Abstract:
One embodiment of the invention provides a system that facilitates designing an integrated circuit using a mask-programmable fabric, which contains both mask-programmable logic and a mask-programmable interconnect. During operation, the system receives a description of a mask-programmable cell, wherein instances of the mask-programmable cell are repeated to form the mask-programmable fabric. The system uses this description of the mask-programmable cell to generate a derived library containing cells that can be obtained by programming the mask-programmable cell. Next, the system receives a high-level design for the integrated circuit. The system then performs a synthesis operation on the high-level design to generate a preliminary netlist for the high-level design, wherein the preliminary netlist contains references to cells in the derived library. Finally, the system converts the preliminary netlist into a netlist that contains references to the mask-programmable cell with the logic appropriately programmed. The netlist is then placed and routed with the mask programmable constraints on the mask programmable fabric. The design, thus implemented on the programmable fabric, can be changed to accommodate logic revisions and bug fixes by changing only a few masks required for its fabrication.

Description:
BACKGROUND 
   1. Field of the Invention 
   The invention relates to the process of designing an integrated circuit. More specifically, the invention relates to a method and an apparatus for designing an integrated circuit using a mask-programmable fabric, which contains both mask-programmable logic and a mask-programmable interconnect. 
   2. Related Art 
   The dramatic improvements in semiconductor integration densities in recent years have largely been achieved through corresponding improvements in process technologies involved in manufacturing the semiconductor chips. As semiconductor feature sizes continue to decrease, the cost of producing a set of masks used to manufacture a semiconductor chip has increased considerably. For example, industry analysts predict that the cost of generating a set of masks (˜30 or more masks are typically required) for a single design in a 90 nm process will exceed one million dollars. 
   Note that mask re-spins are frequently required to correct logic bugs after first silicon, or to deal with changing requirements. Note that requirements frequently change because over the lifetime of a product; various derivatives of a semiconductor chip are required to accommodate changing formats or interfaces. 
   In order to avoid costly mask re-spins to deal with changing requirements, many designers use field-programmable gate arrays (FPGAs) to implement portions of a design that are likely to change. In this way, it is possible to make changes to a system by simply reprogramming the FPGA without incurring the cost of an additional mask re-spin. 
   However, there are a number of problems with using FPGAs. Although FPGAs provide a great amount of flexibility, FPGAs are relatively expensive in terms of area, power and delay. For example, implementing a function using an FPGA can consume up to ten times the area and 500 times the power of a standard cell implementation. Furthermore, FPGAs are typically located off-chip, so a considerable delay is typically involved in communicating with the FPGA. 
   Some developers have considered integrating FPGA circuitry into conventional semiconductor chips along with other types of circuitry. However, a number of interconnection issues still remain. The interconnect architecture in an FPGA depends on the underlying technology used to achieve programmability. In the case of anti-fuse based technologies, programmability is achieved by selectively blowing the anti-fuses. Hence, such an architecture is field-programmable only once and requires programming hardware. In the case of SRAM-based FPGA technologies, the routing programmability needs access to active silicon, which gives rise to additional complications. 
   Hence, what is needed is a method and an apparatus for designing an integrated circuit in a manner that avoids re-spins of the full mask set and without the above-described problems of using an FPGA. (Note that our invention enables the re-spins of only a subset of masks to configure the circuitry.) 
   SUMMARY 
   One embodiment of the invention provides a system that facilitates designing an integrated circuit using a mask-programmable fabric, which contains both mask-programmable logic and a mask-programmable interconnect. During operation, the system receives a description of a mask-programmable cell, wherein instances of the mask-programmable cell are repeated to form the mask-programmable fabric. The system uses this description of the mask-programmable cell to generate a derived library containing cells that can be obtained by programming the mask-programmable cell. Next, the system receives a high-level design for the integrated circuit. The system then performs a synthesis operation on the high-level design to generate a preliminary netlist for the high-level design, wherein the preliminary netlist contains references to cells in the derived library. Finally, the system converts the preliminary netlist into a netlist that contains references to the mask-programmable cell with the logic appropriately programmed. 
   In a variation on this embodiment, the system additionally performs a placement operation and a routing operation on the netlist to produce a layout for the integrated circuit. 
   In a further variation, performing the routing operation involves programming the mask-programmable interconnect. 
   In a variation on this embodiment, the mask-programmable logic and the mask-programmable interconnect that make up the mask-programmable fabric can be programmed by changing inter-metal via layers. 
