National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ - Manual (Page 164)

Register-Level Programming Appendix E
Lab-PC+ User Manual E-8 © National Instruments Corporation
Once the data acquisition operation is started, the operation must be serviced by reading the
A/D FIFO Register every time an A/D conversion result becomes available. To do this,
perform the following sequence until the desired number of conversion results have been
read:
a. Read the Status Register (8-bit read).
b. If the DAVAIL bit is set (bit 0), then read the A/D FIFO Register to obtain the result.
DMA or interrupts can also be used to service the data acquisition operation. These topics are
discussed in the A/D Interrupt Programming and Programming DMA Operation sections later in
this appendix.
Two error conditions may occur during a data acquisition operation: an overflow error or an
overrun error. These error conditions are reported through the Status Register and should be
checked every time the Status Register is read to check the DAVAIL bit.
An overflow condition occurs if more than 16 A/D conversions have been stored in the A/D
FIFO without the A/D FIFO being read; that is, the A/D FIFO is full and cannot accept any more
data. This condition occurs if the software loop reading the A/D FIFO Register is not fast
enough to keep up with the A/D conversion rate. When an overflow occurs, at least one A/D
conversion result is lost. An overflow condition has occurred if the OVERFLOW bit in the
Status Register is set.
An overrun condition occurs if a second A/D conversion is initiated before the previous
conversion is finished. This condition may result in one or more missing A/D conversions. This
condition occurs if the sample interval is too small (sample rate is too high). An overrun
condition has occurred if the OVERRUN bit in the Status Register is set. The minimum
recommended sampling interval on the Lab-PC+ is 16
µ
s.
Both the OVERFLOW and OVERRUN bits in the Status Register are reset by writing to the A/D
Clear Register.
Programming in Freerun Acquisition Mode
Freerun acquisition mode uses only Counter A0 as the sample interval counter. The number of
A/D conversions that have occurred (that is, the sample count) is maintained by software in this
case. With this arrangement, data acquisition operations can acquire more than 65,535 samples.
The following programming steps are required for a data acquisition operation in freerun
acquisition mode: