Analog Input Features
2-21
The way conversions are synchronized depends on whether you are using
an internal or an external clock, as follows:
●
With internal clocking
— The 82C54 stops counting when the gate
signal goes inactive. When the gate signal goes active, the 82C54 is
reloaded with its initial count value and starts counting again;
therefore, with internal clocking, conversions are synchronized to the
gate signal.
●
With external clocking
— The signal from the external clock
continues uninterrupted while the gate signal is inactive; therefore,
with external clocking, conversions are synchronized to the external
clock.
Figure 2-7 illustrates the use of a positive-polarity hardware gate with
both a negative-edge external clock and an internal clock.
Figure 2-7. Hardware Gate
1st conversion
2nd conversion
3rd conversion
gate active
gate inactive;
conversions off
gate active;
conversions on
no conversion
Digital gate
source (positive
polarity)
External clock
source
(negative edge)
Internal clock
source
1st conversion
2nd conversion
3rd conversion
4th conversion
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