HCS12 datasheet(343/452 Pages) FREESCALE

343 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

343

expression corresponds to a specific fuzzy output in RAM. Consequents for

a rule can be encoded as a simple list of pointers to (or addresses of) the

fuzzy outputs to which they refer.

The complete rules are stored in the knowledge base as a list of pointers or

addresses of fuzzy inputs and fuzzy outputs. For the rule evaluation logic to

work, there must be some means of knowing which pointers refer to fuzzy

inputs and which refer to fuzzy outputs. There also must be a way to know

when the last rule in the system has been reached.

• One method of organization is to have a fixed number of rules with a

specific number of antecedents and consequents.

• A second method, employed in Freescale Freeware M68HC11

kernels, is to mark the end of the rule list with a reserved value, and

use a bit in the pointers to distinguish antecedents from consequents.

• A third method of organization, used in the CPU12, is to mark the end

of the rule list with a reserved value, and separate antecedents and

consequents with another reserved value. This permits any number

of rules, and allows each rule to have any number of antecedents and

consequents, subject to the limits imposed by availability of system

memory.

Each rule is evaluated sequentially, but the rules as a group are treated as

if they were all evaluated simultaneously. Two mathematical operations

take place during rule evaluation. The fuzzy and operator corresponds to

the mathematical minimum operation and the fuzzy or operation

corresponds to the mathematical maximum operation. The fuzzy and is

used to connect antecedents within a rule. The fuzzy or is implied between

successive rules. Before evaluating any rules, all fuzzy outputs are set to

zero (meaning not true at all). As each rule is evaluated, the smallest

(minimum) antecedent is taken to be the overall truth of the rule. This rule

truth value is applied to each consequent of the rule (by storing this value to

the corresponding fuzzy output) unless the fuzzy output is already larger

(maximum). If two rules affect the same fuzzy output, the rule that is most

true governs the value in the fuzzy output because the rules are connected

by an implied fuzzy or.

In the case of rule weighting, the truth value for a rule is determined as usual

by finding the smallest rule antecedent. Before applying this truth value to

the consequents for the rule, the value is multiplied by a fraction from zero

(rule disabled) to one (rule fully enabled). The resulting modified truth value

is then applied to the fuzzy outputs.

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