National Instruments USB-621 Series, User Manual

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Chapter 8 Counters
© National Instruments Corporation 8-33 NI USB-621x User Manual
Counter Triggering
Co u nters s u pport three different triggering actions:
• Arm Start Trigger —To begin any co u nter inp u t or o u tp u t f u nction,
yo u m u st first enable, or arm, the co u nter. Software can arm a co u nter
or config u re co u nters to be armed on a hardware signal. Software calls
this hardware signal the Arm Start Trigger. Internally, software ro u tes
the Arm Start Trigger to the Co u nter n HW Arm inp u t of the co u nter.
For co u nter o u tp u t operations, yo u can u se the arm start trigger in
addition to the start and pa u se triggers. For co u nter inp u t operations,
yo u can u se the arm start trigger to have start trigger-like behavior. The
arm start trigger can be u sed for synchronizing m u ltiple co u nter inp u t
and o u tp u t tasks.
When u sing an arm start trigger, the arm start trigger so u rce is ro u ted
to the Co u nter n HW Arm signal.
• Start Trigger —For co u nter o u tp u t operations, a start trigger can be
config u red to begin a finite or contin u o u s p u lse generation. Once a
contin u o u s generation has triggered, the p u lses contin u e to generate
u ntil yo u stop the operation in software. For finite generations, the
specified n u mber of p u lses is generated and the generation stops u nless
yo u u se the retriggerable attrib u te. When yo u u se this attrib u te,
s u bseq u ent start triggers ca u se the generation to restart.
When u sing a start trigger, the start trigger so u rce is ro u ted to the
Co u nter n Gate signal inp u t of the co u nter.
Co u nter inp u t operations can u se the arm start trigger to have start
trigger-like behavior.
• Pause Trigger —Yo u can u se pa u se triggers in edge co u nting and
contin u o u s p u lse generation applications. For edge co u nting
acq u isitions, the co u nter stops co u nting edges while the external
trigger signal is low and res u mes when the signal goes high or vice
versa. For contin u o u s p u lse generations, the co u nter stops generating
p u lses while the external trigger signal is low and res u mes when the
signal goes high or vice versa.
When u sing a pa u se trigger, the pa u se trigger so u rce is ro u ted to the
Co u nter n Gate signal inp u t of the co u nter.