To calculate the values for the contents of the Envelope Period
Coarse and Fine Tune registers, given the input clock and the desired
envelope frequencies, we rearrange the above equations, yielding:
3.5.2 ENVELOPE SHAPE/CYCLE CONTROL
(Register
R15)
The Envelope Generator further counts down the envelope fre-
quency by 16, producing a 16-state per cycle envelope pattern as
defined by its 4-bit counter output, E3 E2 El E0. The particular shape
and cycle pattern of any desired envelope is accomplished by
controlling the count pattern (count up/count down) of the 4-bit
counter and by defining a single-cycle or repeat-cycle pattern.
This envelope shape/cycle control is contained in the lower 4 bits
(B3--B0) of register R15. Each of these 4 bits controls a function in
the envelope generator, as illustrated in the following:
Envelope Shape/Cycle
Control Register (R15)
The definition of each function is as follows:
Hold
when set to logic “1”, limits the envelope to one cycle,
holding the last count of the envelope counter (E3--
E0=0000 or 1111, depending on whether the envelope
counter was in a count-down or count-up mode, respec-
tively).
Alternate when set to logic “1”,
the envelope counter reverses
count direction (up-down) after each cycle.
NOTE: When both the Hold bit and the Alternate bit are ones, the
envelope counter is reset to its initial count before holding.
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