Programmable logic cell

Abstract: A programmable logic cell suitable for use in a programmable gate array and able to produce any logical function of two inputs, operate as a 2 to 1 multiplexor or a data latch is formed by four multiplexors (1,2,3,4), five inverters (12,13,15,16,17) and an OR gate (14) to provide a very fast programmable logic cell.

Description: This invention provides a programmable logic cell, and in particular a programmable logic cell for use in a programmable gate array. Programmable gate arrays comprise a large number of programmable logic cells linked together by a routing network. A programmable logic cell is able to carry out a number of different logic functions on its inputs, and is programmed after manufacture to carry out a particular one of these possible functions. A problem with such logic cells is that if they carry out complex logic functions the time delay between data input and output can become quite significant and this can be a problem, particularly in programmable gate arrays where data will generally be routed in series through a large number of programmable logic cells. An example of this are the programmable logic cells, which, in order to allow complex logical functions to be generated, must use XOR gates, increasing their time delay significantly due to the inherent slowness of such gates. The present invention was intended to produce a programmable logic cell able to carry out a wide range of complex logic functions while having a relatively short delay time. In New Electronics, Vol. 22, No. 10, pages 17 and 18, there is described in outline a programmable logic cell for use in a configurable array, the cell comprising four multiplexors, three of them having two inputs and a fourth one having four inputs, the multiplexors being interconnected to provide an output signal which may be any combinatorial logic function of signals at two inputs X1 and X2, including four types of D latch. The present invention provides a programmable logic cell having a first input, a second input and a third input and an output and comprising a first multiplexor and a second multiplexor each having four inputs, a control signal input and an output, a third multiplexor and a fourth multiplexor each having two inputs, a control signal input and an output, first circuit means arranged to feed a signal at the first input of the logic cell to the third input of the first multiplexor and to the first input of the second multiplexor and to feed said signal at the first input of the logic cell by way of an inverter to the fourth input of the first multiplexor and to the second and third inputs of the second multiplexor, second circuit means arranged to feed a signal at the second input of the logic cell to the fourth input of the second multiplexor and to the second input of the third multiplexor and to feed said signal at the second input of the logic cell by way of an inverter to the first input of the third multiplexor, third circuit means arranged to feed a signal at the third input of the logic cell to the second input of the first multiplexor and to feed said signal at the third input of the logic cell by way of an inverter to the first input of the first multiplexor, means connecting the output of the first multiplexor to the control signal input of the fourth multiplexor, means connecting the outputs of the second and third multiplexors to the first and second inputs of the fourth multiplexor respectively, and means connecting the output of the fourth multiplexor to the output of the logic cell. This provides the advantage that because only multiplexors and inverters may be used, with one OR gate if a latch function is required, the programmable logic gate has a very short delay between data input and data output, particularly compared to a system using XOR gates. It has been realised that any possible logical function of two inputs may be produced by using a 2 to 1 multiplexor with selective input inversion. For example, consider the signals X and Y and a 2 to 1 multiplexor producing the function F = (A.C)+(B.C bar) to produce the function F = X.Y the multiplexor connections required are as follows:- let A = X B = Y C = X bar then F = (X.X bar)+(Y.X)=0+Y.X=X.Y To produce the functions F = X+Y the multiplexor connections required are as follows:- let A = X B = Y C = X then F = (X.X)+(Y.X bar)=X+Y.X bar =X+Y This can be extended to produce any function of two inputs. A system embodying this invention will now be described by way of example only with reference to the accompanying diagrammatic figure in which; Figure 1 shows a programmable logic cell employing the invention. Figure 1 shows a logic cell 5 based on this principle and arranged to produce from two inputs any logical function of the two inputs, a data latch with set and a 2 to 1 multiplexor. The function actually carried out being selected by external programming means. The logic cell 5 comprises an A input 6, a B input 7 and a C input 8. These inputs are supplied to three multiplexors 1,2 and 3 which are controlled by external programming means (not shown), while the outputs of the three multiplexors 1 to 3 are supplied to a 2 to 1 multiplexor 4. The multiplexor 1 is a 4 to 1 multiplexor having inputs A1, B1, C1, D1 and an output out 1 and is controlled by a two bit control signal R1, R2, on control lines 9. The multiplexor 2 is a 4 to 1 multiplexor with inputs A2, B2, C2 and D2 and an output out 2 and is controlled by a two bit control signal R3, R4 along lines 10. The multiplexor 3 is a 2 to 1 multiplexor with inputs A3, B3, and an output out 3 and is controlled by a one bit control signal R5 along control line 11. The 2 to 1 multiplexor 4 is arranged such that a logical 1 from the output of the multiplexor 1 will switch its input A4, which is the output (out 2) of multiplexor 2 to its output (out 4) while a logical 0 from the multiplexor 1 will switch input B4 which is the output (out 3) of the 2 to 1 multiplexor 3 to its output (out 4). The inputs to the three multiplexors 1,2 and 3 are provided as follows, the A input 6 passes through an inverter 12 and the output of the inverter 12 is linked directly to the D1 input of multiplexor 1 and the B2 input of the multiplexor 2. The output of the inverter 12 is also passed through an inverter 13 to the C1 input of the multiplexor 1 and the A2 input of the multiplexor 2, the output of the inverter 12 is also supplied to one input of an OR gate 14. The B input 7 is supplied to an inverter 15 and the output of the inverter 15 is passed into the A3 input of the multiplexor 3 and is also supplied to an inverter 16. The output of the inverter 16 is supplied to the B3 input of the multiplexor 3 and the D2 input of the multiplexor 2. The C input on line 8 is supplied to the B1 input of the multiplexor 1 and to an inverter 17. The output of the inverter 17 is supplied to the A1 input of the multiplexor 1. In order to provide a feed back path for a latch function the output of the 2 to 1 multiplexor 4 is passed back to the second input of the OR gate 14 and the output of this OR gate 14 is supplied to the C2 input of the multiplexor 2. The output from the 2 to 1 multiplexor 8 is passed through an amplifier 17 to provide the output of the logic cell. The multiplexor connections for multiplexors 1,2 and 3 required for simple logical functions of the two inputs A and B are shown in table 1, for each of the functions this shows which input of multiplexors 1,2 and 3 must be connected to their respective outputs. To provide these simple logical functions no control C input along line 8 is required and signals along line 8 will not effect the output. To allow the logic cell to operate as a multiplexor a third control C input along input line 8 is necessary to act as a control input, the necessary multiplexor settings for the available multiplexor functions are shown in Table 2. When the logic cell operates as a latch the A input along line 6 is the set signal while the B input along line 7 is the data signal and the C input along line 8 is the enable signal, the multiplexor settings for the various latch functions are shown in Table 3. It may be desired to produce functions of a single input from the logic cell and in order to do this a connection between the A and B inputs on lines 6 and 7 must be provided outside the cell, such a connection is not shown in Figure 1 but can easily be made. The various single input functions, the required multiplexor settings and whether input A should be connected to input B or vice versa are set out in Table 4. Since this logic cell uses only multiplexors and inverters and a single OR gate it is quick even though it is capable of a wide range of complex functions. The programmable logic cell is intended for use in a programmable gate array and is particularly suitable for use in any field programmable gate array. The control signals used to set the multiplexors can be provided from a RAM memory, in which case the programming of the logic cell can be altered or could be provided in some other way such as a fuse or antifuse permanent programming system. Any type of multiplexor can be used.

