Abstract:
An integrated circuit includes first and second pads that are electrically connected to a circuit inside the integrated circuit. The circuit performs multiple functions which may be selected. A function identification circuit, inside the integrated circuit, is electrically connected to the first and second pads. The function identification circuit operates in multiple modes, wherein each operating mode corresponds to a function performed by the circuit. The function of the circuit may thereby be identified using fewer pads which may allow a reduction in the cost of the integrated circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of integrated circuits in general and more particularly to the performance of multiple functions by integrated circuits. 
     BACKGROUND OF THE INVENTION 
     As the complexity of integrated circuits increases, so may the number of functions provided by the integrated circuits. The function provided by the integrated circuit may be selected and a label placed on the package of the integrated circuit to identify the function provided by the integrated circuit. 
     The function of the integrated circuit may later be identified by applying a voltage to the integrated circuit and observing the resulting current. U.S. Pat. No. 5,103,166 to Jeon et al. discusses identification circuitry including a voltage limiter which limits an input potential difference between a power supply terminal and an input terminal to a predetermined voltage. U.S. Pat. No. 4,480,199 to Varshney et al. discusses an identification circuit wherein, a single identification circuit may be electrically connected to a pad of the integrated circuit. As described above, a voltage may be applied to the pad, causing a current to flow which may be used to identify the function provided by the integrated circuit. As the number of functions provided by the integrated circuit increases, however, so may the number of required pads also increase, which may increase the cost and/or complexity of the integrated circuit. In view of the above, there continues to exist a need to further improve the identification of functions provided by integrated circuits. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to allow a reduction in the cost and/or complexity of integrated circuits including circuits with multiple functions. 
     It is another object of the present invention to allow a reduction in the number of pads that are used to identify the function of circuits. 
     These, and other objects, are provided by a function identification circuit that operates in a plurality of modes. The operating modes correspond to functions performed by a circuit in the integrated circuit. The function performed by the circuit can be identified by response to a series of voltage levels applied to the function identification circuit and observing the predetermined voltage at which current flows. The function which corresponds to the predetermined voltage at which current flows, is the function performed by the circuit. The function of the circuit may thereby be identified using fewer pads which may allow a reduction in the cost of the integrated circuit. 
     In particular, an integrated circuit according to the present invention includes first and seconds pads. A function identification circuit that operates in a plurality of modes is electrically connected to the first and second pads, wherein the plurality of operating modes correspond to the plurality of functions performed by the circuit. 
     In another aspect of the present invention, the function identification circuit includes a plurality of current circuits, that are electrically connected between the first and second pads. The current circuits conduct current when operating. A plurality of selection circuits are electrically connected to the plurality of current circuits, wherein the plurality of selection circuits select which of the plurality of current circuits operate. In one embodiment, the operating modes of function identification circuit are determined by fuses that are cut during a manufacturing process of the integrated circuit. 
     In another aspect of the present invention, the function identification circuit includes a first current circuit, including a first source node that is electrically connected to the first pad and a first gate/drain node. A second current circuit includes a second source node that is electrically connected to the first gate/drain node and a second gate/drain node. A third current circuit includes a third source node that is electrically connected to the second gate/drain node and a third gate/drain node. A fourth current circuit includes a fourth source node that is electrically connected to the third gate/drain node and a fourth gate/drain node that is electrically connected to the second pad. 
     A first selection circuit includes a first primary node that is electrically connected to the first gate/drain node and a first secondary node that is electrically connected to the second pad. A second selection circuit includes a second primary node that is electrically connected to the second gate/drain node and a second secondary node that is electrically connected to the second pad. A third selection circuit includes a third primary node that is electrically connected to the third gate/drain node and a third secondary node that is electrically connected to the second pad. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a block diagram of an integrated circuit including a function identification circuit according to the present invention; 
     FIG. 1B is a schematic diagram of a first embodiment of a function identification circuit according to the present invention; 
     FIG. 2 is a schematic diagram of a second embodiment of a function identification circuit according to the present invention; 
     FIG. 3 is a schematic diagram of a third embodiment of a function identification circuit according to the present invention; 
     FIG. 4 is a schematic diagram of a fourth embodiment of a function identification circuit according to the present invention; and 
     FIG. 5 is a flowchart illustrating operations of a function identification circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. Each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well. 
     FIG. 1A is a block diagram of an integrated circuit  101  including a function identification circuit  103  according to the present invention. According to FIG. 1A, the function identification circuit  103  is electrically connected between a first pad  111  and a second pad  121 . The first and second pads  111 ,  121  provide input and/or output to/from the integrated circuit and the circuit  131 . 
     The circuit  131  can perform a plurality of functions one of which may be selected. For example, the function of the circuit  131  may be selected by severing a fuse during a manufacturing process. The function identification circuit  103  operates in a plurality of modes which correspond to the plurality of functions performed by the circuit  131 . Moreover, the operating modes of function identification circuit  103  are selected to correspond to the functions performed by the circuit  131 . In particular, a series of predetermined voltages may be chosen, each of which corresponds to a function performed by the circuit  131 . The predetermined voltages are applied between the first and second pads  111 ,  121  and the resulting current observed for each voltage applied. The predetermined voltage that results in current flow identifies the function performed by the circuit  131 . 
     FIG. 1B is a schematic diagram of a first embodiment of a function identification circuit  103  according to the present invention. In particular, the function identification circuit  103  comprises a load circuit  140 , including a plurality of current circuits  141 - 144 , and a plurality selection circuits  151 - 153 . The plurality of current circuits  141 - 144  are electrically connected in series between the first and second pads  111 ,  121 . Each of the current circuits  141 - 144  functions as a diode that is forward biased by applying a voltage difference across the anode and cathode of the diode. As shown in FIG. 1B, each current circuit can be a Field Effect Transistor (FET)  161 - 164  that includes a source node, a gate node, and a drain node, such as PMOS transistor, wherein the gate node and the drain node are electrically connected. Accordingly, the transistor functions as a diode as described above, wherein the gate/drain node corresponds to the cathode and the source node corresponds to the anode. Consequently, the transistor will conduct current if the voltage applied at the source node exceeds the voltage applied at the gate/drain node by a corresponding bias voltage. 
     The current circuits  141 - 144  are electrically connected in series between the first and second pads  111 ,  121 . In particular, the source node of the first current circuit  141  is electrically connected to the first pad  111 . The gate/drain node of the first current circuit  141  is electrically connected to the source node of the second current circuit  142 . The gate/drain node of the second current circuit  142  is electrically connected to the source node of the third current circuit  143 . The gate/drain node of the fourth current circuit is electrically connected to the second pad  121 . 
     When a voltage difference is applied between the first and second pads  111 ,  121  a current will flow if the voltage at the first pad  111  exceeds the voltage at the second pad  121  by at least the sum of the voltage drops required to forward bias each of the current circuits electrically connected between the first and second pads  111 ,  121 . In addition, the voltage applied between the first and second pads  111 ,  121  may be in excess of the power supply voltage for the integrated circuit  101 . For exemplary purposes, the forward bias voltage for the current circuits described herein is equal to about 0.7 Volts (V) and the power supply voltage is equal to about 3.3V. 
     The plurality of selection circuits  151 - 153  are electrically connected to the plurality of current circuits  141 - 144  and select which of the plurality of current circuits  141 - 144  is electrically connected between the first and second pads  111 ,  121 . In particular, each selection circuit includes a primary node and a secondary node. When the selection circuit is activated, current may flow between the primary and secondary nodes. If the selection circuit is deactivated, no current will flow between the primary and secondary nodes. The selection circuit can be a fuse that is severed to deactivate the selection circuit or left intact to activate the selection circuit. 
     A primary node of the first selection circuit  153  is electrically connected to gate/drain node of the first current circuit  141 . A secondary node of the first selection circuit  153  is electrically connected to the second pad  121 . A primary node of the second selection circuit  152  is electrically connected to gate/drain node of the second current circuit  142  and a secondary node of the second selection circuit  152  is electrically connected to the second pad  121 . A primary node of the third selection circuit  151  is electrically connected to the gate/drain node of the third current circuit  144  and a secondary node of the third current circuit  144  is electrically connected to the second pad  121 . 
     According to FIG. 1B, the function identification circuit  103  operates in four modes. In a first mode of operation, the first selection circuit  153  is activated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 4.0V, current will flow from the first pad  111  through the first current circuit  141 , through the first selection circuit  153  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     In a second mode of operation, the second selection circuit  152  is activated and the first selection circuit  153  is deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 4.7V, current will flow from the first pad  111  through the first and second current circuits  141 ,  142 , through the second selection circuit  152  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     In a third mode of operation, the third selection circuit  151  is activated and the first and second selection circuits  153 ,  152  are deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 5.4V, current will flow from the first pad  111  through the first, second, and third current circuits  141 - 143 , through the third selection circuit  151  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     In a fourth mode of operation, the first through fourth selection circuits  151 - 153  are deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 6.1V, current will flow from the first pad  111  through the first through the fourth current circuits  141 - 144 , to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     Alternately, the identification circuit  103  may provide the same operating modes described above if the third selection circuit  151  is electrically connected in parallel with the third current circuit  143 , and if the second selection circuit  152  is electrically connected in parallel with the second and third current circuits  142 ,  143 , and the first selection circuit  153  is electrically connected in parallel with the first through the third current circuits  141 - 143 . The term parallel as used herein includes an electrical connection wherein the primary node of the selection circuit is electrically connected to the source node of the corresponding current circuit and the secondary node of the selection circuit is electrically connected to the gate/drain node of the current circuit. 
     FIG. 2 is a schematic diagram of a second embodiment of a function identification circuit  103  according to the present invention. According to FIG. 2, the first through the fourth current circuits  141 - 144  are electrically connected in series between the first and second pads  111 ,  121  as described above. 
     A primary node of a first selection circuit  253  is electrically connected to a gate/drain node of the first current circuit  141  and a secondary node of the first selection circuit  253  is electrically connected to the gate/drain node of the second current circuit  142 . A primary node of a second selection circuit  252  is electrically connected to the gate/drain node of the second current circuit  142  and a secondary node of the second selection circuit  252  is electrically connected to the drain/gate node of the third current circuit  143 . A primary node of a third selection circuit  251  is electrically connected to the gate/drain node of the third current circuit  143  and a secondary node of the third selection circuit  251  is electrically connected to the second pad  121 . 
     As shown in FIG. 2, the function identification circuit  103  operates in four modes. In a first mode of operation, the first selection circuit  253  is activated and the second and third selection circuits are deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 5.4V, current will flow from the first pad  111  through the first current circuit  141 , through the first selection circuit  253 , through the third and fourth current circuits  143 ,  144  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     In a second mode, the first and second selection circuits  253 ,  252  are activated and the third selection circuit  251  is deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 4.7V, current will flow from the first pad  111  through the first current circuit  141 , through the first and second selection circuits  253 ,  252 , through fourth current circuit  144  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     In a third mode, the first through the third selection circuits are activated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 4.0V, current will flow from the first pad  111  through the first current circuit  141 , through the first through the third selection circuits  253 - 251  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     In a fourth mode, the first through the third selection circuits  253 - 251  are deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 6.1V, current will flow from the first pad  111  through the first through the fourth current circuits  141 - 144  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     FIG. 3 is a schematic diagram of a third embodiment of a function identification circuit  103  according to the present invention. According to FIG. 3, the first through the fourth current circuits  141 - 144  are electrically connected in series between the first and second pads  111 ,  121  as described above. 
     A primary node of a first selection circuit  353  is electrically connected to the gate/drain node of the first current circuit  144  and a secondary node of the first selection circuit  353  is electrically connected to the second pad  121 . A primary node of a second selection circuit  352  is electrically connected to gate/drain node of the second current circuit  142  and a secondary node of the second selection circuit  352  is electrically connected to the gate/drain node of the third current circuit  143 . A primary node of a third selection circuit  351  is electrically connected to gate/drain node of the third current circuit  143  and a secondary node of the third selection circuit  351  is electrically connected to the second pad  121 . 
