Abstract:
A collection of logic gates that provide single event upset (SEU) immunity. The family of gates include an inverter, a two-input NOR gate, a two-input NAND gate, a three-input AND-NOR gate, and a three-input OR-NAND as well as a static RAM bit cell. SEU immunity is obtained by constructing each logic element with a redundant set of inputs and using two copies of each such logic element to provide redundant outputs. The design of a logic element is such that when the redundant inputs agree (i.e., each has the same logic value), then the output of the logic element implements the logic function. However, when any pair of redundant inputs disagree, then the output of the logic element is disconnected(tri-state), which preserves the previous output value. SEU events only affect one of the logic elements in the pair, and this upset can not propagate through other logic elements because of the tri-state function.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims the benefit of U.S. Provisional Patent Application Serial No. 60/236,851, filed Sep. 29, 2000 entitled “Single Event Upset Immune Logic Family”. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to single event upset (SEU) prevention in high density electronic circuits. More particularly, the present invention presents a family of SEU immune logic circuits. 
     BACKGROUND 
     Single event upset (SEU) is a phenomenon that sometimes occurs to high density electronics when subjected to radiation fields. When critical dimensions of integrated circuits drop below a certain threshold, Alpha particles and other energetic species impinging on the active integrated circuit or device can cause false bits, lock-up, or catastrophic failure. SEU is a change of state or transient induced by an energetic particle such as a cosmic ray or proton in a device. SEU may occur in digital, analog, and optical components or may have effects in surrounding interface circuitry. This is considered a significant problem in designing electronics for reliable operation in space. 
     Previous SEU immunity for logic families such as those illustrated in U.S. Pat. No. 5,418,473 have relied on internal feedback and signaling which does not drive from rail-to-rail of the power supplies and is not able to provide strong, symmetrical output drive signals. This results in slow logic operation. The present invention overcomes the aforementioned shortcomings without having to rely on internal feedback designs. 
     SEU immunity has mainly been emphasized for memory elements. The present invention, however, expands SEU immunity to all logic functions such that upsets can neither propagate through a logic network, nor corrupt memory storage. 
     SUMMARY 
     The present invention is comprised of a family of logic gates that provide single event upset (SEU) immunity. SEU immunity is obtained by constructing each logic element with a redundant set of inputs and using two copies of each such logic element to provide redundant outputs. The design of a logic element is such that when the redundant inputs agree (i.e., each has the same logic value), then the output of the logic element implements the logic function. However, when any pair of redundant inputs disagree, then the output of the logic element is disconnected (i.e., tri-state), which preserves the previous output value. Since, SEU events only affect one of the logic elements in the pair, an upset can not propagate through other logic elements because of the tri-state function. 
     A single event upset (SEU) immune logic function circuit according to the present invention is comprised of a plurality of logic sub-circuits wherein each logic sub-circuit is comprised of redundant inputs. A chosen implementation circuit functions such that when the redundant inputs are not the same the output becomes tri-state and the output from the previous stage is unchanged. Otherwise, when the redundant inputs are the same the output is driven in accordance with the desired logic function. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an inverter logic circuit in accordance with the present invention. 
     FIG. 2 illustrates a two input NOR gate logic circuit in accordance with the present invention. 
     FIG. 3 illustrates a two input NAND gate logic circuit in accordance with the present invention. 
     FIG. 4 illustrates a static RAM bit cell logic circuit in accordance with the present invention. 
     FIG. 5 illustrates a three input AND-NOR gate logic circuit in accordance with the present invention. 
     FIG. 6 illustrates a three input OR-NAND gate logic circuit in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an inverter logic circuit in accordance with the present invention. The inverter is comprised of a pair of p-channel CMOS transistors T 1  (primary) and T 2  (redundant) and a pair of n-channel CMOS transistors T 3  (primary) and T 4  (redundant). Gate input A 1  to p-channel transistor T 1  is cross-coupled to also be an input to n-channel transistor T 3 . Similarly, redundant gate input A 2  to p-channel transistor T 2  is cross-coupled to also be an input to n-channel transistor T 4 . 
     The source of p-channel transistor T 1  is coupled with a power supply V DD . The drain of p-channel transistor T 1  is coupled with the source of p-channel transistor T 2 . The drain of p-channel transistor T 2  is coupled with the output Y and also with the drain of n-channel transistor T 3 . N-channel transistor T 3  receives its gate input from A 1  and its drain is coupled with the output Y. The T 3  source is coupled with the drain of n-channel transistor T 4 . Lastly, n-channel transistor T 4  receives its gate input from A 2 . Its drain is coupled with the source of n-channel transistor T 3  and its source is sent to ground. 
     A truth table for the inverter logic circuit is presented in TABLE 1. 
     
