Abstract:
Majority voting between triple redundant integrated circuits is used in order to provide an SEU hardened output signal. Accordingly, an input signal is processed in a predetermined manner to provide a first signal, the input signal is processed in the same manner to provide a second signal, and the input signal is also processed in the same manner to provide a third signal. A majority vote is taken between the first, second, and third signals by an SEU immune majority voter circuit, and an output signal is provided corresponding to the majority vote.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed to the hardening of integrated circuits so that such circuits are immune to single event upsets (SEUs). 
     BACKGROUND OF THE INVENTION 
     Integrated circuits are frequently used in the presence of radiation such as x-rays, gamma-rays, photons, particles, etc. A radiation strike can deposit charge in silicon and, therefore, can cause upsets in the integrated circuits. The most common upset causes are from such particles as protons, neutrons, and heavy ions. As a result of such radiation, charges can be collected at circuit nodes that send the nodes to unintended opposite voltage states (e.g., from high to low). When this voltage state change happens to a data storage circuit, for example, the data storage nodes change to the wrong data states. 
     All circuits can tolerate some amount of deposited charge that does not cause a node to change states. However, all circuits also have some deposited charge threshold above which the node state will be changed. This threshold is referred to as the critical charge (i.e., Qcrit) for upset. Such node state changes are defined as radiation induced upsets. When radiation particles, which are particles that are discrete in time and space, cause a data upset, the data upset is referred to as a single event upset (SEU). 
     Various arrangements have been provided to increase the immunity of integrated circuits from single even upsets. For example, co-pending U.S. application Ser. No. 10/034,808 filed on Dec. 28, 2001 gives several examples of SEU hardening techniques for preventing unintended data state changes in storage elements in response to radiation strikes. 
     FIG. 1 shows another technique to increase the immunity of integrated circuits from single event upsets. As shown in FIG. 1, an integrated circuit is provided with triple redundancy as indicated by instantiations  10 ,  12 , and  14  of the same integrated circuit. The instantiation  10  of this integrated circuit is coupled to an input A of a majority voter circuit  16 , the instantiation  12  of the same integrated circuit is coupled to an input B of the majority voter circuit  16 , and the instantiation  14  of the same integrated circuit is coupled to an input C of the majority voter circuit  16 . 
     The majority voter circuit  16  provides an output on an output line  18  based on a majority vote between the inputs A, B, and C. For example, if the inputs A and B are the same but are different from the input C, then the output on the output line  18  is based on the inputs A and B. Alternatively, if the inputs B and C are the same but are different from the input A, then the output on the output line  18  is based on the inputs B and C. However, if the inputs A and C are the same but are different from the input B, then the output on the output line  18  is based on the inputs A and C. 
     The majority voter circuit  16  comprises a first inverter having a p-channel transistor  20  and an n-channel transistor  22  coupled in series between V DD  and ground. The gate of the p-channel transistor  20  and the gate of the n-channel transistor  22  are coupled to the input A, and the junction between the p-channel transistor  20  and the n-channel transistor  22  is coupled to the output line  18 . 
     The majority voter circuit  16  also comprises a second inverter having a p-channel transistor  24  and an n-channel transistor  26  coupled in series between V DD  and ground. The gate of the p-channel transistor  24  and the gate of the n-channel transistor  26  are coupled to the input B, and the junction between the p-channel transistor  24  and the n-channel transistor  26  is coupled to the output line  18 . 
     The majority voter circuit  16  further comprises a third inverter having a p-channel transistor  28  and an n-channel transistor  30  coupled in series between V DD  and ground. The gate of the p-channel transistor  28  and the gate of the n-channel transistor  30  are coupled to the input C, and the junction between the p-channel transistor  28  and the n-channel transistor  30  is coupled to the output line  18 . 
     Accordingly, radiation may strike the sensitive area of one of the instantiations  10 ,  12 , and  14  of the integrated circuit causing the output of that instantiation to assume an incorrect output state radiation. However, it is not likely that radiation will simultaneously strike the sensitive area of a second of the instantiations  10 ,  12 , and  14  of the integrated circuit causing the output of this second instantiation to assume the same incorrect output state. Because it is not likely that radiation will strike the sensitive areas of two or more of the instantiations  10 ,  12 , and  14  at the same time, the output on the output line  18  will be in the correct state because the majority voter circuit  16  will vote on a majority basis to select the inputs unaffected by the radiation and thus control the output on the output line  18  at the intended output state. 