Claims: A programmable logic cell having a first input (6), a second input (7) and a third input (8) and an output and comprising a first multiplexor (1) and a second multiplexor (2) each having four inputs, a control signal input (9, 10) and an output, a third multiplexor (3) and a fourth multiplexor (4) each having two inputs, a control signal input and an output, first circuit means (12, 13, 14) arranged to feed a signal at the first input (6) of the logic cell to the third input of the first multiplexor (1) and to the first input of the second multiplexor (2) and to feed said signal at the first input (6) of the logic cell by way of an inverter (12) to the fourth input of the first multiplexor (1) and to the second and third inputs of the second multiplexor (2), second circuit means (15, 16) arranged to feed a signal at the second input (7) of the logic cell to the fourth input of the second multiplexor (2) and to the second input of the third multiplexor (3) and to feed said signal at the second input (7) of the logic cell by way of an inverter (15) to the first input of the third multiplexor (3), third circuit means (17) arranged to feed a signal at the third input (8) of the logic cell to the second input of the first multiplexor (1) and to feed said signal at the third input (8) of the logic cell by way of an inverter (17) to the first input of the first multiplexor (1), means connecting the output of the first multiplexor (1) to the control signal input of the fourth multiplexor (4), means connecting the outputs of the second and third multiplexors (2, 3) to the first and second inputs of the fourth multiplexor (4), respectively, and means connecting the output of the fourth multiplexor (4) to the output of the logic cell. A programmable logic cell as claimed in Claim 1 wherein said first circuit means (12, 13, 14) includes an OR-gate (14) having its output connected to the third input of the second multiplexor (2), means connecting the first input (6) of the logic cell to a first input of the OR-gate (14) by way of an invertor (12) and means connecting the output of the fourth multiplexor (4) to a second input of the OR-gate (14).

Inventors: HASTIE NEIL STUART, HASTIE, NEIL STUART
Applicants: PLESSEY SEMICONDUCTORS LTD, PLESSEY SEMICONDUCTORS LIMITED
Application year: 1993
Main classification: H03K19
IPC codes: H03K19