User's Manual

Chapter 2 Function Reference — GPCTR_Set_Application
NI-DAQ FRM for PC Compatibles 2-222
© National Instruments Corporation
GPCTR_Watch(deviceNumber, gpctrNumber, ND_COUNT_AVAILABLE,
count_available)
}
until (count_available = ND_YES)
GPCTR_Watch(deviceNumber, gpctrNumber, ND_COUNT, counted_value)
To calculate the measured interval, multiply the counted value by the period corresponding to
the timebase you are using. For example, if your
ND_SOURCE is ND_INTERNAL_20_MHZ, the
interval will be 1/(20 MHz) = 50 ns. If the
ND_COUNT is 4 (Figure 2-15), the actual interval is
4 * 50 ns = 200 ns.
When the counter reaches TC (Terminal Count), it rolls over and keeps counting. To check if
this occurred, use the
GPCTR_Watch function with entityID set to ND_TC_REACHED.
Typically, you will find modifying the following parameters through the
GPCTR_Change_Parameter function useful when the counter application is
ND_SINGLE_PULSE_WIDTH_MSR. You can change the following:
•
ND_SOURCE to ND_INTERNAL_100_KHZ. With this timebase, you can measure pulse
widths between 20 µs and 160 s. The resolution will be lower than if you are using the
ND_INTERNAL_20_MHZ timebase.
•
ND_SOURCE_POLARITY to ND_HIGH_TO_LOW.
•
ND_GATE to any legal value listed in the GPCTR_Change_Parameter function
description.
•
ND_GATE_POLARITY to ND_NEGATIVE. The pulse width will be measured from a
high-to-low to the next low-to-high transition of the gate signal.
You can use the
GPCTR_Change_Parameter function after calling
GPCTR_Set_Application and before calling GPCTR_Control with
action =
ND_PROGRAM or ND_PREPARE.
To provide your timebase, connect your timebase source to one of the PFI pins on the I/O
connector and change
ND_SOURCE and ND_SOURCE_POLARITY to the appropriate values.
You can also configure the other general-purpose counter for
ND_PULSE_TRAIN_GNR and set
ND_SOURCE of this counter to ND_OTHER_GPCTR_TC to measure pulse widths longer than
160 s for E Series and 445X devices and 11.37 hours for 6602 and 455X devices.
Caution Application ND_SINGLE_PULSE_WIDTH_MSR works as described only if the gate
signal stays in the low state when
ND_GATE_POLARITY is ND_POSITIVE, or if the
signal stays in the high state when
ND_GATE_POLARITY is ND_NEGATIVE while
GPCTR_Control is executed with action = ND_ARM or action = ND_PROGRAM. If
this criterion is not met, executing
GPCTR_Control with action = ND_ARM or
!