Abstract:
An integrated circuit having a die with one or more electrical input cells and one or more optical input cells. The cells are arranged together in an array, and the cells may have generally the same size and equivalent geographies, to permit standard electrical-only tools and practices to be used for designing the die.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     This invention relates to integrated circuits which employ optical inputs to the integrated circuit die as well as one or more electrical inputs or outputs. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the invention provides an interface layout for interfacing electrical and optical input signals to the integrated circuit die, wherein the electrical and optical input cells are arranged in one or more arrays, at least one array including at least one electrical input and at least one optical input. 
     The term “optical” is used herein broadly, and is not limited only to visible light. The term is intended to cover any radiation which obeys substantially the laws of optics. 
     The or each array may be linear (e.g. rows and/or columns), or multi-linear or non-linear, or the arrays may includes both linear and non-linear arrays. 
     One or more of the arrays may be at, or adjacent to, the periphery of the integrated circuit die. Alternatively, or additionally, the die may incorporate a “full area” input/output array in which the array is spread over substantially the entire area of the die (or at least a significant portion of the full area). 
     By arranging the optical and electrical inputs in the above manner, the integrated circuit designer can follow standard design practice, without having to pay extra consideration as whether the input cell is an optical cell or an electrical input cell. 
     Preferably, the optical input cell and the electrical input cell have generally equivalent “geographies”. For example, the optical input cell may include a photosensitive region which is positioned at an equivalent position to the electrically conductive input pad of the electrical input cell. Additionally, or alternatively, the optical input cell may include an input (processing) circuit which is positioned at an equivalent position to the electrical input circuit of an electrical input cell. 
     Preferably, an (or at least one) optical input cell occupies approximately the same size of die area as an electrical input cell (or at least one of the electrical input cells if the electrical input cells have a range of different sizes). Typically, the pad size for an electrical contact wire is about 100×100 μm (although depending on the type of semiconductor substrate and the integration technology, this could be as small as about 50×50 μm. Preferably, an optical input cell includes a photosensitive region of about the same size as a pad for the electrical input (or for at least one of the electrical inputs). 
     Preferably, the optical input cell includes conditioning circuitry which occupies approximately the same size area of die as does the input protection/buffer circuitry associated with an electrical input (or at least one of the electrical inputs). 
     Preferably, the or each optical input enters the integrated circuit package from or through the side of the package. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention is now described by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic plan view of an edge portion of an integrated circuit die; 
     FIG. 2 is a schematic side view of an optical input device; 
     FIG. 3 is a schematic view of an alternative arrangement of input and output cells; and 
     FIG. 4 is a schematic view of a further alternative arrangement of input and output devices. 
     FIG. 5 is a schematic view of an array of input and output devices along two edges of an integrated circuit die. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, an integrated circuit  10  comprises a package  12  containing an integrated circuit die  14 . The package comprises a number of peripheral contacts, in the forms of terminal pins  16  in the present embodiment. Each pin is coupled by a respective lead wire  18  which is welded to an electrical input pad  20 , the pads  20  being arranged adjacent to the peripheral circuit edge of the die  14 . Associated with each pad  20  is an input circuit  30  located adjacent to the pad  20 . The input circuit  30  contains input protection, for example, against static build-up, and may also contain input buffer circuitry. Together the pad  20  and the input circuit  30  form an electrical input cell  30 ′. 
     The integrated circuit  10  also has at least one optical input, provided by an optical input coupler  22  located in the wall of the package  12 . For example, the coupler  22  may be an optical fibre coupler. A short length of optical fibre  24  extends from the coupler  22  to a photosensitive area  26  of the die  14 . The photosensitive area may be formed by a photo-diode, or a photo-transistor. The fibre is secured to the photosensitive area  26  by means of an adhesive encapsulation  28 . Associated with each photo-sensitive area  26 , and located adjacent thereto, is an optical input circuit  32 , for conditioning the signal output from the photo-sensitive area  26 . Referring to FIG. 3, the input circuit typically comprises an input amplifier  34  coupled to the output from the photo-transistor or photo-diode  26 , a thresholding circuit  36  for discriminating the input signal level, and an output buffer amplifier  37 . Together, the photosensitive area  26  and the circuit  32  form an optical input cell  32 ′. 
     The electrical input pads  20  and the electrical input circuits  30 , and the optical input areas  26  and the optical input circuits  32 , are arranged in one or more linear arrays, denoted by broken line  38 . In this embodiment, only one array  38  is required to accommodate the number of inputs illustrated. 
     By arranging the inputs in this manner, the integrated circuit can follow normal design conventions for the placement of input and output circuit cells. This enables the designs to be produced to conventional standards, and allows existing circuit design, analysis, and testing tools to be used with little or no modification. 
     In the illustrated embodiment, the electrical input cells and the optical input cells have roughly the same size. In an alternative embodiment, it may be desirable to reduce or increase the size of one type of cell (for example, the electrical cell) with respect to one or more of the other type of cell. For example, such an arrangement is illustrated in FIG. 4, in which the electrical input circuit cell  30  is increased in size to provide increase circuit protection against static electricity. As seen in FIG. 4, the circuit cells are arranged in a linear array type form, which enables existing design tools to be used in the design of the circuit. 
     Referring to FIG. 5, when a large number of input and output circuit cells are required, it may be necessary to provide more than one linear array  38  of cells, and also to stack the electrical contacts on more than one tier, or terminal pin deck  40 . In the arrangement shown in FIG. 5, multi-linear arrays are employed. However, the invention nevertheless enables the die to be designed in accordance with existing conventions, so that the optical inputs do not have to be positioned at awkward positions away from the remainder of the electrical inputs to the die. 
     Although the above embodiments illustrate one or more arrays positioned adjacent to the periphery of the die, it will be appreciated that the same principles can be applied to a full area input/output array which extends over substantially the entire area of the die. Furthermore, although linear arrays may be preferred in some cases (for grid-like circuit designs), the same principles can be applied to a non-linear array of input and/or output circuit cells, which is not limited to a straight-line or grid based layout. 
     It will be appreciated that the invention, particularly as described in the preferred embodiments, enables electrical input cells and optical input cells to be mixed together in a uniform manner, so that existing design conventions for electrical-only input cells can easily be applied. 
     It will be appreciated that the above description is merely illustrative of preferred embodiments of the invention, and that modifications may be made within the scope of the invention.