HCS12 datasheet(370/452 Pages) FREESCALE

370 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

370

Freescale Semiconductor

accumulator represents the ratio of (the x-distance from the beginning of the

line segment to the lookup point) to (the x-distance from the beginning of the

line segment to the end of the line segment). B is treated as an 8-bit binary

fraction with radix point left of the MSB, so each line segment can effectively

be divided into 256 pieces. During execution of the TBL or ETBL instruction,

the difference between the end point y-value and the beginning point

y-value (a signed byte-TBL or word-ETBL) is multiplied by the B

accumulator to get an intermediate delta-y term. The result is the y-value of

the beginning point, plus this signed intermediate delta-y value.

Because indexed addressing mode is used to identify the starting point of

the line segment of interest, there is a great deal of flexibility in constructing

tables. A common method is to break the x-axis range into 256 equal width

segments and store the y value for each of the resulting 257 endpoints. The

16-bit D accumulator is then used as the x input to the table. The upper eight

bits (A) is used as a coarse lookup to find the line segment of interest, and

the lower eight bits (B) is used to interpolate within this line segment.

In the program sequence

LDX

#TBL_START

LDD

DATA_IN

TBL

A,X

The notation A,X causes the TBL instruction to use the Ath line segment in

the table. The low-order half of D (B) is used by TBL to calculate the exact

data value from this line segment. This type of table uses only 257 entries

to approximate a table with 16 bits of resolution. This type of table has the

disadvantage of equal width line segments, which means just as many

points are needed to describe a flat portion of the desired function as are

needed for the most active portions.

Another type of table stores x:y coordinate pairs for the endpoints of each

linear segment. This type of table may reduce the table storage space

compared to the previous fixed-width segments because flat areas of the

functions can be specified with a single pair of endpoints. This type of table

is a little harder to use with the CPU12 TBL and ETBL instructions because

the table instructions expect y-values for segment endpoints to be in

consecutive memory locations.

Consider a table made up of an arbitrary number of x:y coordinate pairs,

where all values are eight bits. The table is entered with the x-coordinate of

the desired point to lookup in the A accumulator. When the table is exited,

the corresponding y-value is in the A accumulator. Figure 9-13 shows one

way to work with this type of table.

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