MC68340 datasheet(171/441 Pages) FREESCALE

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MC68340 USER’S MANUAL

MOTOROLA

The following value has been calculated for independent variable X:

NOT USED

0000000010111101

Since X is an 8-bit value, the upper four bits are used as a table offset and the lower four

bits are used as an interpolation fraction. The following results are obtained from the

subroutine:

Table Entry Offset

⇒ Dx [4:7] = $B = 11

Interpolation Fraction

⇒ Dx [0:3] = $D = 13

Thus, Y is calculated as follows:

Y = 80

+ (13 (64 – 80)) / 16 = 67

If the 8-bit value for X were used directly by the table instruction, interpolation would be

incorrectly performed between entries 0 and 1. Data must be shifted to the left four places

before use:

LSL.W #4, Dx

The new range for X is 0

≤ X ≤ 4096; however, since a left shift fills the least significant

digits of the word with zeros, the interpolation fraction can only have one of 16 values.

After the shift operation, Dx contains the following value:

NOT USED

000010111

010000

Execution of the table instruction using the new value in Dx yields:

Table Entry Offset

⇒ Dx [8:15] = $0B = 11

Interpolation Fraction

⇒ Dx [0:7] = $D0 = 208

Thus, Y is calculated as follows:

Y = 80

+ (208 (64 – 80)) / 256 = 67

5.3.4.4 TABLE EXAMPLE 4: MAINTAINING PRECISION. In this example, three TBL

operations are performed and the results are summed. The calculation is done once with

the result of each TBL rounded before addition and once with only the final result rounded.

Assume that the result of the three interpolations are as follows (a ".'' indicates the binary

radix point).

TBL # 1

0010 0000 . 0111 0000

TBL# 2

0011 1111 . 0111 0000

TBL # 3

0000 0001 . 0111 0000

Freescale Semiconductor, Inc.

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