     As an example, an SEU event may occur in a sensitive area of the instantiation  10  that causes the input A to transition from a low state to a high state so as to turn on the n-channel transistor  22 . However, as long as the sum of the drain currents in the p-channel transistors  24  and  28  is greater than the drain current of the n-channel transistor  22 , the output signal on the output line  18  will not change states. 
     Unfortunately, it is distinctly possible that the drain currents in the p-channel transistors  24  and  28  will not be greater than the drain current of the n-channel transistor  22 , particularly under worst case conditions, in which case the circuit of FIG. 1 is not immune to SEU events. Also, even if the total drain current of the p-channel transistors  24  and  28  is larger than the drain current of the drain current of the n-channel transistor  22 , the speed of the majority voter circuit  16  of FIG. 1 is adversely affected in a significant way due to the competition between the drain currents in the p-channel transistors  24  and  28  and the n-channel transistor  22 . Moreover, if a strong SEU event occurs in one of the transistors  20 - 30 , the affected transistor can turn on so hard that its drain current will overcome the drain currents of the other transistors and produce an erroneous output signal on the output line  18 . Thus, the majority voter circuit  16  itself is not SEU hardened. 
     The present invention is directed to a majority voter circuit that overcomes one or more of these or other problems. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a hardening system comprises first, second, and third integrated circuit blocks and a majority voter circuit. The first, second, and third integrated circuit blocks have substantially identical circuit arrangements with respect to one another, and each of the first, second, and third integrated circuit blocks comprises an output having a signal thereon. The majority voter circuit comprises four transistors coupled to the output of the first integrated circuit block, four transistors coupled to the output of the second integrated circuit block, and two transistors coupled to the output of the third integrated circuit block. The majority voter circuit provides an output signal substantially equal to the signals on the outputs of the first, second, and third integrated circuit blocks that are in the majority. 
     In accordance with another aspect of the present invention, a hardening system comprises first, second, and third integrated circuit blocks and a majority voter circuit. The first integrated circuit block has an output A providing a first signal thereon, the second integrated circuit block has an output B providing a second signal thereon, and the third integrated circuit block has an output C providing a third signal thereon. The majority voter circuit is coupled to the outputs A, B, and C and has transistors such that there is always a redundant off transistor to block the drain current of a transistor that is turned on by an SEU event. 
     In accordance with still another aspect of the present invention, a method of providing an SEU hardened output signal comprises the following: processing an input signal in a manner to provide a first signal; processing the input signal in essentially the same manner to provide a second signal; processing the input signal in essentially the same manner to provide a third signal; determining a majority of the first, second, and third signals by way of an SEU immune majority voter circuit; and, providing an output signal corresponding to the majority. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which: 
     FIG. 1 illustrates a prior art majority voter circuit; and, 
     FIG. 2 illustrates a majority voter circuit according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 shows a majority voting technique according to the present invention that increases the immunity of integrated circuits from single even upsets. As in the case of FIG. 1, an integrated circuit is provided with triple redundancy in the form of three instantiations of the same integrated circuit similar to the instantiations  10 ,  12 , and  14  shown in FIG. 1. A first of these instantiations of the integrated circuit is coupled to an input A of a majority voter circuit  50 , a second of these instantiations of the same integrated circuit is coupled to an input B of the majority voter circuit  50 , and a third of these instantiations of the same integrated circuit is coupled to an input C of the majority voter circuit  50 . 
     The majority voter circuit  50  provides an output on an output line  52  based on a majority vote between the inputs A, B, and C. For example, if the inputs A and B are the same but are different from the input C, then the output on the output line  52  is based on the inputs A and B. Alternatively, if the inputs B and C are the same but are different from the input A, then the output on the output line  52  is based on the inputs B and C. However, if the inputs A and C are the same but are different from the input B, then the output on the output line  52  is based on the inputs A and C. 
