Abstract:
Among other things, the present invention provides an improved decoder section with a noise resistant input. In one embodiment, the section includes a ratioed gate and a stack with at least one transistor. The ratioed gate has an input for receiving a first input signal, which may be noisy, from one or more input signals and an output that generates a true value, when the gate is activated, if the first input signal is true. The stack with at least one transistor is operably connected to the ratioed gate. It has at least one input for receiving the remaining one or more input signals apart from the first input signal. When these remaining input signals are true, the stack activates the ratioed gate. Otherwise, if any of the remaining signals are false, it inactivates the gate. Accordingly, the ratioed gate generates a true output when all of the one or more inputs are true.

Description:
TECHNICAL FIELD 
     The present invention relates generally to integrated circuits and in particular to a decoder for enabling a portion of a memory array. 
     BACKGROUND 
     Various decoder schemes may be used to enable portions of memory in a memory array. A word line decoder generates a word line signal to activate (or enable) a quantity (e.g., a word) of memory within a memory bank. A bank may have numerous words and thus require numerous (e.g., 16) decoders. Each decoder produces a word line signal to enable the word for the implementation of either a store (write) or load (read) operation for a selected word of the memory bank. In doing so, it receives various store enable and load enable signals and generates the word line signal when either a store or load operation is to be performed. 
     FIGS. 1A and 1B show a traditional word line decoder circuit  50 . Decoder circuit  50  receives fully dynamic, full-phase store and load enable signals (STEN 0 , STEN 1 , STEN 2 , LDEN 0 , and LDEN 1 ) from store and load sources SS 0 , SS 1 , SS 2 , LS 0 , and LS 1 , respectively. These signals are triggered from the rising edge of a clock (CLK) signal. Decoder circuit  50  generates a word line enable signal (WL) if either all of the store enable or all of the load enable signals are true (e.g., high) at the same time when the clock is in its first phase (high). 
     One or more gates are depicted in the store and load source blocks to indicate their delay (in terms of gate delay) from the clock=s rising edge. Several gate delays can occur within a single phase of the clock single. That is, signals may occur at different points within the active (e.g., high) phase of the clock, but they all occur within that phase. In FIG. 1, the store enable signals, STEN 0 , STEN 1 , and STEN 2 , are delayed by one, two, and three gate delays from the clock&#39;s rising edge. In contrast, the load enable signals, LDEN 0  and LDEN 1 , are both only delayed by one gate delay from the rising edge. 
     Circuit  50  includes a store decode section  60 , a load decode section  70 , and an output section  80 . Store decode section  60  includes transistors Q 1  through Q 8 ; the load section includes transistors Q 9  through Q 15 ; and the output section includes transistors Q 16  through Q 22 . Transistors Q 1 , Q 6 , Q 7 , Q 9 , Q 13 , Q 14 , Q 16 , Q 20 , and Q 21  are PFET type transistors, and the remaining transistors are NFET type transistors. 
     Store section  60  is configured as a precharged AND gate with inputs at STEN 0 -STEN 2 , and an output at ST. Q 1  through Q 5  are connected in a stacked NAND precharge circuit configuration. Q 1  is a precharge transistor, Q 5  is an evaluate transistor, and Q 2  through Q 4  are the NAND input transistors making up the stack for receiving the STEN 0 , STEN 1 , and STEN 2  signals, respectively. The clock (CLK) signal is connected to the gates of precharge transistor Q 1  and evaluate transistors Q 5 . The drain of precharge transistor Q 1  defines the precharged node, which is labeled NST, and is connected to the drain of Q 2 . Transistors Q 7  and Q 8  are configured as a conventional static inverter that has its input connected to the NST node and its output, which is labeled ST, defining the output node for the store section  60 . Transistor Q 6  serves as half-latch for holding up the NST node when the ST node goes Low. 
