Typewriter User Manual

5- 32 MC68340 USER’S MANUAL MOTOROLA

Table 5-14. Compressed Table Entries

Entry Number X Value Y Value

2 512 1311

3 786 1966

Since the table is reduced from 257 to 5 entries, independent variable X must be scaled

appropriately. In this case the scaling factor is 64, and the scaling is done by a single

instruction:

LSR.W #6,Dx

Thus, Dx now contains the following bit pattern:

31 16 15 0

NOT USED 0000001010001110

Table Entry Offset ⇒ Dx [8:15] = $02 = 2

Interpolation Fraction ⇒ Dx [0:7] = $8E = 142

Using this information, the table instruction calculates dependent variable Y:

Y = 1331 + (142 (1966 – 1311)) / 256 = 1674

The function chosen for Examples 1 and 2 is linear between data points. If another

function had been been used, interpolated values might not have been identical.

5.3.4.3 TABLE EXAMPLE 3: 8-BIT INDEPENDENT VARIABLE. This example shows

how to use a table instruction within an interpolation subroutine. Independent variable X is

calculated as an 8-bit value, allowing 16 levels of interpolation on a 17-entry table. X is

passed to the subroutine, which returns an 8-bit result. The subroutine uses the data listed

in Table 5-15, based on the function shown in Figure 5-9.

Frees

cale Semiconductor,

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

nc...