National Instruments VXI-MIO Series - Manual (Page 90)

Chapter 4 Signal Connections
VXI-MIO Series User Manual 4-44  National Instruments Corporation
The GATE and OUT signal transitions shown in Figure 4-35 are
referenced to the rising edge of the SOURCE signal. This timing
diagram assumes that the counters are programmed to count rising
edges. The same timing diagram, but with the source signal inverted and
referenced to the falling edge of the source signal, would apply when
the counter is programmed to count falling edges.
The GATE input timing parameters are referenced to the signal at the
SOURCE input or to one of the internally generated signals on your
VXI-MIO Series module. Figure 4-35 shows the GATE signal
referenced to the rising edge of a source signal. The gate must be valid
(either high or low) for at least 10 ns before the rising or falling edge of
a source signal for the gate to take effect at that source edge, as shown
by t
gsu
and t
gh
in Figure 4-35. The gate signal is not required to be held
after the active edge of the source signal.
If an internal timebase clock is used, the gate signal cannot be
synchronized with the clock. In this case, gates applied close to a source
edge take effect either on that source edge or on the next one. This
arrangement results in an uncertainty of one source clock period with
respect to unsynchronized gating sources.
The OUT output timing parameters are referenced to the signal at the
SOURCE input or to one of the internally generated clock signals on the
VXI-MIO Series modules. Figure 4-35 shows the OUT signal
referenced to the rising edge of a source signal. Any OUT signal state
changes occur within 80 ns after the rising or falling edge of the source
signal.
FREQ_OUT Signal
This signal is available only as an output on the FREQ_OUT pin. The
FREQ_OUT signal is the output of the VXI-MIO Series module
frequency generator. The frequency generator is a 4-bit counter that can
divide its input clock by the numbers 1 through 16. The input clock of
the frequency generator is software selectable from the internal
10 MHz and 100 kHz timebases. The output polarity is software
selectable. This signal is set to input (High-Z) at startup.