Analog Input Features
2-13
●
External Digital
— Connect the digital trigger to the digital input
DI1 pin (B40) of the 100-pin connector, J1. Trigger types are as
follows:
–
Positive-edge trigger
- Triggering occurs on the rising edge of the
trigger signal.
–
Negative-edge trigger
- Triggering occurs on the falling edge of
the trigger signal.
The actual points at which conversions begin depend on whether the
clock source is internal or external, as follows:
●
Internal clock source
— The 82C54 counter/timer is idle until the
trigger occurs. Within 400ns, the first conversion begins. Subsequent
conversions are synchronized to the internal clock.
●
External clock source
— Conversions are armed when the trigger
occurs; they begin with the next active edge of the external clock
source and continue with subsequent active edges.
Figure 2-3 illustrates conversions enabled with software triggering/gating
and with internal and external clock sources. In the diagram, the delay
between the start of the conversion process by software and the start of
the onboard clock is less than 1µs. Figure 2-4 illustrates the enabling of
conversions with a hardware trigger.
Figure 2-3. Enabling Conversions with Software Triggering/Gating
and with Internal and External Clock Sources
Software enables
conversion process
Conversions begin with
external source (programmed
for negative edge)
idle state
count
count
count
count
Conversions begin with
internal clock source
Internal clock source
External clock source
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