       
         
               
               
               
             
           
               
                   
               
               
                 A1 
                 A2 
                 Y 
               
               
                   
               
             
             
               
                 0 
                 0 
                 1 
               
               
                 0 
                 1 
                 Z 
               
               
                 1 
                 0 
                 Z 
               
               
                 1 
                 1 
                 0 
               
               
                   
               
             
          
         
       
     
     Whenever the redundant input values A 1  and A 2  are the same, then the logic function is implemented. However, if redundant inputs A 1  and A 2  are dissimilar, then the output of the circuit is disconnected (i.e., tri-state) and the previous output value is preserved. A tri-state output is represented in the table using the letter Z. 
     FIG. 2 illustrates a two input NOR gate logic circuit in accordance with the present invention. The two input NOR gate is comprised of four p-channel CMOS transistors (T 1 -T 4 ) and four n-channel CMOS transistors (T 5 -T 8 ). They are connected as follows. Each of the p-channel transistors (T 1 , T 2 , T 3 , T 4 ) receives an input (A 1 , A 2 , B 1 , B 2 ) at its respective gate. Inputs A 1  and B 1  are the primary inputs while inputs A 2  and B 2  are redundant inputs. These inputs are cross coupled with n-channel transistors T 5 -T 8  such that A 1  is coupled with T 8 , A 2  is coupled with T 7 , B 1  is coupled with T 6 , and B 2  is coupled with T 5 . 
     The source of T 1  is coupled with a power supply V DD . The drain of T 1  is coupled with the source of T 2 . Similarly, the source of T 3  is coupled with the drain of T 2  and the source of T 4  is coupled with the drain of T 3 . The drain of T 4  is coupled with the output Y. The drain of T 5  is also coupled with the output Y. The source of T 5  is coupled with the drain of T 6  while the source of T 6  is coupled with ground. The drain of T 7  is coupled with the output Y while the source of T 7  is coupled with the drain of T 8  and the source of T 8  is coupled with ground. 
     A truth table for the two-input NOR gate logic circuit is presented in TABLE 2. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 A1 
                 A2 
                 B1 
                 B2 
                 Y 
               
               
                   
               
             
             
               
                 X 
                 X 
                 1 
                 1 
                 0 
               
               
                 1 
                 1 
                 X 
                 X 
                 0 
               
               
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                 0 
                 0 
                 0 
                 1 
                 Z 
               
               
                 0 
                 0 
                 1 
                 0 
                 Z 
               
               
                 0 
                 1 
                 0 
                 0 
                 Z 
               
               
                 1 
                 0 
                 0 
                 0 
                 Z 
               
               
                   
               
             
          
         
       
     
     A NOR gate sets the output to “0” whenever one of the inputs is “1” regardless of the other input. Thus, in the table above when A 1 =A 2 =1 or B 1 =B 2 =1, then the output Y is set to “0” regardless of the other input. An “X” as an input value in the table above indicates that any value will suffice, even a corrupted value. The table produces tri-state outputs Z whenever A 1 ≠A 2  when B 1 =B 2 =0 or B 1 ≠B 2  when A 1 =A 2 =0. In such cases, the output of the circuit is disconnected and the previous output value is preserved. 
     FIG. 3 illustrates a two input NAND gate logic circuit in accordance with the present invention. The two input NAND gate is comprised of four p-channel CMOS transistors (T 1 -T 4 ) and four n-channel CMOS transistors (T 5 -T 8 ). They are connected as follows. Each of the p-channel transistors (T 1 , T 2 , T 3 , T 4 ) receives an input (A 1 , A 2 , B 1 , B 2 ) at its respective gate. Again, inputs A 1  and B 1  are the primary inputs while inputs A 2  and B 2  are redundant inputs. These inputs are cross coupled with n-channel transistors T 5 -T 8  such that A 1  is coupled with T 8 , A 2  is coupled with T 7 , B 1  is coupled with T 6 , and B 2  is coupled with T 5 . 
     The source of T 1  is coupled with a power supply V DD . The drain of T 1  is coupled with the source of T 2 . The drain of T 2  is coupled with the output Y. The source of T 3  is coupled with power supply V DD . The drain of T 3  is coupled with the source of T 4  and the drain of T 4  is coupled with the output Y. The drain of n-channel transistor T 5  is also coupled with the output Y while the source of T 5  is coupled with the drain of T 6 . The source of T 6  is coupled with the drain of T 7 . the source of T 7  is coupled with the drain of T 8  and the source of T 8  is couple with ground. 
     A truth table for the two-input NAND gate logic circuit is presented in TABLE 3. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 A1 
                 A2 
                 B1 
                 B2 
                 Y 
               
