the output of the first rail is coupled to the input of the pull-up device of the second rail and vice versa. The cross-coupling allows the self cut-off mechanism of the static gate to function properly and provides for components which have lower capacitance than conventional static gates. The lower capacitance results in a faster static gate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of U.S. patent application Ser. No. 10/021,544, filed on Oct. 22, 2001 now U.S. Pat. No. 6,717,441. 

   FIELD 
   The embodiments disclosed herein relate generally to logic circuits, and more particularly to domino logic circuits. 
   BACKGROUND 
   With the growing complexity of modern computer systems, designers are constantly seeking more efficient methods to reduce power and cost, while increasing speed. Generally, the major components in a computer system are formed from the combination of millions of logic gates. Typically, the power, cost, and speed of the components correlate to the operation efficiency of these logic gates. By significantly improving the performance of the logic gate, the overall performance of the computer system can be improved. 
   One type of well known logic circuit is a domino logic circuit which has a series of logic gates coupled together. Specifically, domino logic circuits have dynamic gates and static gates coupled together in a serial fashion such that the gates alternate between dynamic and static. Typically, the dynamic gates are simple and fast because they do not use p-type metal oxide semiconductor (“PMOS”) transistors to propagate an input signal. 
   Rather, the dynamic gates use a PMOS transistor only for precharging each of the dynamic gates. Conversely, conventional static gates are more complex and include a complementary PMOS network, which is comprised of a plurality of interconnected PMOS transistors. The PMOS network results in an increase in capacitance experienced during the evaluation phase. The increased capacitance results in slower switching speeds, which results in lower system performance. 
   Moreover, conventional static gates often include two or more PMOS which are stacked together, which requires that the transistors be upsized, which further increases the capacitance experienced through the gate. Therefore, conventional static gates are known to act as a bottle neck for the domino logic circuit. 

   
     DESCRIPTION OF THE DRAWINGS 
     Various embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an,” “one,” or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
       FIG. 1  is a domino logic circuit according to an embodiment. 
       FIG. 2  is a timing chart which shows the behavior of the input and output signals of an embodiment in relation to the clock signal. 
       FIG. 3  is a schematic of the static gate shown in FIG.  1 . 
       FIG. 4  is an alternative embodiment of the static gate shown in FIG.  3 . 
   