   In a variation on this embodiment, the system additionally performs a packing operation on the netlist to combine cells that can use free resources from other cells. During this packing operation, the system can consider, drive strengths of output pins for mask-programmable cells, routability of pins for mask-programmable cells, net count for mask-programmable cells, and active pin count for mask-programmable cells. 
   In a variation on this embodiment, the mask-programmable cell includes a sequential logic portion, and the derived library contains a sequential cell, which corresponds the sequential logic portion of the mask-programmable cell. 
   In a variation on this embodiment, the description of the mask-programmable cell defines one or more pins. In this variation, a pin may be specified as being tied to, power, ground, a route segment or another pin. Furthermore, a pin may be associated with a logic function, and a pin may be specified as part of a sequential element. 
   In a variation on this embodiment, the description of the mask-programmable cell defines route segments for routing signals within the mask-programmable fabric. In this embodiment, route segments may be horizontal, vertical or at any angle. Moreover, several route segments may be collinear, and route segments may be coupled to pins or to other route segments. 
   In a variation on this embodiment, the description of the mask-programmable fabric includes information related to timing for route segments and connections, wherein this timing information can be used while performing a routing operation for the mask-programmable fabric. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  presents a design flow for both a mask-programmable fabric and a standard cell design in accordance with an embodiment of the invention. 
       FIG. 2  illustrates how pins are accessed within a mask-programmable cell in accordance with an embodiment of the invention. 
       FIG. 3  illustrates a programmable connection to a neighboring cell in accordance with an embodiment of the invention. 
       FIG. 4  illustrates a mask-programmable fabric in accordance with an embodiment of the invention. 
       FIG. 5A  illustrates an exemplary mask-programmable cell in accordance with an embodiment of the invention. 
       FIG. 5B  illustrates metal layer  3  segments for the exemplary mask programmable cell in accordance with an embodiment of the invention. 
       FIG. 5C  illustrates metal layer  4  segments for the exemplary mask programmable cell in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, the invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   The data structures and code described in this detailed description are typically stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet. 
     FIG. 1  presents a design flow for both a mask-programmable fabric and a standard cell design in accordance with an embodiment of the invention. The standard cell design flow (in a very simplistic view) follows the bold arrows. It starts by creating a library of standard cells and defining their power, delay, physical and electrical characteristics (step  106 ). Next, given a high-level design  108  (for example, in a register-transfer level format), the standard cell design flow performs a synthesis operation on the high-level design (step  110 ) to produce a netlist  118 , which references gates from the library of standard cells. Next, the system performs standard floor-planning (step  120 ), physical synthesis (step  122 ) and routing (step  126 ) operations. The system then performs an extraction operation (step  128 ) for interconnect networks and back-annotates them in a static timing analyzer for analysis. This makes it possible to sign-off on timing for the design (step  130 ). Note that the above-described process can use industry-standard tools for the standard-cell design flow. 
   In  FIG. 1 , the MPF flow follows the dashed arrows wherever possible. Once again we can use industry standard tools for the design flow, except for the MPF portions (where we use a custom tool). Note that we can use a custom language (called the MPF language) to create a description of an MPF cell  102 . This language specifies information on the nature of the logic and routing programmability. The MPF flow mimics the standard-cell flow with the following exceptions:
         (1) The MPF library contains a description of single MPF cell  102 . Using the MPF library and the description of the MPF cell  102 , we generate a derived library  104  for synthesis. Cells in the derived library are generated by enumerating logic functions obtained by tying a subset of inputs to power or ground.   (2) After synthesis, preliminary netlist  112  contains instances from the derived library. We then perform a reverse transformation, which converts each cell in the derived library to an instance of the MPF cell with its inputs appropriately tied (step  114 ). We also pack the logic to fully utilize unused resources in a cell (step  116 ). The output of this process becomes part of netlist  118 .   (3) Instead of a standard-cell router, which assumes that all metal and via layers are available for realizing physical connections, we use a special MPF router, which performs an MPF routing operation (step  124 ). This MPF router uses symbolic constraints called connections (between a pair of pins, a pin and metal segment or between a pair of segments) to realize the physical connectivity. This information can be provided in the MPF language.       

   Note that we can create a baseline implementation flow for a set of designs using standard cells. This baseline implementation defines performance metrics (area, delay and power dissipation) for the standard cell implementation. Each instance of an MPF cell will result in performance that is inferior to standard cell due to the constraints of mask programmability. Hence, the ratio of performance metric in an MPF library to a standard cell library will be invariably greater than one. 