     The load section  70  is also configured as a precharge AND gate with inputs at LDEN 0  and LDEN 1 , and an output at LD. Q 9 -Q 12  form a precharged NAND stack having a precharged node at NLD. Q 9  and Q 12  are the precharge and evaluate transistors, respectively, with the clock signal connected to their gates. Q 11  and Q 12  serve as the NAND stack input transistors receiving the input LDEN 0  and LDEN 1  load enable signals, respectively. Q 14  and Q 15  form a static inverter having its input connected to the NLD node and an output providing the LD output node. With its drain connected to the NLD node and its gate connected to the LD node, Q 13  serves as half-latch for holding up the NLD node when the LD node is low. 
     Output section  80  performs an OR function with its inputs being the ST and LD signals and its output being the word line (WL) signal. Q 16 -Q 19  are connected in a precharge OR circuit configuration. Q 16  and Q 19  are precharge and evaluate transistors, respectively, with their gates connected to the clock signal. Q 17  and Q 18  are in a conventional NOR configuration with their commonly connected drains connected to the drain of precharge transistor Q 16  and their commonly connected sources connected to the drain of evaluate transistor Q 19 . The gate of Q 17  is connected to the LD node, and the gate of Q 18  is connected to the ST node. Their commonly connected drains define the precharged node labeled NWL. Transistors Q 21  and Q 22  are connected in a conventional static inverter arrangement with the input connected to the NWL node and the output providing the WL node, which is the output of the decoder circuit  50 . Q 20  functions as half-latch with its drain connected to the NWL node for holding it up when the WL node is low. 
     With regard to the operation of the store section  60 , during a precharge state, (when the clock is low), Q 1  is turned on and Q 5  is off. This causes node NST to precharge to a high value without regard to the values of STEN 0 -STEN 2 . Thus, during the precharge state, the Q 7 /Q 8  inverter output at ST is low. When the clock transitions to a high (during an evaluate state), Q 1  turns off and Q 5  turns on. If all of the STEN 0 , STEN 1 , and STEN 2  signals are true (or high) at the same time, node NST is discharged low, which causes ST to go high. Conversely, if any of these store enable signals are false (or low), the NST node remains charged high, and the ST output remains low. Accordingly, when STEN 0 , STEN 1 ,and STEN 2  are all high at the same time in an evaluate state, NST goes low and ST goes high indicating that a store operation is to occur. 
     The load section  70  functions in the same way. If LDEN 0  and LDEN 1  are high at the same time during an evaluate state (when CLK is high), the NLD node discharges and goes low. This causes the LD node to go high indicating that a load operation is to occur. Conversely, if any of LDEN 0  or LDEN 1  are not true, the NLD node remains charged high, and the LD node remains at a low value. 
     With regard to the output section  80 , precharge transistor Q 16  and evaluate transistor Q 19  operate to precharge the NWL node during the precharge (clock low) state. During the evaluate state, if either ST or LD are high, node NWL is discharged low, which causes WL to go high. This corresponds to a word line activation. If neither ST or LD are high during the evaluation state, the WL node remains low, and the word line is not fired. 
     Unfortunately, there are several problems associated with this decoder solution. To begin with, circuit  50  uses fully clock dynamic gates for each of the decoder sections. That is, each of the sections has precharge and evaluate transistors that are continuously drawing clock power in their respective states. This clock loading problem is exaggerated in typical applications that require numerous word line decoders (e.g., 16, 32) for each memory bank. In addition, with a three input NAND stack in the store decoder section, the Q 2 -Q 4  NFETs must be excessively large in order to operate sufficiently fast for most present-day applications. Moreover, the dynamic NAND inputs (at Q 2 -Q 4  and Q 10 -Q 11 ) can be especially sensitive to noisy inputs. This can be problematic—especially when some (or all) of the store enable and/or load enable signals are received from distant sources within a system such as a microprocessor. For example, with the decoder of FIG. 1, the STEN 0  and LDEN 0  signals are received from first-stage decoder sources that are 1200 microns away from the decoder circuit  50 . 
     Accordingly, what is needed is an improved decoder scheme that will solve any or all of these problems. 
