Page 338 - Programming Microcontrollers in C

Table Look-Up 323

{
int i;
for(i=0;i< 256; i++)
printf(“ i = %d r = %d\n”,i,table_look_up(i));
}

where tlu.h contains the function and the table above. This code
compiles and runs on a MS-DOC PC, and the result is as expected.
Here, the program scans the entire input range and prints out the
resultant value at each point. A check of the outputs will show that
the function hits the break value at each break, and creates a linear
interpolation between all points. For values above or below the range,
the proper output is observed.
In a normal control system, there will often be need for several table
look-up operations. The above code is not too good in this case because
the code to do the look-up must be repeated with each table. This extra
code can be eliminated if the function table_look_up( ) is passed
two parameters: the first parameter is the interpolation value, and the
second value is a pointer to the look-up table as is shown below:

char table_look_up( char x_in, struct lut* table)
{
int i;
for(i=0;x_in>table->data[i].x && i<=table->entries; i++) ;
if(i>=table->entries)
return table->data[table->entries-1].y;
else if (i==0)
return table->data[0].y;
else
return table->data[i-1].y+
(((x_in-table->data[i-1].x)*table->data[i- 1].slope)>>8);
}

Listing 6-7: Table Look-Up, Version 2
This form of the table look-up should probably be used in all
cases. Here is another example of where it is wise to examine the
code generated by a compiler. It will not be listed here, but the as­
sembly code required for Listing 6-6 is 182 bytes long, and that for
listing 6-7 is but 142 bytes. This greatly improved utility automati­
cally gives a substantial savings in code.
It was pointed out that the compiler optimizer will sometimes
change the basic code dictated by the programmer. Perhaps, it should