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TC OUTPUT
One output form avai"lable is called Terminal Count (TC) and represents
the period of time that the counter reaches an equivalent value of
zero.
Figure 3-18 shows a Terminal Count pulse and an example context
that generated it.
Not
i
ce that the TC wi dth is determi ned by the
period of the counting source.
Figure 3-18 assumes active-high source
polarity, counter armed, gate active, counter decrementing and a reload
va 1 ue of K.
The counter wi 11 al ways be updated when TC occurs; the user can choose
the source location and the value.
If a non-zero value is picked, the
counter will never really attain a zero state and TC will indicate the
counter state that would have been zero had no parallel transfer
occurred.
COUNT CONTROL
Counter Mode ,bits CM3 through CM7 specify the various options available
for direct control of the counting process.
Most of the control bits
operate rel ati vely
ii
ndependently of the others so that they can be
combined freely to fonn many types of counting configurations.
Bit CM3 speci fi es i ncrementi ng or decrementi ng of the counter.
For
many timing and frequency division purposes, down-counting is preferred
since the key value is identified when the counter reaches zero.
Alternati vely,
for
even
counti ng
or
el apsed
time
appl ications,
up-counting will
be
preferred so that
accumulated
values can be
conveniently handledQ
SOURCE
JV\J\J\AF'--f\J\J\J\J\-/\-
-_I\---1J'---~~
TC TOGGLED
I--J
I
\
r -
OUTPUT
----1
~~
I
Figure 3-18.
TC Waveform Format
3-29
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