     SUMMARY OF THE INVENTION 
     Among other things, the present invention provides an improved decoder section with a noise resistant input. In one embodiment, the section includes a ratioed gate and a stack with at least one transistor. The ratioed gate has an input for receiving a first input signal, which may be noisy, from one or more input signals and an output that generates a true value, when the gate is activated, if the first input signal is true. The stack with at least one transistor is operably connected to the ratioed gate. It has at least one input for receiving the remaining one or more input signals apart from the first input signal. When these remaining input signals are true, the stack activates the ratioed gate. Otherwise, if any of the remaining signals are false, it inactivates the gate. Accordingly, the ratioed gate generates a true output when all of the one or more inputs are true. 
     In addition, the present invention provides a decoder circuit with an improved output section. In one embodiment, the improved output section includes a first input for receiving a first signal, a second input for receiving a second signal, and an output for providing a dynamic output signal that is true during an evaluate state if either the first or second signal is true during the evaluate state. The output section also includes a precharged node and a first transistor that is operably connected to the first input and between the precharged node and ground to discharge the precharged node if the first signal is true during the evaluate state. The first transistor is adapted to receive the first signal in order to turn off when the first signal is false and turn on when the first signal is true. This occurs because the first signal is pulsed so that it will only be true in the evaluate stat, which ensures that the first transistor is off during the precharged state. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIGS. 1A and 1B show a schematic diagram of a prior art word line decoder circuit; 
     FIGS. 2A and 2B show a schematic diagram of one embodiment of a word line decoder circuit of the present invention; 
     FIG. 3A shows a timing diagram for a store operation with the circuit of FIG. 2; and 
     FIG. 3B shows a timing diagram for a load operation with the circuit of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     FIGS. 2A and 2B show one embodiment of a decoder circuit  150  of the present invention. Decoder  150  receives store enable signals STEN 0 , STEN 1 , and STEN 2  and load enable signals LDEN 0  and LDEN 1  from sources SS 0 , SS 1 , SS 2 , LS 0 , and LS 1 , respectively. Based on these signals, it generates a word line signal WL for activating a word of memory within a memory bank. Note that in the depicted application, the store enable signals are out of phase with one another. As a result, they are clocked with full phase clocks and maintained as full phase signals. In contrast, the load enable signals are substantially in-phase with each other (albeit they may be delayed from the clock&#39;s rising edge). This allows them to be transmitted with a pulse clock that is generated from the clock signal. This causes them to be converted into pulses rather than full phase signals. In the depicted embodiment, the pulses occur for about 25% of a phase of the clock. The relevance of this will be addressed below. It is also worth reminding at this time that in the depicted embodiment, the STEN 0  and LDEN 0  signals are received from relatively distant sources, which makes them fairly noisy. 
     Circuit  150  generally includes store section  160 , load section  170  and output section  180 . The store section includes transistors Q 30  through Q 38 ; the load section includes transistors Q 39  through Q 44 ; and the output section includes transistors Q 45  through Q 51  and inverter U 1 . Q 31 , Q 35 -Q 37 , Q 39 , Q 42 , Q 43 , Q 45 , Q 48 , and Q 50  are PFET type transistors, and the remaining transistors are NFET type transistors. U 1  is a conventional inverter; it may be implemented, for example. with a stacked PFET/NFET static inverter configuration, which is actually used throughout circuit  150 . 
     The store section  160  performs an AND function with respect to inputs STEN 0 , STEN 1 , and STEN 2  and provides the logical output at the node marked ST. Q 31 -Q 36  are configured as a 3 input static NAND gate. The gates of Q 31  and Q 32  are tied together as the input for STEN 0 , and their drains are tied together at the output marked NST. The source of Q 31  is connected to V DD . The source of Q 32  is connected to the drain of Q 33 . Providing the other NAND gate inputs (for STEN 1  and STEN 2 ), Q 33  and Q 34  are stacked together with the source of Q 33  connected to the drain of Q 34 , and the source of Q 34  being connected to ground. The STEN 0  signal is connected to the gates of Q 31 /Q 32 ; the STEN 1  signal is connected to the gate of Q 33 ; and the STEN 2  signal is connected to the gate of Q 34 . The gate of Q 35  is connected to the gate of Q 33 , and its drain is connected to the NST node. Similarly, the gate of Q 36  is connected to the gate of Q 34 , and the drain of Q 36  is also connected to the NST node. Q 37  and Q 38  are configured as a conventional static inverter with their gates tied together as an input for the NST node and their drains tied together for providing the ST output. 
     Q 31  through Q 36  function as a static ratioed NAND gate. Q 35  and Q 36  serve as pull-up transistors for pulling up the NST node when either STEN 1  or STEN 2  is low. Thus, when STEN 0 , STEN 1 , and STEN 2  are all high, the output at NST is low. If any of STEN 0  through STEN 2  are low, NST will be high. Inverter Q 37 /Q 38  causes node ST to go high when NST goes low, which (as will be addressed below) causes the word line (WL) to fire for a store operation. Conversely, ST remains low if NST remains high, which is its nominal value for this dynamic, monotonic circuit. 
     (It is worth noting at this time that in contrast to the prior art, the STEN 0  signal now feeds into a static, ratioed input, i.e., at the gates of Q 31 /Q 32 . In general, a ratioed input is one that requires a transistor to turn off, as well as for one to turn on, in order for an output to change in response to a sufficient change at the input. The ratioed input of Q 31 /Q 32  is much less sensitive to noise, which is important with the depicted embodiment where STEN 0  is received from a distant, noisy source.) 
     The load section  170  also performs an AND function. It logically ANDs input signals LDEN 0  and LDEN 1  and provides the logical result at the output marked LD. Load section  170  is similarly configured with the store section  160 . Q 39 -Q 42  are configured as a 2 input static NAND gate. The gates of Q 39  and Q 42  are tied together and provide one of the NAND gate inputs for receiving the LDEN 0  signal. The drains of Q 39  and Q 40  are tied together forming the NAND output node, which is labeled NLD. The source of Q 39  is connected to V DD  and the source of Q 40  is connected to the drain of Q 41 . The gate of Q 41  serves as the other NAND gate input and is connected to the LDEN 1  signal. Its source is connected to ground. Q 42  functions as a pull-up transistor for NLD. Its gate is connected to the gate of Q 41 ; its drain is connected to the NLD node; and its source is connected to V DD . Q 43  and Q 44  are connected as a conventional static inverter with their gates connected together at the NLD node and their drains connected together and defining the LD output node. 
     As with the STEN 0  input, the LDEN 0  input at the gates of Q 39 /Q 40  is ratioed, which makes it less sensitive to noise. This is important because the LDEN 0  signal is also relatively noisy. When LDEN 0  and LDEN 1  are both high, the output at NLD goes low, which makes the LD output go high. Conversely, if either input signal is low, the NLD output remains high, which causes the LD output to remain low. 
     Output section  180  performs an OR function on the ST and LD signals and provides as its output the WL signal. The source of precharge transistor Q 45  is connected to V DD , its gate is connected to the clock (CLK), and its source is connected to the precharged node, which is marked NWL. Transistors Q 46  and Q 47  are each configured with their drains connected to the NWL node. The gate of Q 46  is connected to the LD signal, and the gate of Q 47  is connected to the ST signal. The source of Q 46  is connected to ground. In contrast, the source of Q 47  is connected to a virtual ground node marked DNG, which is provided by the output of inverter U 1 . The input of U 1  is connected to the clock. Q 48  and Q 49  are connected as a conventional, static inverter. Likewise, Q 50  and Q 51  are connected as a conventional inverter. The input of the Q 48 /Q 49  inverter is connected to the NWL node, and its output provides the WL node. The Q 50 /Q 51  inverter, which serves as a latch for the NWL node, is inversely connected across the Q 48 /Q 49  inverter with its input being connected to the WL node and its output being connected to the NWL node. 