   Programmable Fabric Cell Design 
   There are several aspects to the design of a programmable fabric cell. We consider each of these in a systematic manner in the following sections. We also outline a generic scheme that enables easy routing of the interconnect fabric. While reading the following sections, note there is a tight link between the choices made during cell design and the impact on tools. 
   Programmable Layers 
   Deciding on layers that provide programmability is key. It is advantageous to leave the mask layers in front end of line set untouched. From the back end of line set, we have the metal and via layers available. If we pick layers close to the top, the number of foundry steps that remain to be completed upon re-programming is small. However from a routing perspective, all connections to pins will have to be brought up to the higher programmable layers, which consumes scarce vertical routing resources. Moreover, it renders layers above the MPF implementation unavailable for the rest of the chip. Hence, in one embodiment of the invention, we restrict programmability to the lower layers of metal and vias (metal  2  to metal  5 ). Note that we selectively leave the lowest metal layer (metal  1 ) for intra-cell routing. The number of layers that we use depends on the cell design. For example, we can pick via 2 , via 3  and via 4  to be the programmable layers for our cell. 
   Combinational Logic 
   The nature of programmable combinational logic is important to a specific design. If the design is “rich” in arithmetic operations, it is important to have gates such as 3-input XORs. In such a case, a look-up table serves well. On the other hand, if the design is dominated by control logic, then a multiplexer tree structure that enables complex and-or-invert gates fares better. Hence, in an exemplary embodiment of the invention, we use a multiplexer tree for our cell. Furthermore, note that even other structures, such as universal logic blocks, can be used. 
   Local Inversions 
   In a typical design, the ratio of inverters to the total number of gates is at least 10%. Note that implementing an inverter using a large programmable gate is an extremely inefficient use of resource. In a look-up table architecture, a majority of the inversions can be absorbed into the table functions. However, it is not possible to do this for other architectures. 
   An alternative solution is to create small inverters in the programmable cell that can only drive input pins within the cell. In this way, signal inversion takes place locally where it is needed. In order to implement this solution, we need to determine the number of such inverters needed within a cell. Hence, from a tools perspective, it is desirable to convert individual inverters (after synthesis) into local inverters as much as possible, thereby reducing programmable cell count. This process can be accomplished in the packing step described below. In an exemplary embodiment of an MPF cell, we can assign two local inverters for each combinational logic function in the cell. 
   Sequential Logic 
   The nature of sequential MPF elements is to a large extent dictated by the choice of sequential behavior required on a per-design basis. The user decides if a flip-flop with set, reset, and multiplexer-enable signals is required. In addition, the user decides if support for scan test is required. Unlike traditional standard cells, which provide both Q and  Q  signals, it is sometimes advantageous to only provide only a Q. This is especially true if local inversion is supported. Also note that each cell output has to be designed with sufficient strength to drive the programmable interconnect fabric. Consequently, the output stages may have larger transistors than typical standard cells. Note that by not providing  Q , we avoid having a large output stage in the cell and can thus save some area. 
   Ratio of Combinational to Sequential Logic 
   Sequential content in a design can vary significantly. If the number of sequential elements in a design (compared to the number of combinational gates) is small, having a combinational to sequential ratio of 1:1 in the MPF cell is unfavorable because a large number of sequential elements will be unused. Depending on the design, the user can configure an MPF cell so that the ratio of programmable combinational logic to programmable sequential logic varies from one to three. Software support to take advantage of this is can be provided in the packing step described below. 
   Another approach is to create an MPF cell that allows the construction of sequential elements from combinational elements in the cell. However, this can lead to sub-optimal delay performance and a significant increase in programmable routing resources. 
   Programmable Interconnect 
   It is desirable to design a generic interconnect scheme that enables the router to operate efficiently independent of the decisions made earlier in this section. We next provide a set of guidelines that facilitate such a design. 
   Programmable Interconnect: Pin Access and Pin Tying 
   In one embodiment of the invention, all pins are designed to be available on metal  2  as vertical segments that span a few metal  3  tracks on the north and south edges of the cell. This enables access from pins to the interconnect fabric by simply using a via. 
   Note that since logic programmability is achieved by tying pins to power and ground, it is desirable to achieve this efficiently. Moreover, it is also desirable for unused input pins to be tied. We can solve this problem by dedicating horizontal metal  3  tracks that span all input pins to power and ground. In this way, tying a pin to logic zero (one) involves using a via to connect the pin to the segment on metal  3  that is tied to ground (power). 
     FIG. 2  shows six pins, three on the north side and three on the south side of a cell. The dashed lines represents metal  3  tracks. These pins span six metal  3  tracks. Two of them are used to facilitate tying pins to power and ground (VDD and VSS). Note that it is desirable for pins to span sufficiently many tracks (excluding those used for power and ground) to allow all active pins on that side to be connected. In  FIG. 2 , the number of open tracks spanned is four. Failure to do so can leave pins orphaned during routing, i.e. there is no way to connect to them. Note that during routing, we can use an analysis step called “pin reservation” to detect poor pin access in a design. 
   Programmable Interconnect: Interconnect Architecture 
   In one embodiment of the invention, the MPF cell uses a crossbar architecture on metal  3 , metal  4  and metal  5  layers. Each metal segment spans the cell (almost) in the preferred direction. Moreover, a segment can couple to metal segments on neighboring layers by means of an appropriate via. Note that an MPF cell can have up to four MPF cells as its neighbors. Since an interconnect fabric is defined within a cell, connectivity with the interconnect in the neighboring cell is also achieved through vias. 
     FIG. 3  shows an example of how a metal  3  segment can be extended to the cell on the right. We provide metal  2  segments that connect to neighbors by abutment. Hence, a signal that needs to propagate to the right requires a programmable via 2  in the current cell and a similar via in the neighbor cell. Within a cell, we reserve a fixed number of metal  3  and metal  4  segments for routing within the cell. These are utilized to connect signals reaching the cell to the appropriate pin. For a crossbar architecture, a safe number to use is the number of active pins. The remaining segments are available for routing signals through a cell. Note that managing the number of active pins in a cell is an effective means of controlling local routability. We use this as a constraint during the packing step described below. 
   Programmable Fabric Design Tools 
   As can be seen from  FIG. 1 , we have restricted the extra tool support required to enable an MPF flow to be very minimal. 
   Language for Programmable Fabric Description 
   In one embodiment of the invention, we use textual format to describe an MPF cell. During library generation, the logic function information in the description is used to create a set of functions for use in the derived library for synthesis. The language also contains constraints on how multiple instances of elements from the derived library can be packed onto a single MPF cell. This is used in the packing step. Finally, the language contains constructs to describe MPF routing constraints. 
   In one embodiment of the invention, the MPF router understands three objects: pins, segments and connections. The interconnect fabric defines the segments. A connection is a mechanism to realize an electrical equivalence between either a pair of pins, a pair of segments or a pin and a segment. 
   Library Generation 
   Library generation involves enumerating all distinct (equivalent up to permutation of inputs) functions of the programmable logic function. In addition, a sequential cell that models the circuit in the MPF cell is also added. 
   Packing 
   The packing operation involves multiple steps. Each of these steps ensures that the active pin count in an MPF cell does not exceed a user-specified threshold. This is implicit in the graph constructions described below. 
   We first merge each sequential cell with either a cell driving it or a cell driven by it. We formulate this as a graph-matching problem on a graph derived from the connectivity of the netlist. Each sequential cell and its immediate combinational fanin and combinational fanout cells (in the netlist) are nodes in the graph. There is an edge between a sequential cell and each of its combinational fanin and fanout cells. Packing a sequential and a combinational cell can be realized by solving a maximum matching problem on the graph. Since this graph is often sparse and not connected, an optimum graph-matching technique can be used efficiently. 
   The second step involves packing multiple combinational cells if we have a combinational to sequential ratio exceeding one. We consider packing a combinational cell with one of its immediate combinational fanin or one of its immediate combinational fanout in the netlist. This too can be formulated as a graph-matching problem, wherein each combinational cell forms a node in a graph, and an edge indicates a fanin/fanout relationship in the netlist. Unfortunately, this results in large connected graphs. Consequently, although the optimum matcher finds excellent solutions, it can require a significant amount of computational time. Note that we can resort to a heuristic to solve this problem, which yields an approximate solution (with an observed quality nearly 10% away from the optimum) in a fraction of the runtime. 