               
                   
               
             
             
               
                 0 
                 0 
                 X 
                 X 
                 1 
               
               
                 X 
                 X 
                 0 
                 0 
                 1 
               
               
                 1 
                 1 
                 1 
                 1 
                 0 
               
               
                 0 
                 1 
                 1 
                 1 
                 Z 
               
               
                 1 
                 0 
                 1 
                 1 
                 Z 
               
               
                 1 
                 1 
                 0 
                 1 
                 Z 
               
               
                 1 
                 1 
                 1 
                 0 
                 Z 
               
               
                   
               
             
          
         
       
     
     A two-input NAND gate will only set the output to “0” when both inputs are “1”. Conversely, if either input is “0” then the output will be set to “1” regardless of the other input. Thus, in the table above, if A 1 =A 2 =0 or B 1 =B 2 =0, then the output Y will be set to “1” regardless of the other input. The output can only be “0” if A 1 =A 2 =1 and B 1 =B 2 =1. All other combinations will disconnect because A 1 ≠A 2  while not B 1 =B 2 =0 or B 1 ≠B 2  while not A 1 =A 2 =0. 
     FIG. 4 illustrates a static RAM bit cell logic circuit in accordance with the present invention. The inverters used are those described in FIG. 1 above. Thus, the static RAM bit cell logic circuit is SEU immune because the inverters comprising the circuit are SEU immune. The static RAM bit cell logic circuit includes multiple I/O pins to eliminate the potential of an SEU while reading or writing to the memory cell. This circuit is not an inverter with decoupling transistor in the power supply terminals. Rather, this circuit implements a set of redundant inverter circuits. 
     FIG. 5 illustrates a three input AND-NOR gate logic circuit in accordance with the present invention. The three input AND-NOR gate is comprised of six p-channel CMOS transistors (T 1 -T 6 ) and six n-channel CMOS transistors (T 7 -T 12 ). They are connected as follows. Each of the p-channel transistors (T 1 , T 2 , T 3 , T 4 , T 5 , T 6 ) receive an input (A 1 , A 2 , B 1 , B 2 , C 1 , C 2 ) at the respective gates. Inputs A 1 , B 1 , and C 1  are the primary inputs while inputs A 2 , B 2 , and C 2  are redundant inputs. These inputs are cross coupled with n-channel transistors T 7 -T 12  such that A 1  is coupled with T 12 , A 2  is coupled with T 11 , B 1  is coupled with T 10 , and B 2  is coupled with T 9 , C 1  is coupled with T 8 , and C 2  is coupled with T 7 . 
     The source of T 1  is coupled with a power supply V DD while its drain is coupled with the source of T 2 . The drain of T 2  is coupled with the source of T 4 . The source of T 3  is coupled with power supply V DD . The drain of T 3  is coupled with the source of T 4  while the drain of T 4  is coupled with the source of T 2 . The source of T 5  is coupled with the drains of T 2  and T 4 . The drain of T 5  is coupled with the source of T 6  while the drain of T 6  is coupled with the output Y. The drain of n-channel transistor T 7  is coupled with the output Y. The source of T 7  is coupled with the drain of T 8  while the source of T 8  is coupled with ground. The drain of T 9  is coupled with output Y while its source is coupled with the drain of T 10 . The source of T 10  is coupled with the drain of T 11 . The source of T 11  is coupled with the drain of T 12  and the source of T 12  is coupled with ground. 
     A truth table for the three input AND-NOR gate logic circuit is presented in TABLE 4. 
     
