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

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Chapter 3 Theory of Operation
© National Instruments Corporation 3-11 AT-MIO-64F-5 User Manual
A SCANCLK signal is generated from the sample-interval counter. This signal pulses once at
the beginning of each A/D conversion and is supplied at the I/O connector. During
multiple-channel scanning, the configuration memory location pointer is incremented repeatedly,
thereby sequencing through the memory and automatically selecting new channel settings during
data acquisition. The signal used to increment the configuration memory location pointer is
generated from the SCANCLK signal. Incrementing can be identical to SCANCLK, sequencing
the configuration memory location pointer once after every A/D conversion, or it can also be
generated by dividing SCANCLK by Counter 1 of the Am9513A Counter/Timer. With this
method, the location pointer can be incremented once every
N A/D conversions so that N
conversions can be performed on a single-channel configuration selection before switching to the
next configuration memory selection.
Continuous Scanning Data Acquisition Timing
Continuous scanning data acquisition uses the configuration memory register to automatically
sequence from one analog input channel setting to another during the data acquisition sequence.
Continuous scanning cycles through the configuration memory without any delays between
cycles. Scanning is similar to the single-channel acquisition in the programming of both the
sample-interval counter and the sample counter. Scanning data acquisition is enabled through a
register in the AT-MIO-64F-5 register set. Figure 3-7 shows the timing for a continuous
scanning data acquisition sequence.
Trigger*
DAQPROG
DAQCMPLT
DAQCLEAR*
Interrupt
0 1 2 0 1 2 0 Channel
SCANCLK
CONVERT
Figure 3-7. Scanning Posttrigger Data Acquisition Timing
In this sequence, the timing is the same as the single-channel acquisition except for the addition
of the channel sequencing and the generation of the SCANCLK signal. The first sampled
channel is Channel 0, followed in time by Channel 1, and finally Channel 2. After this, the
sequence is repeated. For this example, the sequence consists of Channels 0, 1, and 2 which are
cycled through twice to generate six values of conversion data. After the six samples have been
acquired, the sample counter terminates the data acquisition sequence.
The SCANCLK signal is generated to indicate when the input signal can be removed from the
conversion channel. This signal is available at the I/O connector and can be used to control
external multiplexers for higher channel-count applications. The rising edge of SCANCLK
signals when the ADC has acquired the input signal and no longer needs to have it held
available. In the scanning acquisition modes, this signal pulses for every conversion.