Pad Synchronization - National Instruments NI 6602 User Manual

Chapter 2 Device Overview
External Signal
Prescaler Rollover
(Used as Source
by Counter)
Counter Value

Pad Synchronization

NI 660x User Manual
Figure 2-1 shows an example of prescaling.
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Prescaling is intended for use with two counter period and frequency
measurements where the measurement is made on a continuous, repetitive
signal. The prescaling counter cannot be read, so you cannot determine how
many edges have occurred since the previous roll-over. You can also use
prescaling for counting edges if it is acceptable to have an error of up to
seven when using 8X prescaling or one when using 2X prescaling.
The NI 660x devices allow synchronization of their PFI lines and RTSI
lines at the I/O pads. This is called pad synchronization in this document,
and digital synchronization in the NI-DAQmx API. You cannot use digital
filtering while enabling this feature.
Pad synchronization is useful when several counters are measuring or
operating off the same external signal. For example, suppose
counters 0 and 1 are configured for triggered pulse generation and each
counter uses the same external trigger (this external signal is connected to
PFI 38 on the I/O connector and both counters have PFI 38 selected as their
GATE). After the trigger signal propagates through the I/O pad of the
ASIC, the time for the signal to reach the GATE of each counter within the
ASIC may differ by a few nanoseconds.
This signal is sampled at the counters' GATEs using the selected SOURCE.
Because of different propagation times for the paths to the two GATEs,
it is possible for the counters to detect the trigger on different edges on
SOURCE. Thus, one counter could see the trigger one SOURCE period
after the other. If you want to allow the counters to see the changes in the
signal at the same instance, you should use pad synchronization. During
pad synchronization, the signal is offset by one clock cycle.
Figure 2-1. Prescaling Example
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