Semiconductor integrated circuit

Abstract: This invention is concerned with semiconductor integrated circuits of the type comprising configurable logic circuit arrays of the type disclosed in specification No. GB B-2180382. Such an array may be programmed to configure a plurality of NAND-gates (G1) in the array to perform various and different logic functions. This invention is particularly concerned with the provision of an additional logic circuit (c) at each discrete site or cell which additional logic circuit (c) is controllable by a control means (GCS) to cause the additional logic circuit (c) and the logic circuit to operate as either a simple NAND logic function or a simple LATCH logic function.

Description: The present invention relates to electronic integrated circuits of the type comprising configurable logic circuit arrays. GB-A-2202356 discloses a configurable combinational logic circuit for a configurable logic array which comprises a plurality of logic gates. Each circuit can be configured to perform a particular logic function under the control of binary input signals. Each circuit can be connected to any other circuit through a universally programmable connection network. Each circuit contains sufficient logic gates to enable it to perform any desired logic function. This invention finds particular utility in a configurable logic circuit array of the type as disclosed in our Specification No. GB-B-2180382 in which the logic circuit array comprises a matrix of discrete sites or cells at each of which is a logic circuit which is adapted to perform a simple logic function. Typically the simple logic function is implemented by means of a two-input NAND gate. An array of this type is capable of being programmed in such a manner as to configure the various NAND gates, as required, to perform various and different logic functions. One such function is known as a latching function and in the logic array as disclosed in Specification No. GB-B-2180382, a latching function may be implemented using four NAND gates. This has the disadvantage that the greater number of latching functions that may be required from any logic array the fewer NAND gates remain for other required functions. This has the effect of reducing the overall effectiveness of the array. An object of this invention is to overcome this disadvantage by providing an additional logic circuit for inclusion in each of the logic circuits at each discrete site to enable each site to have a greater programmable facility and thereby increase the overall utilisation of the array. Reference is also made to EP-A-0270145 which discloses an emitter coupled logic circuit which includes a by-pass circuit for providing a conductive path for current when a programmable fuse is blown. In a first mode, a clock signal is provided each time an input value is to be written to a latch and in a second mode (when the fuse is blown) input data is transmitted independently of the state of a clock signal so that the circuit functions as a combinatorial circuit. According to the present invention there is provided a semiconductor integrated circuit including a configurable logic array formed on an area of said integrated circuit, said logic array comprising: a plurality of discrete sites, each site having at least one input and at least one output; a plurality of first logic circuits individually defined at each of said sites, each first logic circuit only being capable of implementing a first simple logic function; and a signal translation system between inputs and outputs of the sites, said signal translation system providing a plurality of direct connection paths between said inputs and outputs wherein each connection path is selectable as to its conduction state, and wherein said signal translation system is a restricted signal translation system by virtue of said direct connection paths extending, for each of said sites, from at least one output of that site to inputs of a respective first set of some of other said sites and from at least one input of that site to outputs of a respective second set of some of other said sites, and wherein each first set for that site is different from the first set of any other site, and wherein each second set for that site is different from the second set of any other site; characterised in that the first logic circuit at each site comprises a logic gate and a first inverter coupled to an output of said logic gate; said logic array further comprises an additional logic circuit at each discrete site, said additional logic circuit including: a second inverter in parallel with and in a back-to-back connection with said first inverter with the input of the first inverter connected to the output of the second inverter thereby to serve as a latching mechanism, and first switching means connected between the output of the logic gate and the input of the first inverter; the first switching means having an input for receiving an input signal for controlling the state of the first switching means; second switching means are connected between the output of the logic gate and the input of the first inverter; and selection control means are provided at each discrete site arranged to supply a control signal to the second switching means so that with the control signal in a first state the second switching means is conductive and enables the first logic circuit to function by itself to implement said first simple logic function and, with the control signal in a second state, the second switching means is non-conductive, causing the first logic circuit to function in combination with the first switching means and the second inverter to implement a latching function. The logic gate is conveniently a NAND gate. Preferably, the first switching means is a transmission gate comprising first and second transistors, the transmission gate being controllable to closed and open operative states by signals input to the transmission gate. The second switching means can comprise a single transistor whose conduction state is controlled by the said control signal. When this is the case, the control signal when present causes the single transistor to be in a conduction state as a consequence of which the transmission gate becomes short circuited and inoperative and thereby the cell operates exclusively as a NAND gate