Abstract:
An apparatus and method for selectively enhancing the soft error rate (SER) immunity of a dynamic logic circuit. The apparatus includes a bootstrap capacitor coupled to a precharge input signal and a dynamic node of the dynamic logic circuit, and a device, such as an FET, for selectively connecting the bootstrap capacitor to the dynamic node.

Description:
BACKGROUND OF INVENTION 
   1. Technical Field 
   The present invention relates generally to integrated circuits, and more particularly, to a method and apparatus for enhancing the soft error rate immunity of dynamic logic circuits. 
   2. Related Art 
   A dynamic complementary metal oxide semiconductor (CMOS) logic gate includes a logic structure having a precharge or output node, hereafter referred to as a “dynamic node,” that is precharged to the supply voltage using a clocked p-channel device (e.g., a precharge PFET). The dynamic node is conditionally discharged (evaluated) to external ground (GND) through a clocked n-channel device (e.g., an evaluate NFET) by a set of devices forming the logic structure. 
   The clocked p-channel device has its gate coupled to an input precharge signal (PC). The dynamic node is “precharged” through a p-channel device to the supply voltage when PC is low. When PC goes high, the dynamic node is conditionally discharged (evaluated) through the clocked n-channel device to GND. The dynamic node is discharged if the input signals to the devices forming the logic structure (e.g., an AND or OR logic structure) have configured a conducting path to GND, otherwise the dynamic node stays charged high. 
   The amount of charge on a dynamic node of a dynamic logic gate is sensitive to soft errors caused by natural radiation sources, such as alpha particles and cosmic rays. For example, the charge on a dynamic node may be reduced below the charge threshold (Qcrit) necessary to alter the state of the node from high (logic 1) to low (logic 0) as charge is “knocked off” the dynamic node as a result of the circuit&#39;s interaction with an alpha particle. This is especially problematic for dynamic nodes having a low Qcrit. 
   Qcrit will continue to decrease as feature size and power requirements are scaled downward as a result of advances in CMOS process technology. As a result, soft error rates (SER) are likely to increase. Accordingly, there is a need in the art for a method and apparatus for enhancing the soft error rate immunity of dynamic logic circuits. 
   One technique that has been used to increase the SER robustness of a dynamic logic circuit involves increasing the capacitance on the dynamic node. This increases Qcrit because the charge Q on the dynamic node is proportional to capacitance (i.e., Q=C*V). Unfortunately, by increasing the capacitance, the speed of the dynamic logic circuit is often reduced. Accordingly, there is a need in the art for an apparatus for enhancing the SER immunity of dynamic logic circuits by increasing the capacitance on the dynamic node, which is capable of being selectively deactivated to increase the performance (e.g., speed) of the dynamic logic circuit. 
   SUMMARY OF INVENTION 
   The present invention provides a method and apparatus for enhancing the soft error rate (SER) immunity of dynamic logic circuits. The invention utilizes a bootstrap capacitor tied between the dynamic node of a dynamic logic circuit and an input precharge signal to increase the Qcrit on the dynamic node. An FET, coupled between the bootstrap capacitor and the dynamic node of the dynamic logic circuit, allows the bootstrap capacitor to be selectively connected to the dynamic node. 
   A first aspect of the present invention is directed to an apparatus for enhancing soft error rate (SER) immunity of a dynamic logic circuit, comprising a bootstrap capacitor coupled to a precharge (PC) input signal and a dynamic node the dynamic logic circuit and a device for selectively connecting the bootstrap capacitor to the dynamic node. 
   A second aspect of the present invention is directed to a method for enhancing soft error rate (SER) immunity of a dynamic logic circuit, comprising coupling a bootstrap capacitor to a precharge input signal and a dynamic node of the dynamic logic circuit, and selectively connecting the bootstrap capacitor to the dynamic node. 
   A third aspect of the present invention is directed to an integrated circuit including an apparatus for enhancing soft error rate (SER) immunity, the apparatus comprising a bootstrap capacitor coupled to a precharge input signal and a dynamic node of a dynamic logic circuit, and a device for selectively connecting the bootstrap capacitor to the dynamic node. 
   The foregoing and other features of the invention will be apparent from the following more particular description of embodiments of the invention. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
       FIG. 1  illustrates a dynamic logic OR gate including a bootstrap capacitor arrangement in accordance with the present invention. 
       FIG. 2  shows a domino logic configuration in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   A dynamic logic OR gate  10  including a bootstrap capacitor  12  in accordance with the present invention is illustrated in FIG.  1 . The OR gate  10  includes a precharge PFET  14  coupled between VDD and a dynamic node  16  of the gate. A logic structure  18  is coupled between the dynamic node  16  of the OR gate  10  and an evaluate NFET  20  that is tied to ground (GND). A precharge signal (PC) is applied to the gates of the precharge PFET  14  and the evaluate NFET  20 . In this example, the logic structure  18  comprises a pair of NFETS  22 ,  24 , arranged in parallel to provide an OR logic function. An input “A” is applied to the gate of NFET  22 , while an input “B” is applied to the gate of NFET  24 . Although shown as including only two NFETS  22 ,  24 , it should be clear that the logic structure  18  could include more than two NFETS arranged in parallel. In addition, it should be clear that the logic structure  18  may alternately provide an AND logic function (e.g., NFETS  22 ,  24 , arranged in series) or other known logic function, and may include any number of NFETS. 
   When the PC signal is low during a precharge phase, the evaluate NFET  20  is turned off and the precharge PFET  14  is turned on. As such, the dynamic node  16  is pulled high (i.e., precharged) to VDD. A keeper circuit  26  of a type known in the art is provided to keep the dynamic node  16  pulled high (unless the dynamic node  16  is pulled down to ground through the logic structure  18  during an evaluate stage). In this embodiment, the keeper circuit  26  comprises an inverter  28  and a PFET  30  that is coupled between VDD and the dynamic node  16 . In operation, when the dynamic node  16  is charged high, the inverter  28  outputs a low signal which turns on PFET  30 , pulling the dynamic node  16  to VDD. The dynamic node  16  will remain high, therefore, until the logic structure  18  provides a path to GND through the evaluate NFET  20  (i.e., PC must also be high). 
