Appendix B
Timing Diagrams
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National Instruments Corporation
B-5
Internal Timing
AI Timing Clocks
The analog input timing engine has two levels of timing that control an AI
acquisition. The first level is the convert level. This is the timing that
controls when the analog to digital conversions take place. The SC, DIV,
and SI2 counters run on this timing level. The signal that clocks this timing
level is called Convert Clock Timebase. This signal can come from an
internal source (for example, an internal timebase) or an external signal.
It can be divided down using the SI2 counter, or it can be used directly
(in external convert mode). In order to synchronize triggers to the Convert
Clock Timebase signal, another related signal is generated called Sync
Convert Clock Timebase. Sync Convert Clock Timebase is generated
differently depending on the mode the AI timing engine is operating on:
•
When Convert Clock Timebase is a signal that will be divided down
using the SI2 counter (either internal or external), it is considered to be
a free-running clock. In this case, the Sync Convert Clock Timebase is
the inverted version of the Convert Clock Timebase signal. The idea is
to use the falling edge of the original signal to synchronize external
signals before the rising edge of the Convert Clock Timebase occurs
(after polarity selection). This case is the one described in this section.
•
When Convert Clock Timebase is not going to be divided by the
SI2 counter (in the case of an external convert signal), this signal is
assumed to be not free-running and highly irregular. In this case, Sync
Convert Clock Timebase is selected to be the actual external signal,
and Convert Clock Timebase is a delayed version of the external
signal. This delay is long enough so that external signals can be
synchronized with Sync Convert Clock Timebase and used by Convert
Clock Timebase. For timing diagrams and parameters for this case,
refer to the
section.
Table B-1.
Input Timing
Time
From
To
Min (ns)
Max (ns)
t
1
*
PFI
PFI_i
4.2
6.4
15.2
19.2
RTSI
RTSI_i
0.9
2.2
2.0
3.0
STAR
STAR_i
0.9
—
—
2.8
*
The delay ranges given for PFI and RTSI represent the fastest and slowest terminal routing within the trigger group for a
given condition (maximum or minimum timing). This difference can be useful when two external signals will be used
together and the relative timing between the signals is important.
