National Instruments AT-MIO-64F-5, User Manual

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Theory of Operation Chapter 3
AT-MIO-64F-5 User Manual 3-8 © National Instruments Corporation
Data Acquisition Timing Circuitry
This section details the different methods of acquiring A/D data from a single channel or
multiple channels. Prior to any of these operations, the channel, gain, mode, and range settings
must be configured. This is accomplished through writing to a register in the AT-MIO-64F-5
register set.
Single-Read Timing
The simplest method of acquiring data from the A/D converter is to initiate a single conversion
and then read the resulting value from the ADC FIFO buffer after the conversion is complete. A
single conversion can be generated three different waysÐapplying an active low pulse to the
EXTCONV* pin of the I/O connector, generating a falling edge on the sample-interval counter
output pin (Counter 3 of the Am9513A Counter/Timer), or strobing the appropriate register in
the AT-MIO-64F-5 register set. Any one of these operations will generate the timing shown in
Figure 3-4. The ADC_BUSY* signal status can be monitored through a status register on the
AT-MIO-64F-5.
CONVERT*
ADC_BUSY*
FIFO_LD*
Figure 3-4. ADC Conversion Timing
When the ADC value is shifted into the ADC FIFO buffer by FIFO_LD*, a signal is generated
that indicates valid data is available to be read. Single conversion timing of this type is
appropriate for reading channel data on an ad hoc basis. However, if a sequence of conversions
is needed, this method is not very reliable because it relies on the software to generate the
conversions in the case of the strobe register. If finely timed conversions are desired that require
triggering and gating, then it is necessary to program the board to automatically generate timed
signals that initiate and gate conversions. This is known as a data acquisition sequence.
A data acquisition operation refers to the process of taking a sequence of A/D conversions with
the sample interval (the time between successive A/D conversions) carefully timed. The data
acquisition timing circuitry consists of various clocks and timing signals. Three types of data
acquisition are available with the AT-MIO-64F-5 boardÐsingle-channel data acquisition,
multiple-channel data acquisition with continuous scanning, and multiple-channel data
acquisition with interval scanning. All data acquisition operations work with pretrigger and
posttrigger modes with either internal or external timing signals. Pretriggering acquires data
before a software or hardware trigger is applied. Posttriggering acquires data only after a
software or hardware trigger is received.
Single-Channel Data Acquisition Timing
The sample-interval timer is a 16-bit down counter that can be used with the six internal
timebases of the Am9513A to generate sample intervals from 0.4 µsec to 6 sec (see the
Timing
I/O Circuitry section later in this chapter). Conversion intervals of less than 5 µsec will result in
an overrun condition. Counter 3 of the Am9513A Counter/Timer is used to generate conversion