Data Translation DT9857, User Manual, Page: 101 / 192

Principles of Operation
101
• If the input subsystem sample frequency is slower than the selected input frequency, then
the new measure count value is stored as each period measurement completes. When an
input subsystem sample occurs, then the most recently stored measure count is written
into the input data stream.
When the selected start edge is not the same as the selected stop edge, the internal 48 MHz
counter starts incrementing when it detects the selected start edge and stops incrementing
when it detects the next selected stop edge; at that point, the counter stores and resets the
count. The stored count is maintained until it is read as part of the input data stream or until a
new count is stored. The value of the measure count depends on the input subsystem sample
frequency, described as follows:
• If the input subsystem sample rate is faster than the selected measurement period, then
the stored count retains the current value when the count is read by the input subsystem.
• If the input subsystem sample rate is slower than the selected measurement period, then a
new count value is stored as each period measurement completes. When an input
subsystem sample occurs, the most recently stored measure count is written into the input
data stream.
A data pipeline is used in the hardware to compensate for the A/D group delay and
synchronizes the value of the measure counters with the analog input measurements, so that
all measurements are correlated in time. The measure counters are treated like any other
channel in the analog input channel list; therefore, all the triggering and conversion modes
supported for analog input channels are supported for the measure counters.
When you read the value of the measure counters as part of the analog input data stream, you
might see results similar to the following:
Table 4: An Example of Reading a Measure Counter as Part of the Analog Input Data Stream
Time A/D Value
Measure Counter
Values Status of Operation
10 5002 0 Operation started, but is not complete
20 5004 0 Operation not complete
30 5003 0 Operation not complete
40 5002 12373 Operation complete
50 5000 12373 Next operation started, but is not complete
60 5002 12373 Operation not complete
70 5004 12373 Operation not complete
80 5003 14503 Operation complete
90 5002 14503 Next operation started, but is not complete