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Chapter 5
Counters
Master Timebase Signal
The Master Timebase (MasterTimebase) signal, or Onboard Clock, is the timebase from which
all other internally generated clocks and timebases on the board are derived. It controls the
timing for the analog output and counter subsystems. It is available as on output on the I/O
connector, but you must use one or more counters to do so.
The maximum allowed frequency for the MasterTimebase is 20 MHz, with a minimum pulse
width of 23 ns high or low. There is no minimum frequency limitation.
The two possible sources for the MasterTimebase signal are the internal 20MHzTimebase signal
or an external signal through RTSI 7. Typically the 20MHzTimebase signal is used as the
MasterTimebase unless you wish to synchronize multiple devices, in which case, you should use
RTSI 7. Refer to Chapter 8,
which signals are available through RTSI.
Figure 5-10 shows the timing requirements for MasterTimebase.
Figure 5-10. MasterTimebase Timing Requirements
Getting Started with Counter Applications in
Software
You can use the AO Series device in the following counter-based applications.
•
Counting Edges
•
Frequency Measurement
•
Period Measurement
•
Pulse Width Measurement
•
Semi-Period Measurement
•
Pulse Generation
You can perform these measurements through programmed I/O, interrupt, or DMA data transfer
mechanisms. The measurements can be finite or continuous in duration. Some of the
applications also use start triggers and pause triggers.
For more information about programming counter applications and triggers
Note
in software, refer to the NI-DAQmx Help.
5-8 | ni.com
Real-Time System Integration Bus
t
p
t
t
w
w
t
= 50 ns minimum
p
t
= 23 ns minimum
w
(RTSI), for more information on
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