HCS12 datasheet(354/452 Pages) FREESCALE

354 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

354

Freescale Semiconductor

The Y index register is set to the base address for the fuzzy inputs and

outputs (in working RAM). Each rule antecedent is an unsigned 8-bit offset

from this base address to the referenced fuzzy input. Each rule consequent

is an unsigned 8-bit offset from this base address to the referenced fuzzy

output. The Y index register remains constant throughout execution of the

REV instruction.

The 8-bit A accumulator is used to hold intermediate calculation results

during execution of the REV instruction. During antecedent processing, A

starts out at $FF and is replaced by any smaller fuzzy input that is

referenced by a rule antecedent (MIN). During consequent processing, A

holds the truth value for the rule. This truth value is stored to any fuzzy

output that is referenced by a rule consequent, unless that fuzzy output is

already larger (MAX).

Before starting to execute REV, A must be set to $FF (the largest 8-bit

value) because rule evaluation always starts with processing of the

antecedents of the first rule. For subsequent rules in the list, A is

automatically set to $FF when the instruction detects the $FE marker

character between the last consequent of the previous rule and the first

antecedent of a new rule.

The instruction LDAA #$FF clears the V bit at the same time it initializes A

to $FF. This satisfies the REV setup requirement to clear the V bit as well

as the requirement to initialize A to $FF. Once the REV instruction starts, the

value in the V bit is automatically maintained as $FE separator characters

are detected.

The final requirement to clear all fuzzy outputs to $00 is part of the MAX

algorithm. Each time a rule consequent references a fuzzy output, that fuzzy

output is compared to the truth value for the current rule. If the current truth

value is larger, it is written over the previous value in the fuzzy output. After

all rules have been evaluated, the fuzzy output contains the truth value for

the most-true rule that referenced that fuzzy output.

After REV finishes, A will hold the truth value for the last rule in the rule list.

The V condition code bit should be one because the last element before the

$FF end marker should have been a rule consequent. If V is zero after

executing REV, it indicates the rule list was structured incorrectly.

HCS12 Datasheet(PDF) 354 Page - Freescale Semiconductor, Inc