Inverters and Logic Gates Employing Inverters

This invention relates to inverters and logic gates employing inverters.

According to the invention, there is provided a circuit element having an input, an output, programmable means providing either a direct or an inverse signal path between the said input and said output and programming means responsive to a programming signal for determining whether said direct or said inverse path is subsequently operative. Such a circuit element is hereinafter referred to as a "programmable inverter."

According to a preferred form of the invention, there is provided a logic gate having at least one input and at least one output, at least one of said inputs and said output incorporating a programmable inverter.

The logic gate may take the form of any gate of the vertex type (AND/OR/NAND/NOR) and may also be an EXCLUSIVE-OR or an EXCLUSIVE-NOR gate.

Preferably the state of the or each programmable inverter is determined by the combinations of electrical signals applied to the inputs or the outputs of such inverters usually in combination with a separate programming signal.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawing, in which:

FIGS. 1 and 2 are schematic circuit diagrams of a programmable inverter in accordance with the invention,

FIG. 3 is a schematic diagram of a logic gate incorporating programmable inverters of the type shown in FIG. 1 or FIG. 2,

FIG. 4 is a schematic circuit diagram of an alternative implementation of the gate shown in FIG. 3, and

FIG. 5 is a side view of a socket for an integrated circuit package.

Referring to FIG. 1, a programmable inverter in accordance with the invention has its input terminal 10 directly connected to one input of an EXCLUSIVE-OR gate 12 the output of which forms the output of the inverter. The input terminal 10 is also connected to the other input of the EXCLUSIVE-OR gate 12 by a diode stack 14. This second input of the EXCLUSIVE-OR gate is also connected to a reference voltage +V (logical 1) by a resistor 16 and to reference voltage 0 (logical 0) by a fusible link 18.

If no programming signal is applied, the fusible link 18 holds the second input of the EXCLUSIVE-OR gate at logic 0. The current in the fusible link 18 is insufficient to cause fusing and the diode stack 14 has an overall forward conduction voltage which exceeds the switching threshold of the gate 12. Consequently the same logical signal appears at the output of the EXCLUSIVE-OR gate 12 as is applied to the input 10.

The inverter can be programmed into the invert mode by applying to the input 10 a negative voltage so as to cause a current to flow through the diode stack 14 and the fusible link 18, such current being of sufficient magnitude to fuse the latter so that it becomes open circuit. Thereafter, in normal use, the resistor 16 holds the second input of the EXCLUSIVE-OR gate 12 at logical 1 so that the logical output of the gate 12 is the inverse of the signal applied at the input 10.

The diode stack 14 may be replaced by a diode stack 17 connected between the second input of the EXCLUSIVE-OR gate 12 and the output 19 thereof, as shown in FIG. 2. The circuit operates as before except that, when it is desired to fuse the link 20, the negative voltage is applied to the output 19 instead of to the input 10.

FIG. 3 shows a logic gate 20 employing programmable inverters in accordance with the invention.

The gate 20 has its two inputs A and B connected to the outputs of respective programmable inverters 21 and 22 and its output Z connected to a programmable inverter 23. All of the programmable inverers 21, 22 and 23 may be as illustrated in FIG. 1. Input signals A' and B' are applied to the inverters 21 and 22 and the inverter 23 produces and output Z'. The possible functions synthesised by this circuit when the logic gate 20 is either an AND gate or an EXCLUSIVE-OR gate are shown in the following table:

FIG. 4 shows an alternative implementation of the circuit shown in FIG. 2 for use when the logic gate 20 is to be an AND gate. The two inputs D and E are connected to respective invert/non-invert circuits 30 and 32, each of which has two outputs comprising signals which are the same as and the inverse of the input respectively. Each of these four outputs is connected by a respective fusible link 34, 36, 38, 40 to a node 42. The node 42 is also connected to a reference voltage +V by a resistor 44, to a programming terminal 46 by a diode 48, and directly to the input of an output buffer 50, the output of which forms the output of the circuit.

In use, a positive programming voltage is applied to the terminal 46 and, at the same time, each of the inputs D and E is set to either logical 1 or logical 0. If the input D is logical 1, the link 36 is fused, while if it is logical 0, the link 34 is fused. Similarly if the input E is logical 1, the link 40 is fused, while if it is logical 0, the link 38 is fused. Thus, logical 1 is applied to the invert/non-invert circuit of an input which is required to be an inverting input while logical 0 is applied to the invert/non-invert circuit of an input which is required to be a direct input.

In subsequent use, either the direct or inverted output from each of the circuits 30 and 32 is applied by the corresponding surviving fusible link to the node 42 which, together with the resistor 44, forms a wired-AND gate. The diode 48 prevents signals on the node 42 from being affected by the signals on nodes of other logic gates supplied from the same programming voltage source.

FIG. 5 shows an integrated circuit 52 suitable for connection to a socket 54 mounted on a printed circuit board 56. Each connector of the socket, except for the power supply connectors, incorporates a programmable inverter, for example as illustrated in FIG. 1. Since such a programmable inverter is a one-way device, it is necessary to predesignate some of the connectors of the socket as input connections and others as output connections and install the programmable inverter in the socket accordingly. Power for the programmable inverters is taken from the power supply connections of the socket. This arrangement not only eliminates the need to provide inverter packages on the printed circuit board, thereby simplifying the layout thereof, but also enables a better utilisation to be made of existing integrated circuits. For example, an OR gate can be converted to an AND gate by providing inverters as its output and/or of its input. Thus, only one type of integrated circuit package is needed to provide all vertex functions and, in general, fewer packages will be only partially utilised.