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Chapter 2

Function Reference — GPCTR_Set_Application

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NI-DAQ FRM for PC Compatibles

ND_PROGRAM

GPCTR_Watch

ND_TRIG_PULSE_WIDTH_MSR

In this application, the counter is used for a single measurement of the time interval between 
two transitions of the opposite polarity of the gate signal. By default, the measurement is 
performed between a low-to-high and a high-to-low transition on the I/O connector default 
gate pins (see Table 2-25 for default gate pin selection). The counter counts the 20 MHz 
internal timebase (

INTERNAL_20_MHZ)

, so the resolution of measurement is 50 ns. The 

counter counts up starting from 0.

Unlike 

ND_SINGLE_PULSE_WIDTH_MSR

, your gate signal can change state during counter 

arming. However, the counter will start counting only after a high-to-low edge on the gate if 
the gate polarity is positive, or after a low-to-high edge on the gate if the gate polarity is 
negative. This transition is the trigger from this application’s name.

The default 20 MHz timebase, combined with the counter width (24 bits), lets you measure 
the duration of a pulse between 100 ns and 0.8 s long. For the 6602 and 455X devices with 
counter width 32 bits, you can measure pulse duration between 100 ns and 214 s long.

Figure 2-17 shows one possible scenario of a counter used for 

ND_TRIG_PULSE_WIDTH_MSR

 

after the following programming sequence:

GPCTR_Control(deviceNumber, gpctrNum, ND_RESET) 

GPCTR_Set_Application(deviceNumber, gpctrNum, ND_TRIG_PULSE_WIDTH_MSR)

GPCTR_Control(deviceNumber, gpctrNum, ND_PROGRAM) 

In Figure 2-17, the following behavior is present:

•

Gate is the signal present at the counter gate input.

•

Source is the signal present at the counter source input.

•

Count is the value you would read from the counter if you called the 

GPCTR_Watch

 

function with entityID =

ND_COUNT

. The different numbers illustrate behavior at 

different times.

•

Count Available is the value you would read from the counter if you called the 

GPCTR_Watch

 function with entityID =

ND_COUNT_AVAILABLE

. The different values 

illustrate behavior at different times.