Abstract:
The present invention provides an array of customizable functional cells having high density and high drive capacity. It further provides an architecture that maximizes the width of P-channel transistors in an array of standard cells to compensate for the lower speed operation of P-type devices. More particularly, the invention discloses a digital circuit comprising a plurality of inputs for receiving respective logic signal and circuitry, coupled to the inputs, for passing one of the signals responsive to the order in which a transition is received on each of the inputs.

Description:
This application claims priority under 35 U.S.C. §119(e)(1) of provisional application Ser. No. 60/175,553, filed on Jan. 11, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to the field of customized or application specific integrated circuits. More specifically, the present invention relates to an architecture for providing a customized application specific device having high functional density with high operational speed. 
     2. Description of the Related Art 
     There are many conflicting demands on manufacturers of application specific integrated circuits. Customers demand more complexity, but also demand faster development time. In integrated circuit design, the maximum layout density (and thus highest complexity per integrated circuit) is provided by wholly custom layouts. However, custom generation of integrated circuits is very time consuming. It is not possible to meet the customer&#39;s need for quick turn-around with custom layouts. To meet this need while providing good functional density, the use of arrays of standard cells has emerged as a useful architecture. 
     Standard cell arrays are generally arranged in rows having a fixed width. The cells may have varying length to provide the necessary cell functionality. For example, the simplest cells are inverters. In complementary metal oxide semiconductor (CMOS) designs, an inverter will have one N-channel and one P-channel transistor. In between the rows are routing areas for interconnecting the cells. Power leads may also run through the routing areas or may have designated areas overlying the cell areas. Standard cell arrays have been combined with powerful computerized design tools to provide high functional density with fast order turn-around time. An example of this type of device is the GS30 series provided by Texas Instruments. 
     However, the standard cell system provides inherent design compromises. To provide high density, an array may be laid out using the minimum row width. For example, a minimum width may be six squares. A square is normalized unit equal to the minimum feature size that can be formed on the integrated circuit. Six square rows provide a very dense array. However, after applying half of the row to P-type transistors and the other half to N-type, the maximum transistor width is about two squares (after including isolation structures between devices). A transistor this small does not provide adequate drive capacity to provide high-speed operation. On the other hand, providing wide rows for high drive transistors reduces the density of the array. The present invention solves this trade-off by providing an architecture that allows for high density and high drive transistors. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an array of customizable functional cells having high density and high drive capacity. 
     It is a further object of the present invention to provide an architecture that maximizes the width of P-channel transistors in an array of standard cells to compensate for the lower speed operation of P-type devices. 
     These and other objects are provided by one embodiment of the present invention that includes an integrated circuit having a plurality of first circuit elements, the first circuit elements having a first width. These circuit elements are arranged in a plurality of rows in a semiconductor substrate. The integrated circuit also includes a plurality of second circuit elements having a width of twice the width of the first circuit elements. The second circuit elements are placed in the plurality of rows and occupy the width of two of the first circuit elements. 
     An additional embodiment of the present invention includes an integrated circuit comprising a plurality of first circuit elements having a first width. The first circuit elements are arranged in a plurality of rows having a plurality of rows in a semiconductor substrate. The rows are divided into a first area of a first conductivity type and a second area of a second conductivity type. The first and second areas alternate in at least two adjacent rows such that the first areas of the at least two adjacent rows are positioned adjacent to each other. The integrated circuit includes a plurality of second circuit elements having a width of twice the first circuit elements. The second circuit elements positioned in the plurality of rows and occupying the width of two of the first circuit elements. At least one of the second circuit elements spans two adjacent rows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference should be made to the following Detailed Description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a layout diagram of a prior art array; 
     FIG. 2 is a layout diagram comparing two prior art cells from a standard cell array; 
     FIG. 3 is a layout diagram of a portion of a standard cell array that is one embodiment of the present invention; 
     FIG. 4 is another view of FIG. 3 with the grid lines removed; 
     FIG. 5 is layout view of another embodiment of the present invention; 
     FIG. 6 is layout view of an additional embodiment of the present invention including; 
     FIG. 7 is a layout of a standard cell suitable for use with the present invention; 
     FIG. 8 is a layout of a another standard cell suitable for use with the present invention; and 
     FIG. 9 is a layout view of a high drive cell suitable for use with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a layout diagram of a prior art cell array. Array  10  consists of a plurality of rows  12  for placement of circuit elements, often called standard cells. FIG. 2 is a layout diagram of example cells as used in the prior art. Cell  20  is designed for high density. The row width for an array including cell  20  is six squares. Cell  20  is shown with a length of four squares. However, cells used with cell  20  may have a number of lengths. Cell  30  is designed for a high speed array. Cell  30  has a width of eight squares with a length of five squares. In addition, cell  30  provides the same functionality as cell  20 , but the transistors in cell  30  are wider. As is commonly used in the industry, transistor width is the surface dimension perpendicular to the flow of carriers in the transistor&#39;s channel region. A wider transistor has a greater current drive capability. Greater current drive capability allows cell  30  to operate faster than cell  20 . However, an array including cell  30  will be larger for the same functionality or some functionality must be excluded if the arrays are the same size. The present invention avoids the need to compromise array size and array speed. 
     FIG. 3 is a layout diagram of an array designed using the principles of the present invention. Array  40  is preferably a CMOS or Bipolar CMOS integrated circuit fabricated using a process such as that shown in Smayling et al., U.S. Pat. No. 5,767,551, which is assigned to the assignee of this application and which is hereby incorporated by reference. The portion of array  40  shown in FIG. 3 is two rows  41  and  43  of a standard cell array. The complete layout area of cells  42 ,  44 ,  46  and  48  are shown. In addition, a portion of cells  50  and  52  are shown. These cells are compact cells with a width of six squares. Routing areas  58  and  60  are provided for inter-cell routing of leads, including power (V DD ) and ground (V SS ) leads where appropriate. 