     As shown in FIG. 3, the function identification circuit  103  operates in four modes. In a first mode of operation, the first selection circuit  353  is activated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 4.0V, current will flow from the first pad  111  through the first current circuit  141 , through the first selection circuit  353  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     In a second mode of operation, the second selection circuit  352  is activated and the first and third selection circuits  353 ,  351  are deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 5.4V, current will flow from the first pad  111  through the first and second current circuits  141 ,  142 , through the second selection circuit  352 , through the fourth current circuit  144  to the second pad  121 . Alternately, the third selection circuit  351  may be activated and the second selection deactivated to provide the same mode of operation described above. The function performed by the circuit  131  may thereby be identified. 
     In a third mode of operation, all of the selection circuits  353 - 351  are deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 6.1V, current will flow from the first pad  111  through the first through the fourth current circuits  141 - 144  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     In a fourth mode of operation, the second and third selection circuits  352 ,  351  are activated and the first selection circuit  353  is deactivated. Consequently, if the voltage applied between the first and second pads  111 ,  121  exceeds about 4.7V, current will flow from the first pad  111  through the first and second current circuits  141 ,  142 , through the second and third selection circuits  352 ,  351  to the second pad  121 . The function performed by the circuit  131  may thereby be identified. 
     The identification circuit  103  may provide the same operating modes described above if the first selection circuit  353  is electrically connected in parallel with the second through the fourth current circuits  142 - 144 , and the second selection circuit  352  is electrically connected in parallel with the second current circuit  142 , and the third selection circuit  351  is electrically connected in parallel with the third current circuit  143 . 
     The identification circuit  103  may also provide the same operating modes described above if the first selection circuit  353  is electrically connected in parallel with the second through the fourth current circuits  142 - 144 , and the second selection circuit  352  is electrically connected in parallel with the first current circuit  141 , and the third selection circuit  351  is electrically connected in parallel with the second current circuit  142 . 
     Furthermore, the identification circuit  103  may provide the same operating modes described above if the first selection circuit  353  is electrically connected in parallel with the first through the third current circuits  141 - 143 , and the second selection circuit  352  is electrically connected in parallel with the first current circuit  141 , and the third selection circuit  351  is electrically connected in parallel with the second current circuit  142 . 
     FIG. 4 is a schematic diagram of a fourth embodiment of a function identification circuit  103  according to the present invention. According to FIG. 4, the first through the fourth current circuits  141 - 144  are electrically connected in series between the first and second pads  111 ,  121  as described above and may be NMOS transistors  461 - 464 . 
     According to FIG. 4, a primary node of a first selection circuit  453  is electrically connected to the first pad  111  and a secondary node of the first selection circuit  453  is electrically connected to the gate/drain node of the first current circuit  144 . A primary node of a second selection circuit  452  is electrically connected to the gate/drain node of first current circuit  141  and a secondary node of the second selection circuit  452  is electrically connected to the gate/drain node of the second current circuit  142 . A primary node of a third selection circuit  451  is electrically connected to the gate/drain node of second current circuit  142  and a secondary node of the second selection circuit  451  is electrically connected to the gate/drain node of the third current circuit  143 . 
     As shown in FIG. 4, the function identification circuit  103  operates in four modes. In particular, the four operating modes associated with the embodiment in FIG. 4 are analogous to the operating modes described above in conjunction with FIG.  2 . 
     FIG. 5 is a flowchart illustrating operations of a function identification circuit according to the present invention. During a manufacturing process, a number of selection circuits, described herein, may be activated to identify the function performed by the circuit  131  (Block  501 ). The activated selection circuits may reduce the voltage difference required between the first and second pads to cause a current to flow into the integrated circuit. 
     A series of predetermined voltages may be applied between the first and second pads of the integrated circuit. A current flow may be detected as a result of one of the applied predetermined voltages. The applied predetermined voltage which causes the current flow may be used to determine which selection circuits are activated, thereby determining the function performed by the circuit (Block  511 ). 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.