2-12
Functional Description
The 82C54 contains three counter/timers: C/T0, C/T1, and C/T2. If
you are using a hardware internal clock source, the time base logic
uses C/T1 and C/T2 in either normal or cascaded mode, as follows:
–
Normal Mode
- A software-selectable count is loaded into C/T2
of the 82C54 counter/timer circuitry. Each time C/T2 reaches
terminal count, a conversion is initiated. The time interval
between conversions ranges from 25
µ
s to 65.536ms.
–
Cascaded Mode
- A software-selectable count is divided between
C/T2 and C/T1 of the 82C54 counter/timer circuitry. When C/T2
counts down to 0, C/T1 decrements by 1. C/T2 is reloaded with
its count value and begins counting down again. Each time C/T2
counts down to 0, C/T1 decrements by 1. Each time both C/T2
and C/T1 reach terminal count, a conversion is initiated. The time
interval between conversions ranges from 25
µ
s to 1.2 hours.
Refer to page 2-20 for more information about the 82C54
counter/timer circuitry.
•
Hardware (external clock source)
— An external clock source is
useful if you want to sample at rates not available with the 82C54
counter/timer circuitry, if you want to sample at uneven intervals, or if
you want to sample on the basis of an external event. An external
clock also allows you to synchronize conversions on multiple boards
to a common timing source.
The external clock source is an externally applied TTL-compatible
signal, which you attach to the INT_IN / XCLK pin (pin 24) of the
main I/O connector. When you start an analog input operation,
conversions are armed. At the next falling edge of the external clock
source (and at every subsequent falling edge of the external clock
source), a conversion is initiated.
Note:
If you are using a hardware external clock source, you cannot
use the INT_IN / XCLK pin (pin 24) to generate interrupts.
Figure 2-3 illustrates how conversions are initiated when using an internal
and an external clock source. (Note that Figure 2-3 assumes that you are
not using a hardware trigger; refer to Figure 2-4 for an illustration of
conversions when using a hardware trigger.)
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