       
         
               
               
               
               
               
               
               
             
               
               
             
           
               
                   
               
               
                 A1 
                 A2 
                 B1 
                 B2 
                 C1 
                 C2 
                 Y 
               
               
                   
               
             
             
               
                 X 
                 X 
                 X 
                 X 
                 1 
                 1 
                 0 
               
               
                 1 
                 1 
                 1 
                 1 
                 X 
                 X 
                 0 
               
               
                 X 
                 X 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                 0 
                 0 
                 X 
                 X 
                 0 
                 0 
                 1 
               
             
          
           
               
                 ALL OTHERS 
                 Z 
               
               
                   
               
             
          
         
       
     
     A three input AND-NOR gate performs an AND function on inputs A and B and then performs a NOR function on input C and the output of the A/B AND. Since a NOR gate sets the output to “0” whenever one of the inputs is “1” regardless of the other input, the output of the AND function is irrelevant if C 1 =C 2 =1. Similarly, if the output of the AND operation yields a 1 (i.e., A 1 =A 2 =1 and B 1 =B 2 =1), then the values of C 1  and C 2  are irrelevant. A NOR gate can only return a “1” when both inputs are “0”. Thus, if C 1 =C 2 =0 and if A 1 =A 2 =0 or B 1 =B 2 =0, then the AND-NOR output Y will be “1”. All other combinations will cause a tri-state disconnect. 
     FIG. 6 illustrates a three input OR-NAND gate logic circuit in accordance with the present invention. The three input OR-NAND gate is comprised of six p-channel CMOS transistors (T 1 -T 6 ) and six n-channel CMOS transistors (T 7 -T 12 ). They are connected as follows. Each of the p-channel transistors (T 1 , T 2 , T 3 , T 4 , T 5 , T 6 ) receives an input (A 1 , A 2 , B 1 , B 2 , C 1 , C 2 ) at its respective gate. Again, inputs A 1 , B 1 , and C 1  are the primary inputs while inputs A 2 , B 2 , and C 2  are redundant inputs. These inputs are cross coupled with n-channel transistors T 7 -T 12  such that A 1  is coupled with T 12 , A 2  is coupled with T 11 , B 1  is coupled with T 10 , and B 2  is coupled with T 9 , C 1  is coupled with T 8 , and C 2  is coupled with T 7 . 
     The source of T 1  is coupled with a power supply V DD while its drain is coupled with the source of T 2 . The drain of T 2  is coupled with the source of T 3 . The drain of T 3  is coupled with the source of T 4 . The drain of T 4  is coupled with the output Y. The source of T 5  is coupled with power supply V DD . The drain of T 5  is coupled with the source of T 6  while the drain of T 6  is coupled with the output Y. The drain of T 7  is coupled with the output Y. The source of T 7  is coupled with the drain of T 8 . The source of T 8  is coupled with the drain of T 9 . The source of T 9  is coupled with the drain of T 10  while the source of T 10  is coupled with ground. The drain of T 11  is coupled with the source of T 8  and the drain of T 9  while the source of T 11  is coupled with the drain of T 12 . The source of T 12  is coupled with ground. 
     A truth table for the three input OR-NAND gate logic circuit is presented in TABLE 5. 
     
       
         
               
               
               
               
               
               
               
             
               
               
             
           
               
                   
               
               
                 A1 
                 A2 
                 B1 
                 B2 
                 C1 
                 C2 
                 Y 
               
               
                   
               
             
             
               
                 X 
                 X 
                 X 
                 X 
                 0 
                 0 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 X 
                 X 
                 1 
               
               
                 X 
                 X 
                 1 
                 1 
                 1 
                 1 
                 0 
               
               
                 1 
                 1 
                 X 
                 X 
                 1 
                 1 
                 0 
               
             
          
           
               
                 ALL OTHERS 
                 Z 
               
               
                   
               
             
          
         
       
     
     A three input OR-NAND gate performs an OR function on inputs A and B and then performs a NAND function on input C and the output of the A/B OR. Since a NAND function will return a “1” only when one of the inputs is a “0”, the OR-NAND gate will return a “1” when either C 1 =C 2 =0 or when A 1 =A 2 =0 and B 1 =B 2 =0. The OR-NAND gate will return a “0” when C 1 =C 2 =1 and either A 1 =A 2 =1 or B 1 =B 2 =1. All other combinations will cause a tri-state disconnect. 
     The present invention has been illustrated using CMOS transistor logic. Other forms of transistor logic, however, could be used with the concepts of the present invention. Thus, the present invention is not limited to CMOS transistor logic only. Moreover, the present invention has illustrated a family of gates that include an inverter, a two-input NOR gate, a two-input NAND gate, a three-input AND-NOR gate, and a three-input OR-NAND. The present invention is not limited to this particular family. The concepts of the present invention can be applied by those of ordinary skill in the art to achieve SEU immunity for other logic functions. 
     In the following claims, any means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.