     The majority voter circuit  50  comprises p-channel transistors  54 ,  56 ,  58 ,  60 , and  62  and n-channel transistors  64 ,  66 ,  68 ,  70 , and  72 . The source terminals of the p-channel transistors  54 ,  56 , and  58  are coupled to V DD . The drain terminals of the p-channel transistors  54  and  56  are coupled to the source terminal of the p-channel transistor  60 , and the drain terminal of the p-channel transistor  58  is coupled to the source terminal of the p-channel transistor  62 . 
     The drain terminals of the p-channel transistors  60  and  62  are coupled to the output line  52  and to the source terminals of the n-channel transistors  64  and  66 . The drain terminal of the n-channel transistor  64  is coupled to the source terminals of the n-channel transistors  68  and  70 , and the drain terminal of the n-channel transistor  66  is coupled to the source terminal of the n-channel transistor  72 . Finally, the drain terminals of the n-channel transistors  68 ,  70 , and  72  are coupled to ground. 
     The gates of the p-channel transistors  54  and  58  and the gates of the n-channel transistors  66  and  68  are coupled to the input A. The gates of the p-channel transistors  56  and  62  and the gates of the n-channel transistors  70  and  72  are coupled to the input B. The gate of the p-channel transistor  60  and the gate of the n-channel transistor  64  are coupled to the input C. 
     Accordingly, radiation may strike the sensitive area of one of the instantiations of the integrated circuit coupled to the inputs A, B, and C causing the output of that instantiation to assume an incorrect output state radiation. However, because it is not likely that radiation will also strike the sensitive area of a second of the instantiations at the same time, the output on the output line  52  will be in the correct state because the majority voter circuit  50  will vote on a majority basis to select the two inputs unaffected by the radiation to control the output on the output line  52 . 
     Moreover, it is noted above in connection with FIG. 1 that the drain currents provided by two of the p-channel transistors in two of the invertors of the majority voter circuit  16  may not be greater than the drain current provided by the n-channel transistor in the other of the inverters under worst case conditions. If so, it can be seen that the circuit of FIG. 1 is not immune to SEU events. This circumstance cannot occur in the majority voter circuit  50  because the majority voter circuit  50  ensures that there is always adequate drive current to maintain the output line  52  in the proper state. 
     It is also noted above in connection with FIG. 1 that, even if the total drain current provided by two of the p-channel transistors in two of the invertors of the majority voter circuit  16  is greater than the drain current provided by the n-channel transistor in the other of the inverters under worst case conditions, the speed of the circuit of FIG. 1 is adversely affected in a significant way due to the competition between the drain currents in the three inverters. However, the speed of the majority voter circuit  50  is not adversely affected in a significant way due to the competition between drain currents. 
     It is finally noted above that the majority voter circuit of FIG. 1 is somewhat SEU hardened. However, if a strong SEU event occurs in one of the transistors of the majority voter circuit  16 , the affected transistor can turn on so hard that its drain current will overcome the drain currents of the other transistors and produce an erroneous output signal on the output line  18 . This situation cannot occur in the circuit of FIG. 2 because, in the majority voter circuit  50 , there is always a redundant off transistor to block the drain current of a transistor that is turned on by an SEU event. Accordingly, the majority voter circuit  50  is immune to SEU events. 
     Certain modifications of the present invention will occur to those practicing in the art of the present invention. For example, in the majority voter circuit  50 , the transistors  54 ,  56 ,  58 ,  60 , and  62  are p-channel transistors and the transistors  64 ,  66 ,  68 ,  70 , and  72  are n-channel transistors. Instead, the transistors  54 ,  56 ,  58 ,  60 , and  62  may be n-channel transistors and the transistors  64 ,  66 ,  68 ,  70 , and  72  may be p-channel transistors. 
     Also, instantiations  10 ,  12 , and  14  are described above as being instantiations of an integrated circuit. However, instantiations  10 ,  12 , and  14  may instead be instantiations of only portions of an integrated circuit. Thus, an integrated circuit block as used herein means either an entire integrated circuit or a portion of an integrated circuit. 
     Moreover, the majority voter circuit  50  as shown above has only two transistors coupled to the input C. However, the majority voter circuit  50  may instead have four transistors coupled to the input C as it does in the case of inputs A and B. 
     Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.