   DETAILED DESCRIPTION 
   Various embodiments disclosed herein overcome the problems in the existing art described above by replacing the conventional static gate of a domino logic circuit with a self cut-off pseudo static gate which uses ratio logic. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments. It will be apparent, however, to one skilled in the art that the embodiments may be practiced without some of these specific details. For example, various signals, layout patterns and logical circuits may be modified according to the teachings of the various embodiments. 
   The following description and the accompanying drawings provide examples for the purposes of illustration. However, these examples should not be construed in a limiting sense as they are not intended to provide an exhaustive list of all possible implementations. In other instances, well known structures and devices are omitted or simplified in order to avoid obscuring the details of the various embodiments. 
   Referring now to  FIG. 1 , a portion of domino logic circuit  10  is shown according to an embodiment. Domino logic circuit  10  includes a plurality of dynamic gates  11  and a plurality of static logic gates  13  coupled to dynamic logic gates  11  such that dynamic gates  11  and static gates  13  are alternately connected in series. Each static logic gate  13  comprises first pull-down device  12  which has first input line  14  coupled thereto and second pull-down device  16  which has second input line  18  coupled thereto. In addition, each static gate  13  includes first pull-up device  20  which has an input-to be driven by output  22  of second pull-down device  16  and second pull-up device  24  which has an input to be driven by output  26  of first pull-down device  12 . 
     FIG. 3  shows static gate  13  of FIG.  1 . In such an embodiment, first pull-down device  12  and second pull-down device  16  each comprise an n-type metal oxide semiconductor (“NMOS”) pull-down network, which is comprised of a plurality of interconnected NMOS transistors. First pull-up device  20  and second pull-up device  24  each comprise a single PMOS transistor, and a clock may be coupled to a gate of first pull-up device  20  by first logical NAND gate  28 . Likewise, the clock may also be coupled to a gate of second pull-up device  24  by second logical NAND gate  30 . 
   In embodiments which include the clock coupled to the pull-up devices as described above, output  22  of second pull-down device  16  may be coupled to the gate of first pull-up device  20  by first inverter  32  and first logical NAND gate  28 . Likewise, output  26  of first pull-down device  12  may be coupled to the gate of second pull-up device  24  by second inverter  34  and second logical NAND gate  30 . 
   In other embodiments, first pull-up device  20  and second pull-up device  24  each comprise a plurality of PMOS transistors. An example of this embodiment is shown in FIG.  4 . In the embodiment shown, a clock is coupled to a gate of first transistor  36  of first pull-up device  20 , and the clock is also coupled to a gate of first transistor  40  of second pull-up device  24 . In addition, output  22  of second pull-down device  16  is coupled to a gate of second transistor  38  of first pull-up device  20  by plurality of inverters  44 , and output  26  of first pull-down device  12  is coupled to a gate of second transistor  42  of second pull-up device  24  by plurality of inverters  46 . 
   In various embodiments, static gate  13  further comprises first NMOS transistor  48  having a drain coupled to output  26  of first pull-down device  12  and a gate to be driven by output  22  of second pull-down device  16 . Likewise, second NMOS transistor  50  has a drain coupled to output  22  of second pull-down device  16  and a gate to be driven by output  26  of first pull-down device  12 . These embodiments include the NMOS transistors to act as keepers to maintain the outputs of the two pull-down devices in a complementary state during the evaluation phase. 
   Similarly, in various embodiments first PMOS transistor  52  has a drain coupled to first input line  14  and a gate to be driven by second input line  18 . In addition, second PMOS transistor  54  has a drain coupled to second input line  18  and a gate to be driven by first input line  14 . These PMOS transistors also act as keepers to maintain complementary functioning of domino logic circuit  10  during the evaluation phase. 
   Turning now to  FIG. 2 , the input/output waveforms of static gate  13  are shown. During the precharge phase, the clock is low and the outputs of dynamic gate  11  are both high (e.g. input lines  14  and  18 ). As a result, outputs  26  and  22  are both low. In addition, pull-up devices  20  and  24  are both OFF since the outputs of NAND gates  28  and  30  are both high (since clock is low and outputs  26  and  22  are both low). 
   Once the clock goes high, the pseudo logic (or ratio logic) phase begins. This pseudo logic phase is very short relative to a clock period and occurs before the complementary inputs D′ (input line  14 ) and D′# (input line  18 ) commence their final complementary state during the evaluation phase. During the pseudo logic phase, pull-up devices  20  and  24  and precharged pull-down devices  12  and  16  are all ON and conducting. 
   Thus, the voltage levels of outputs  26  and  22  are determined by the DC-gain ratio of the pull-up/pull-down devices. The gain ratio is designed such that outputs  22  and  26  are still within the margins to be evaluated as low signals for the next dynamic gates. The output waveform of  FIG. 2  shows the effects of this pseudo logic phase. 
   Specifically, the pseudo logic phase effect on static gate  13  is evidenced by the slight raise in Out. (output  26 ) and Out# (output  22 ) when the clock goes high, but despite the slight raise, both signals are still considered low. Once inputs D′ and D′# begin to act in a complementary fashion during the evaluation phase, the output signals also begin to behave in a complementary nature since one of the pull-down networks stops conducting. 
   The self cut-off of one of the pull-down networks of static gate  13  to achieve complementary functioning of the outputs is accomplished, in part, by cross coupling the output of one rail with the input of the pull-up device of the other rail and vice versa. Such a cross coupling can be seen in  FIGS. 1 ,  3  and  4 . 
   By utilizing static gates with a self cut-off mechanism as disclosed herein, circuit performance increases up to 30% over conventional domino logic circuits, which do not implement the self cut-off pseudo static gates disclosed herein. 
   It is to be understood that even though numerous characteristics and advantages of various embodiments have been set forth in the foregoing description, together with details of structure and function of the various embodiments, this disclosure is illustrative only. Changes may be made in detail, especially matters of structure and management of parts, without departing from the scope of the various embodiments as expressed by the broad general meaning of the terms of the appended claim.