   Finally, we seek to push inversions to be local to a cell. Each independent inverter competes for an unused local inverter in its&#39; fanout cell with other inverters in the fanins of the fanout cell. We create a graph with a node for each inverter in the design (assuming all fanouts of the inverter can accommodate it as a local inverter). An edge exists between two inverters if they have a common immediate fanout cell or if one drives the other. We now solve an instance of a Maximum Independent Set on this graph. The decision problem to check if there is a Maximum Independent Set of size k in a graph is NP-Complete (reducible to Maximum Clique problem on the dual graph). However, it is trivial to devise a heuristic to compute the Maximal Independent Set using node degrees. Furthermore, using randomization, it is possible to find larger sets. 
   Routing 
   From the placement information, we build a routing graph for the whole design. The nodes in the graph include pins in the MPF cells and the segments in the interconnect fabric. For each top-level port in the design, we pick a set of “closest” metal segments in the interconnect fabric. The router will connect to one of these segments when it attempts to complete the routing of the net associated with the port. Euclidean distance can be used to drive the measure of nearness to a port. The connections between a pair of segments and a segment and a pin are available from the MPF description. The connections between neighboring cells are inferred from the placement and the MPF description. Note that each connection forms an edge in the routing graph between the pair of objects it connects. 
   Much like in standard cell routing, MPF routing problem can be broken into multiple steps to manage the complexity. In the first step, we reserve metal segments that can connect to an active pin in the design. This step, called “pin reservation,” detects any pins that are orphaned due to a poor cell design. Also, the mechanism of reserving a segment for a pin prevents nets other than the one connected to the pin from using the segment. This ensures that the pin is not orphaned due to poor detailed routing (due to its sequential nature in processing nets). 
   Global routing is carried out in a similar manner to standard cells. We can use each MPF cell as the granularity of coarseness during global routing. Thus, the detail router is only restricted to routing within an MPF cell. During global routing, we discount the horizontal and vertical capacities of each MPF cell by the maximum number of active pins. At the end of global routing, we check if an embedding of the global routing into the fabric is possible without any capacity violations. If we find a violation, the solution is to augment the interconnect fabric in the MPF cell by one more routing resource and redo the steps. Once we succeed at this step, the interconnect architecture and the routing process ensures a solution exists after detail routing. In a cross-bar routing architecture, it is easy to ensure that the detail router will not fail, by ensuring that the global routing can be successfully embedded and if that each MPF cell has at least as many horizontal and vertical tracks free as the number of active pins. 
   Note that we embed the horizontal and vertical segments that span more than one MPF cell using an approach based on track assignment. Finally, detailed routing is carried out the design, focusing on one MPF cell at a time. 
   Example Routing Fabric 
     FIG. 4  presents an exemplary mask-programmable fabric  400  in accordance with an embodiment of the invention. Mask programmable fabric  400  includes a two-dimensional array of MPF cells  401 - 412 , which can be programmed by modifying vias between metal layer as described above. 
   Example Cell 
     FIGS. 5A-5C  illustrate the structure of a simple MPF cell. This exemplary cell is architected so that all internal connections are on metal  1 , and pins appear as vertical strips on metal  2  as shown in  FIG. 5A . The power and ground pins appear on the north and south edges of the cell on metal  1  and are not shown. In addition, we route power and ground on two horizontal metal  3  segments each, that intersect the vertical spans of the pins. This supports logic programming efficiently because each pin can be easily tied to power or ground by means of a via from metal  2  to metal  3 . Moreover, note that each pin can connect to a horizontal metal  3  segment that intersects its vertical span for routing. 
   In this example, the metal  3  and metal  4  routing fabric is comprised of metal segments and is shown in  FIG. 5B  and  FIG. 5C , respectively. Note that four segments on metal  3  are reserved for power and ground. Hence, they are unavailable for interconnect routing. 
   Metal  5  segments are created similarly to metal  3 . However, unlike in the case of metal  3 , all segments on metal  5  are available for interconnect routing. A metal  3  segment can connect to a vertical metal  4  segment with a via for interconnect purposes. Similarly, a connection between metal  4  and metal  5  can be realized through a via. 
   Connections between neighboring cells can be accomplished by, (1) providing tiny metal segments (called “abutment segments”) that connect by abutment between neighbor cells, and (2) connecting metal segments to abutment segments through vias. In this way, a connection between two metal  4  vertical segments in neighboring cells (one cell to the north side of the other) and having the same x coordinate can be realized through two via connections. 
   The foregoing descriptions of embodiments of the invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the invention. The scope of the invention is defined by the appended claims.