     With regard to the operation of the output section  180 , during a precharge (phase  2  or low) phase of the clock, precharge transistor Q 45  charges the NWL node to a high value with Q 46  and Q 47  being off at this time. Q 46  will be off because the pulsing of the load enable signals ensures that the LD signal is low during the precharge state (clock phase  1 ). Alternatively, Q 47  will be off as a result of inverter U 1 . With the clock being low in this precharge state, inverter U 1  generates a high at the DNG node (or source of Q 47 ), which ensures that Q 47  will not discharge the NWL node. When the clock transitions to its first phase (high), the virtual ground node at DNG becomes low providing a virtual ground to the source of Q 47 . This allows the NWL precharged node to discharge low if either ST or LD fires high during this time. If the NWL node discharges low, the Q 48 /Q 49  inverter provides a high at the WL output for activating the word line. The Q 50 /Q 51  inverter serves to latch the WL output high for the entire evaluate state responsive to the NWL node going low. This will come into play when the output section  180  is fired from the LD pulse. If the NWL node is not discharged during the evaluate state, the WL output remains low. 
     FIG. 3A is a timing diagram illustrating the firing of the WL signal in circuit  150  for a load operation. As mentioned above, the LDEN 0  and LDEN 1  signals, prior to reaching the depicted load sources, LS 0  and LS 1 , are sufficiently in phase with one another that they can be pulsed (i.e., converted into pulses) at the load sources and yet overlap one another to generate an active LD signal. Thus, the LDEN 0  and LDEN 1  signals are pulsed within the load sources from a rising edge of the clock. In one embodiment, these pulses are approximately one-fourth of a clock phase. The load enable pulses cause the load section  170  to generate an LD pulse, which causes Q 46  to turn on and discharge the NWL precharged node. For this embodiment, it can be assumed that the load enable and store enable signals will not be active at the same time. Thus, the ST node is low during this time. When the NWL node discharges, the output at WL is flipped high and remains so until the clock transitions to its low phase, which causes the NWL precharge node to go high again. The Q 50 /Q 51  inverter, which is configured as a latch, is used to latch the WL output high after the LD pulse goes low, which causes Q 46  to turn off. 
     FIG. 3B is a timing diagram showing the various timing signals in circuit  150  when a store operation is performed. Again, during the clock&#39;s second phase (precharge state), precharged node NWL is charged to a high value. However, because STEN 0 , STEN 1 , and STEN 2  are delayed from each other, they are not pulsed like the load enable signals. Rather, they are transmitted in full phase to ensure that if they are all active for a clock cycle, they will overlap with one another for at least some portion within the first clock phase to activate the store section  160 . When each of the store enable signals has gone high, the ST node fires high, which causes input gate transistor Q 47  to turn on and discharge the NWL precharged node, which causes WL to fire high. 
     With the delayed store enable signals not being pulsed, which results in the ST signal lasting through the falling clock edge, the virtual ground at DNG is used so that Q 47  turns off when the clock goes low. If an actual ground were used (as is the case with the load side of output section  180 ), the ST signal would cause Q 47  to remain on and allow the WL signal to be high in the precharge (second clock phase state), which would not be tolerable. However, with U 1 , when clock goes low, the virtual ground node DNG goes high and inhibits Q 47  from being on. In fact, with ST still high when the clock transitions low, the high DNG node actually turns on Q 47  in the reverse direction, which assists in pre-charging the NWL node. 
     The various advantages of the present invention, as embodied in decoder  150 , will be discussed. In this embodiment, sixteen decoders are used for each bank. However, only one decoder fires at any time. Thus, in one embodiment of the invention, only one of the virtual ground inverters is used to supply a virtual ground for the sixteen different decoders, which reduces consumed transistor space. 
     In addition, with regard to the output section, the dynamic evaluation NFETs for the load side have been completely removed. This is made possible with the use of the pulsed load enable signals, which alleviates the need for an evaluation transistor since the input gate transistor (Q 46 ) is assured of being off in a precharge state. This also reduces the required size of the circuit components. 
     Moreover, the precharge and evaluation transistors have been removed from the store and load sections. That is, these sections no longer have to use precharged circuits. The removal of these components significantly reduces the loading on the clock, which was a problem in the prior art designs. However, circuit  150  still has the same performance as a dynamic circuit from a time and evaluation point of view and still produces a dynamic output. 
     Finally, in the prior art circuit, simple ratio-less gates were used as inputs for the noisy store and load enable signals. In contrast, decoder  150  uses static, ratioed inputs, which are much less sensitive to noise. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.