When creating microcontroller firmware, you often need to work with data arrays. Tables make easy work of data arrays, such as those for digital-code transformation, correction for sensor linearity, sophisticated calculations, and multiple output organization. Table 1 shows how you can organize data in a table. Outputs A, B, and C have values based on the input value, V.

Next, put the data from Table 2 in either the beginning or the end of ROM, which makes the data easy to find. For definition of 1-byte data storage, use pseudo operators FCB or DB. For storage of data comprising 2 bytes, use FDB or DW, as in the following example:

When working with tables, you should always use indexed addressing mode. It provides access to data using variable addresses. Most microcontrollers have two index registers, X and H. Register X contains the low byte of the conditional address of the operand; H contains the high byte. The algorithm of working with tables is straightforward. After you detect the input value, you should then compare it with the table’s input data. The X index determines this value, starting with X=0 and ending with X=N. In this example, N=4. When you find table data equal to the input value, you use the corresponding X as an index to load the output registers with their values. In the case of 2-byte numbers, you should load the output registers separately, first with a high byte and then with a low one. Figure 1 illustrates this process.

In the listing of assembler code, you can double-check the table content in memory at addresses $F800 through $F813. The listing uses Freescale assembler because most of the appropriate applications employ inexpensive, 8-bit microcontrollers from Freescale’s HC08 Nitron family. You can, however, use this approach with any type of microcontroller and assembly language.