HCS12 datasheet(422/452 Pages) FREESCALE

422 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

422

Freescale Semiconductor

These instructions can also be used to limit a data value prior to using it as

an input to a table lookup or other routine. Suppose a table is valid for input

values between $20 and $7F. An arbitrary input value can be tested against

these limits and be replaced by the largest legal value if it is too big, or the

smallest legal value if too small using the following two CPU12 instructions.

HILIMIT

FCB

$7F

;comparison value needs to be in mem

LOWLIMIT

FCB

$20

;so it can be referenced via indexed

MINA

HILIMIT,PCR

;A=MIN(A,$7F)

MAXA

LOWLIMIT,PCR

;A=MAX(A,$20)

;A now within the legal range $20 to $7F

The “,PCR” notation is also new for the CPU12. This notation indicates the

programmer wants an appropriate offset from the PC reference to the

memory location (HILIMIT or LOWLIMIT in this example), and then to

assemble this instruction into a PC-relative indexed MIN or MAX instruction.

B.8.6 Fuzzy Logic Support

The CPU12 includes four instructions (MEM, REV, REVW, and WAV)

specifically designed to support fuzzy logic programs. These instructions

have a very small impact on the size of the CPU and even less impact on

the cost of a complete MCU. At the same time, these instructions

dramatically reduce the object code size and execution time for a fuzzy logic

inference program. A kernel written for the M68HC11 required about 250

bytes and executed in about 750 milliseconds. The CPU12 kernel uses

about 50 bytes and executes in about 16 microseconds (in a 25-MHz

HCS12).

B.8.7 Table Lookup and Interpolation

The CPU12 instruction set includes two instructions (TBL and ETBL) for

lookup and interpolation of compressed tables. Consecutive table values

are assumed to be the x coordinates of the endpoints of a line segment. The

TBL instruction uses 8-bit table entries (y-values) and returns an 8-bit result.

The ETBL instruction uses 16-bit table entries (y-values) and returns a

16-bit result.

An indexed addressing mode is used to identify the effective address of the

data point at the beginning of the line segment, and the data value for the

end point of the line segment is the next consecutive memory location (byte

for TBL and word for ETBL). In both cases, the B 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

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