function. When the control signal is not present, the single transistor would be in a non conduction state enabling the transmission gate to be controlled to the open and closed operative states and the cell operates exclusively as a LATCH circuit function. It may be arranged that the signal transient time for an input signal to pass to the transmission gate is shorter than the transient time for the same signal to pass through the logic gate to thereby effect rapid switching of the transmission gate between the closed to the open state. A further aspect of this invention envisages the cascading of a plurality of individual cells which are configured for the LATCH circuit function to form a shift register. The invention will be more readily understood from the following description of an exemplary embodiment which should be read in conjunction with the accompanying drawing. The drawing illustrates a schematic diagram of two interconnected logic circuits or cells each of which incorporates the additional logic circuit in accordance with this invention. Referring to the drawing, for convenience like components in each cell have been identically designated. The two cells shown are master MC and slave SC latch cells. Each cell comprises a basic two-input NAND gate G1 together with an output buffer I1 in the form of an inverter and a further inverter I2. Referring to the master cell MC, one of the inputs CK of the gate G1 carries clock signals from a multiplexer means MUX1 to the cell while the other input D carries data signals from a further multiplexer means MUX2 to the cell. As disclosed in GB-B-2180382, the multiplexers (MUX1, MUX2) receive signals from neighbouring cells and/or from directly connected bus lines and select an appropriate input to the cell from these signals. In the Figure, the site MC has two inputs (to MUX1 and MUX2 respectively) and an output from the inverter 11. The site SC has two inputs (to MUX1 and MUX2 respectively) and one output from inverter I1. The sites are used in an array where the sites are interconnected by a signal translation system between inputs and outputs as described more fully in GB 2180382. The signal translation system provides a plurality of direct connection paths between said inputs and outputs wherein each connection path is electrically selectable as to its conduction state, and wherein said signal translation system is a restricted signal translation system by virtue of said direct connection paths extending, for each of said sites, from at least one output of that site to inputs of a respective first set of some of other said sites and from at least one input of that site to outputs of a respective second set of some of other said sites, and wherein each first set for that site is different from the first set of any other site, and wherein each second set for that site is different from the second set of any other site. An additional logic circuit C is connected between the NAND gate G1 and the output buffer I1. By the utilisation of control signals from a bit store BS, which also provides control signals for the multiplexers MUX1 and MUX2, the additional logic circuit is arranged to change the NAND function of the cell into a LATCH circuit function. The additional logic circuit comprises an additional inverter I3 which is connected in parallel and in back-to-back relationship with inverter I2 in the existing logic circuit and is so arranged to perform a latching mechanism. A further inverter I4 is included to invert the clock signals from input CK of cell MC to the transistor T2 of cell SC. Two switching means are provided, the first comprises two transistors T1 and T2 of opposite polarity types connected in parallel to form a transmission gate, and the second a single transistor T3 serving as a simple switch. In operation, selection of the cell (comprising the logic circuit together with the additional logic circuit) to operate either as a NAND gate function or as a LATCH function, is controlled by a gate control signal GCS which emanates from the bit store BS. When the signal GCS is present the transistor T3 is caused to conduct and the switch short circuits transistors T1 and T2 forming the transmission gate making it inoperative. In these circumstances the cell operates as a NAND gate function exclusively. On the other hand when the signal GCS is not present, transistor T3 is not conducting and transistors T1 and T2 are now controlled by the clock signals from the input CK of the NAND gate G1, being applied directly to transistor T1 on the one hand and through an inverter I4 to transistor T2 on the other hand, together with the prevailing output OP of the gate G1 itself to the transmission gate. The signals now passed by the transmission gate (T1 + T2) from the output OP are effectively latched by the latching mechanism comprising inverters I2 and I3. In these circumstances the cell operates as a LATCH circuit function exclusively. The operation of the latching function and control of the transmission gate are summarised in the following table: When the input signal CK is at a binary one condition and the transmission gate is closed, the LATCH (I2 and I3) follows the data signal on input D. However, in order to prevent the signal stored in the latch being lost when the transmission gate is about to be opened the signals to the transmission gate must be fast enough to effect a change of state of the switch before the signal at the input of gate G1 is effective at the LATCH (I2 + I3). Thus, a rapid switching of the LATCH (I2 + I3) is achieved as a consequence of the shorter signal transient times for signals to pass to the transmission gate than through the NAND gate G1 itself. In the utilisation of this invention including cells operating in a latch mode as described herein and further interconnected in the manner of a shift register (two such stages being shown as master cell MC and slave cell SC), it is convenient to use the inverted output by way of the inverter I4 from a preceding cell to serve as an inverted clock input to the succeeding cell. In such an arrangement, it will be apparent to those skilled in the art that using a number of latches cascaded together will form a suitable shift register which not only give an increase in operating speed but will also contribute to a more effective use of the array itself.