   When the PC signal goes high during the evaluate stage, the evaluate NFET  20  is turned on and the precharge PFET  14  is turned off. The dynamic node  16  may be discharged to GND if either (or both) of the inputs A and B to NFETS  22 ,  24  within the logic structure  18  have configured a conducting path to GND. Otherwise, the dynamic node  16  stays charged high. 
   In accordance with the present invention, a bootstrap capacitor arrangement comprising a bootstrap capacitor  12  (hereafter “SER-CAP”  12 ) is provided to increase the Qcrit of the dynamic node  16  and, therefore, the SER immunity of the OR gate  10 . In particular, as shown in  FIG. 1 , SER-CAP  12  is connected to the PC signal and is selectively coupled to the dynamic node  16  via NFET  32 . A signal “SER-HI” is used to selectively gate the NFET  32  to connect SER-CAP  12  to the dynamic node  16 . For higher performance and lower capacitance, SER-CAP  12  can be selectively disconnected from the dynamic node  16  by turning off NFET  32 . For improved SER immunity, SER-CAP  12  can be selectively connected to the dynamic node  16  by turning on NFET  32  using SER-HI. 
   The PC signal grounds the source of SER-CAP  12  during precharge (i.e., PC is low). The drain of SER-CAP  12  is connected to the dynamic node  16 . When the PC signal goes high (evaluate) and NFET  32  is turned on by SER-HI, the potential of SER-CAP  12  gets pushed up from underneath, and the dynamic node  16 &#39;s potential rises, adding Qcrit. Due to the impedance of NFET  32 , most of the SER improvement is provided by the extra charge and dynamic node  16  potential, rather than simply from the added capacitance of SER-CAP  12 . 
   The present invention allows the SER immunity of a circuit (e.g., an integrated circuit such as a microprocessor) to be selectively/dynamically changed based on its intended use. For example, in many consumer desktop applications, the performance of a microprocessor is often more important than SER reliability. In these types of applications, therefore, the bootstrapped capacitors  12  within the microprocessor can be turned off to boost performance. In applications where reliability is more important than performance (e.g., in an air traffic controller computer), the bootstrapped capacitors  12  within the same microprocessor can be turned on to boost SER reliability. Using the present invention, the failures in time rate (FIT) of the microprocessor could be improved, for example, by 100% by turning on the bootstrapped capacitors  12 , while the performance of the microprocessor could be improved, for example, by 20% by turning off the bootstrapped capacitors  12 . These values are only exemplary of the performance/SER reliability tradeoff afforded by the present invention, and are not intended to be limiting. The exact values are application/circuit specific and may vary widely based on many factors. 
   A domino logic configuration  40  is illustrated in FIG.  2 . The domino logic configuration  40  typically comprises dynamic logic circuits  10  arranged in a plurality of domino stages. Each domino stage includes a logic structure  18  (e.g., an arrangement of AND or OR gates), with each domino stage separated by an inverting stage  42 . In this arrangement, input signal(s)  44  applied to the logic structure  18  of the first domino stage, while the PC input signal is high, triggers operation of the remaining stages in sequence. This yields a domino-like signal propagation effect within the logic configuration  40 . 
   In accordance with the present invention, a bootstrap capacitor arrangement, comprising a SER-CAP  12 , and an NFET  32  for selectively connecting SER-CAP  12  to the dynamic node  16  of the corresponding logic structure  18 , may be added to each stage of the domino logic configuration  40  to provide selective SER immunity enhancement. 
   As detailed above, the bootstrap capacitor arrangement of the present invention is configured to selectively increase Qcrit on the dynamic node  16  of a dynamic logic circuit  10 . The results of circuit simulations showing the resultant increase in Qcrit are provided in Tables 1 and 2. These simulations were run at different temperatures and a constant VDD of 1.20 volts. Table 1 shows the results of the simulation with SER-HI (HIGH), while Table 2 shows the results of the simulation with SER-HI (LOW). By comparing Tables 1 and 2, it should be readily apparent that the bootstrap capacitor arrangement of the present invention significantly increases Qcrit on the dynamic node  16  of a dynamic logic circuit  10 . 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               (SER-HI (HIGH)) 
             
           
        
         
             
                 
               Qcrit 
               Temp 
               Qcrit 
               Temp 
             
             
                 
                 
             
           
        
         
             
                 
               4.82 
               0 
               4.23 
               50 
             
             
                 
               4.72 
               10 
               4.06 
               60 
             
             
                 
               4.63 
               20 
               3.95 
               70 
             
             
                 
               4.49 
               30 
               3.82 
               80 
             
             
                 
               4.36 
               40 
               3.71 
               90 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               (SER-HI (LOW)) 
             
           
        
         
             
                 
               Qcrit 
               Temp 
               Qcrit 
               Temp 
             
             
                 
                 
             
           
        
         
             
                 
               2.66 
               0 
               2.38 
               50 
             
             
                 
               2.59 
               10 
               2.32 
               60 
             
             
                 
               2.52 
               20 
               2.25 
               70 
             
             
                 
               2.43 
               30 
               2.19 
               80 
             
             
                 
               2.44 
               40 
               2.13 
               90