Chapter 2
Analog Input Timing/Control
©
National Instruments Corporation
2-93
DAQ-STC Technical Reference Manual
Figure 2-20.
External CONVERT_SRC Timing
2.7.7 External Triggers
Each of the external signals will be latched and recognized by the DAQ-STC in one of two
configurations. The latching of the external signal generates an internal version of the signal,
which the internal control circuit uses. The control circuit then can recognize the internal
signal and change state. The latching and recognition of the external signal are software
programmable to occur in synchronous mode or asynchronous mode. In synchronous mode,
the DAQ-STC is internally synchronizing the signal to prevent metastability. In asynchronous
mode, the external signal must already be synchronized to the state clock, allowing the
DAQ-STC to use the signal directly.
Each of the external inputs to the DAQ-STC can be edge or level sensitive. The level-sensitive
signals are passed directly to the latch and recognition circuitry. The edge-sensitive mode
generates an intermediate internal signal by prelatching the external signal at its active edge.
This intermediate signal is routed to the latching and recognition circuitry. The edge-sensitive
mode has only a pulsewidth requirement, but in order to guarantee recognition by a specific
clock edge it must also meet the setup time to the latching state clock edge.
The four modes resulting from the combination of the above options are shown in Figures
2-21 through 2-26. The four modes are asynchronous-level sensitive, asynchronous-edge
sensitive, synchronous-level sensitive, and synchronous-edge sensitive.
In synchronous mode, the synchronizing edge depends on whether you select internal
CONVERT or external CONVERT. In the internal CONVERT mode, the external signal
synchronizes to the inactive edge of the SI2 source. In the external CONVERT mode, the
external signal synchronizes to the active edge of CONVERT_SRC.
Name
Description
Minimum
Maximum
Tds
CONVERT_SRC to CONVERT asserted
21
65
Textw
CONVERT_SRC width
50
—
Texti
CONVERT_SRC interval
100
—
All timing values are in nanoseconds.
CONVERT
CONVERT_SRC
Texti
Textw
Tds
Tds
Tds