Claims: A semiconductor integrated circuit including a configurable logic array formed on an area of said integrated circuit, said logic array comprising: a plurality of discrete sites (MC,SC), each site having at least one input and at least one output; a plurality of first logic circuits (G1,I2) individually defined at each of said sites, each first logic circuit (G1,I2) only being capable of implementing a first simple logic function; and a signal translation system between inputs and outputs of the sites, said signal translation system providing a plurality of direct connection paths between said inputs and outputs wherein each connection path is electrically selectable as to its conduction state, and wherein said signal translation system is a restricted signal translation system by virtue of said direct connection paths extending, for each of said sites (MC,SC), from at least one output of that site to inputs of a respective first set of some of other said sites and from at least one input of that site to outputs of a respective second set of some of other said sites, and wherein each first set for that site is different from the first set of any other site, and wherein each second set for that site is different from the second set of any other site; characterised in that the first logic circuit (G1,I2) at each site comprises a logic gate (G1) and a first inverter (I2) coupled to an output of said logic gate (G1); said logic array further comprises an additional logic circuit (C) at each discrete site (MC, SC), said additional logic circuit (C) including: a second inverter (I3) in parallel with and in a back-to-back connection with said first inverter (I2) with the input of the first inverter (I2) connected to the output of the second inverter (I3) thereby to serve as a latching mechanism, and first switching means (T1,T2) connected between the output of the logic gate (G1) and the input of the first inverter (I2), the first switching means having an input for receiving an input signal for controlling the state of the first switching means; second switching means (T3) are connected between the output of the logic gate (G1) and the input of the first inverter (I2); and selection control means (BS) are provided at each discrete site arranged to supply a control signal (GCS) to the second switching means so that with the control signal in a first state the second switching means is conductive and enables the first logic circuit (G1,I2) to function by itself to implement said first simple logic function and, with the control signal in a second state, the second switching means is non-conductive, causing the first logic circuit (G1,I2) to function in combination with the first switching means and the second inverter (I3) to implement a latching function. An integrated circuit according to claim 1, wherein the first switching means (T1, T2) is a transmission gate comprising first and second transistors (T1,T2), the transmission gate (T1,T2) being controllable between closed and open operative states by said input signal. An integrated circuit according to claim 2, wherein the logic gate of each logic circuit (G1,I2;C) is arranged to receive said input signal and is such that the transient time for said input signal to pass to the transmission gate (T1,T2) is shorter than the transient time for the same signal to pass through the logic gate (G1) to thereby effect rapid switching of the transmission gate (T1,T2) between the closed and open state. An integrated circuit according to claim 1, 2 or 3, wherein the second switching means (T3) comprises a single transistor (T3) having a control electrode connected to receive the control signal from the selection control means (BS). An integrated circuit according to any preceding claim, wherein the first logic function is a NAND gate function. An integrated circuit according to any preceding claim wherein a plurality of individual sites which are configured for the latching function are cascaded to form a shift register.

Inventors: AUSTIN KENNETH, CLIFF RICHARD GUY MAUD COTTAGE, AUSTIN, KENNETH, CLIFF, RICHARD GUY MAUD COTTAGE
Applicants: PILKINGTON MICRO ELECTRONICS, PILKINGTON MICRO-ELECTRONICS LIMITED
Application year: 1989
Main classification: H03K19
IPC codes: H03K19, H01L21