     Array  40  includes cells  54  and  56  with widths of twelve squares that span both rows  41  and  43 . These cells are included in the array when high drive is needed to maintain circuit speed. For example, a cell may need to drive inputs to several other cells. If a low drive cell, such as cell  46 , were used for this function, the lower drive current would require too much time to charge or discharge the inputs of down-stream cells to the desired signal value. By using cells  54  and  56 , high drive cells can be used when needed to maintain speed, but small cells ( 42 ,  44 ,  46 ,  48 ,  50  and  52 ) can be used for the majority of the array&#39;s functionality. This provides a high speed array with high density. 
     FIG. 4 is another view of array  40  with the grid lines removed to more clearly see the layout of the cells and the routing areas. FIG. 5 is a layout of an array  140 , which is another embodiment of the present invention. Like numbered components in FIG. 5 provide the same function as those shown in FIG.  3 . The embodiment of FIG. 5 includes N-well  142  that spans rows  41  and  43 . In addition, P-well  144  is formed in row  41  and P-well  146  is formed in row  43 . N-well  142  is for the formation of P-channel transistors in accordance with known fabrication techniques for making P-channel transistors such as those shown in Smayling et al. 
     N-well  142  is actually two N-wells formed adjacently. One for row  41  and one for row  43 . Of importance, in cells  54  and  56 , N-well  142  forms one contiguous area. This allows for the formation of transistors that include the full width of N-well  142  less the area needed for isolation from devices formed in P-wells  144  and  146 . This structure allows cells  54  and  56  to have very wide P-channel transistors. As is well known in the art, P-channel transistors inherently have lower drive capability than N-channel transistors because holes are the primary carrier mechanism in P-channel transistors. Electrons are the primary carrier mechanism in N-channel devices. Holes are less mobile than electrons. Thus, an N-channel transistor will provide less drive current for the same transistor size, characteristics and drive voltages. The advantages of providing wide P-channel transistors in the embodiment of FIG. 5 will be explained more fully below. 
     FIG. 6 is a layout diagram of another embodiment of the present invention. Like numbered components in array  240  perform the same function as those of array  40 . As with array  140  of FIG. 5, array  240  is designed for CMOS cells. N-well  242  provides an area for P-channel transistors in row  41 . P-well  244  provides an area for N-channel transistors in row  41 . N-well  248  provides an area for P-channel transistors in row  43 . P-well  246  provides an area for N-channel transistors in row  43 . Thus, complete CMOS cells can be formed in each row. 
     FIG. 7 is a layout diagram of a D-type flip-flop cell  300  suitable for use with the present invention. Flip-flop  300  uses a row width of seven squares and is thus suitable for use in a single seven square row. V DD  is provided in routing area  58 . A ground bus overlying the border between rows provides V SS . The D input signal is provided at terminal  310  and a clock signal is input at terminal  312  and an output on terminal  316  as the Q output. Area  344  is an N-well for P-channel transistors and area  342  is a P-well for N-channel transistors. 
     FIG. 8 is an inverter  400  suitable for use in one row in the present invention. The active components of inverter  400  are P-channel transistor  410  and N-channel transistor  412 . P-channel transistor  410  is formed in N-well  444 . N-channel transistor  412  is formed in P-well  442 . V DD  is provided to the source of transistor  410 . V SS  is provided to the source of transistor  412 . The drains of transistors  410  and  412  are tied together using lead  414  and provided to output terminal  416 . The input terminal  418  is tied to gate  420 , which serves as the gate for both transistors  410  and  412 . 
     In contrast to inverter  400 , inverter  500  of FIG. 9 is high drive invertor suitable for use in a two row cell. The source of P-channel transistor  510  is connected to V DD  via lead  516 . Lead  516  is a common bus overlying the border of rows  541  and  543 . The sources of transistors  512  and  514  are connected to V ss  by leads  518  and  520 , respectively. Gate  522  serves as a common gate for transistors  510 ,  512  and  514 , and as and input terminal. The drains of transistors  510 ,  512  and  514  are tied together using leads  524  and  526 , which serve as output terminals. In a preferred embodiment, leads  524  and  526  will be one lead formed in a multilevel metal system. P-channel transistor  510  is formed in N-well  542 . N-channel transistors  512  and  514  are formed in P-wells  544  and  546 , respectively. Of importance, the width W of P-channel transistor  510  is equal to the width of an entire row less the area used for isolation from transistors  512  and  514 . This is more than twice the channel width of transistor  410  of FIG. 8 because there is no need for isolation between the two halves of transistor  510 . On the other hand, transistor  410  must have isolation devices on both the top and bottom of its source and drain diffusions. Thus, the described embodiment of the present invention allows the use of selected transistors that are more than twice the width achievable using the prior art. 
     Although specific embodiments of the present invention are described herein, they are not to be construed as limiting the scope of the invention. For example, although specific circuits and device fabrication techniques are described and referred to herein, many specific devices and fabrication techniques may be advantageously used within the scope of the invention. Many embodiments of the invention will become apparent to those skilled in the art in light of the teachings of this specification. For example, although the described embodiments use adjacent N-well regions to provide wide P-channel transistors, the teachings of the invention may be used to provide wide N-channel transistors in adjacent P-well regions. As another example, although the described embodiments use CMOS transistors, the teachings of the invention may be advantageously applied to circuits using bipolar transistors or circuits using only P or N type transistors. The scope of the invention is only limited by the claims appended hereto. 
     Having thus described my invention, what I claim as new and desire to secure by letters patent is set forth in the following claims: