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A technique of minimizes circuit area on programmable logic with fracturable logic elements by using "balancing" in the technology mapping stage of the programmable logic computer-aided-design flow. A fracturable LE can be used for logic implementation in many ways, such as being used as one maximum

A technique of minimizes circuit area on programmable logic with fracturable logic elements by using "balancing" in the technology mapping stage of the programmable logic computer-aided-design flow. A fracturable LE can be used for logic implementation in many ways, such as being used as one maximum-sized look-up table (LUT) or multiple smaller LUTs. One of more inputs of the multiple smaller LUTs may be shared. By balancing, this means mean that the technology mapping algorithm is tuned to use more small LUTs and fewer maximum-sized LUTs to implement the circuit. Although this is counterintuitive since the larger LUTs are more effective at absorbing gates, the technique achieves a smaller final circuit area by packing small LUTs into fracturable LEs.

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What is claimed is: 1. A method comprising: mapping a logical function into a logic block comprising a look-up table structure configurable in a first configuration having a first look-up table having X inputs, or in a second configuration having a second look-up table having Y inputs and a third l

What is claimed is: 1. A method comprising: mapping a logical function into a logic block comprising a look-up table structure configurable in a first configuration having a first look-up table having X inputs, or in a second configuration having a second look-up table having Y inputs and a third look-up table having Z inputs, wherein U inputs of the second and third look-up tables are shared, X, Y, Z, and U are integers 1 or greater, X is larger than Y and Z, and a total number of inputs to the second and third look-up tables is X plus U; and selecting one of the first configuration or second configuration of the look-up table structure for the logic block based upon a cost function comprising a product Wp*P, where P is a measure of an availability for packing and Wp is a coefficient. 2. The method of claim 1 wherein Wp is a user-selectable value. 3. The method of claim 1 wherein the look-up table structure further comprises a third configuration having a fourth look-up table and fifth look-up table, each having V independent inputs, wherein V is less than X, Y, and Z. 4. The method of claim 1 wherein the cost function comprises Wa*A-Ws*S+Wp*P, where A is a cost measure of look-up table area input and S is a measure of sharing possibility, and Wa and Ws are coefficients. 5. The method of claim 4 wherein Wp is 0 or Wp is 쩍 of Wa. 6. The method of claim 4 wherein Wp is 7.5*Wa. 7. The method of claim 1 wherein X is 6, Y and Z are 5, and U is 2. 8. A method comprising: providing a plurality of logic blocks comprising a look-up table configurable in a first configuration as a look-up table having at most X inputs or in a second configuration as one or more look-up tables, in which at least one of these look-up tables in the second configuration has at most Y inputs, wherein X and Y are integers 1 or greater, and Y is less than X; mapping a logic function into one of the plurality of logic blocks, wherein based on a first selected value for a coefficient in a cost function, the logic function is mapped into I look-up tables of the first configuration, wherein I is an integer 1 or greater; and mapping a logic function into one of the plurality of logic blocks, wherein based on a second selected value for the coefficient in a cost function, the logic function is mapped into J look-up tables of the second configuration, wherein J is an integer greater than I. 9. The method of claim 8 wherein the cost function comprises Wa*A-Ws*S+Wp*P, where A is a cost measure of look-up table area input, S is a measure of sharing possibility, P is a measure of an availability for packing, and Wa, Ws, and Wp are coefficients. 10. The method of claim 9 wherein Wp is 0, Wp is 쩍 of Wa, or Wp is 7.5*Wa. 11. The method of claim 9 wherein X is 6 and Y is 5. 12. The method of claim 8 wherein at least one input is shared between a first look-up table and a second look-up table of the second configuration. 13. The method of claim 12 wherein a number of total number of inputs to the J look-up tables in the second configuration is greater than X. 14. A method comprising: providing a plurality of logic blocks comprising a look-up table configurable in a first configuration having a look-up table with at most X inputs and a second configuration having a plurality of look-up tables, at least one of the look-up tables of the second configuration having at most Y inputs, wherein X and Y are integers 1 or greater, and Y is less than X; evaluating a cost function to obtain a first cost value when mapping a logic function using a logic block of the first configuration; evaluating the cost function to obtain a second cost value when mapping the logic function using a logic block of the second configuration; and mapping the logic function to a logic block having the first configuration if the first cost value is less than the second cost value. 15. The method of claim 14 further comprising: mapping the logic function to a logic block having the second configuration if the second cost value is less than the first cost value. 16. The method of claim 14 wherein for the second configuration of the logic block, at least one of the input to the plurality of look-up tables is shared. 17. The method of claim 14 wherein the cost function comprises a variable representing a packability value. 18. The method of claim 17 wherein the packability value is higher when using the look-up table in the first configuration than when using the look-up tables in the second configuration. 19. The method of claim 14 wherein the cost function comprises a variable representing a fan out value. 20. The method of claim 14 wherein evaluating a cost function to obtain a first cost value comprises: consulting a look-up table in a database to find a packability value corresponding to a look-up table having X inputs. 21. The method of claim 20 wherein evaluating a cost function to obtain a first cost value further comprises: consulting a look-up table in a database to find an area value corresponding to a look-up table having X inputs. 22. A computer program product having a computer usable medium having computer readable code embodied therein, the computer program product comprising: computer readable code configured to cause a computer to effect evaluating a cost function to obtain a first cost value when mapping a logic function using a logic block of a first configuration having a look-up table with at most X inputs wherein X is integer 1 or greater, and; computer readable code configured to cause a computer to effect evaluating the cost function to obtain a second cost value when mapping the logic function using a logic block of a second configuration having a plurality of look-up tables, one of the look-up tables having at most Y inputs wherein Y is integer 1 or greater, and where Y is less than X; and computer readable code configured to cause a computer to effect mapping the logic function to a logic block having the first configuration if the first cost value is less than the second cost value.