HCS12 datasheet(363/452 Pages) FREESCALE

363 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

363

In cycle 2.0, the first element of the rule list (a 16-bit address) is read from

memory. Due to propagation delays, this value cannot be used for

calculations until two cycles later (cycle 4.0). The X index register, which is

used to access information from the rule list, is incremented by two to point

at the next element of the rule list.

The operations performed in cycle 4.0 depend on the value of the word read

from the rule list. $FFFE is a special token that indicates a transition from

antecedents to consequents or from consequents to antecedents of a new

rule. The V bit can be used to decide which transition is taking place, and V

is toggled each time the $FFFE token is detected. If V was zero, a change

from antecedents to consequents is taking place, and it is time to apply

weighting (provided it is enabled by the C bit equal one). The address in

TMP2 (derived from Y) is used to read the weight byte from memory. In this

case, there is no bus access in cycle 5.0, but the index into the rule list is

updated to point to the next rule element.

The old value of X (X0) is temporarily held on internal nodes, so it can be

used to access a rule word in cycle 7.2. The read of the rule word is timed

to start two cycles before it will be used in cycle 4.0 of the next loop pass.

The actual multiply takes place in cycles 6.2 through 8.2. The 8-bit weight

from memory is incremented (possibly overflowing to $100) before the

multiply, and the upper eight bits of the 16-bit internal result is used as the

weighted result. By using weight+1, the result can range from 0.0 times A to

1.0 times A. After 8.2, flow continues to the next loop pass at cycle 4.0.

At cycle 4.0, if Rx is $FFFE and V was one, a change from consequents to

antecedents of a new rule is taking place, so accumulator A must be

reinitialized to $FF. During processing of rule antecedents, A is updated with

the smaller of A, or the current fuzzy input (cycle 6.0). Cycle 5.0 is usually

used to read the next rule word and update the pointer in X. This read is

skipped if the current Rx is $FFFF (end of rules mark). If this is a weight

multiply pass, the read is delayed until cycle 7.2. During processing of

consequents, cycle 6.1 is used to optionally update a fuzzy output if the

value in accumulator A is larger.

After all rules have been processed, cycle 7.0 is used to update the PC to

point at the next instruction. If weights were enabled, Y is updated to point

at the location that immediately follows the last rule weight.

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