Creating a C code lookup function

Creating a C code lookup function

If you have a logic function that you need to implement in software or firmware you can create a fast, compact C code lookup function for it.

The lookup function is generated as a stand-alone module that can be added to your source files. You call the function with arguments representing the input variable values, and the function returns 1 or 0.

To generate a C code lookup function:

1. Select Operation | Generate C Lookup Function....
2. In the dialog that appears, select the size of type unsigned int on the target processor and compiler. This information is needed to properly distribute the output truth values over the bits of a data table.
3. Choose whether you want the function to have parameters for each individual input variable, or a single parameter that will be the truth table row number (minterm). For example, suppose the logic function has output F and two input variables, A and B. If you choose individual parameters the lookup function header will be
       int lu_F( int A, int B )
   If you choose the single parameter the header will be
       int lu_F( unsigned int nTerm )
   The parameter nTerm can take arguments of 0, 1, 2, and 3.
4. Click OK and the "Save As" dialog will appear. After you save the file the new function will be displayed in a Notepad window.

Notes:

• If the problem contains multiple outputs, an independent lookup function will be generated for each output.
• If the logic function contains "don't care" outputs for any rows, they are converted to logic 1, or true.
• Lookup functions do not include any error checking, and in general may need editing to compile on the target compiler.
• You should always verify that the lookup function works correctly on the target system.