Chapter 4 Register Map and Descriptions
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National Instruments Corporation 4-3 AT-MIO-16X User Manual
Register Sizes
Two different transfer sizes for read-and-write operations are available
on the PC: byte (8-bit) and word (16-bit). Table 4-1 shows the size of
each AT-MIO-16X register. For ex am ple , read in g t h e ADC F IFO
Register requires a 16-bit (word) read operation at the selected address,
whereas writing to the RTSI Strobe Register requires an 8-bit (byte)
write operation at the selected address. These registe r size ac cess es
must be adhered to for proper board operation. Performing a byte access
on a word location is an invalid operation and should be avoided. The
converse is also true. Performing a word access on a byte location is
also an invalid operation and should be avoided. You should pa y
particular attention to the register sizes—they are very important.
Register Description Format
The remainder of this register description chapter discusses each of the
AT-MIO-16X registers in the order shown in Table 4-1. Each register
group is introduced, followed by a detailed bit description of each
register. The individual register description gives the address, type,
word size, and bit map of the register, followed by a description of
each bit.
The register bit map shows a diagram of the register wi th the MSB
shown on the left (bit 15 for a 16-bit register, bit 7 for an 8-bit register),
and the LSB shown on the right (bit 0). A square is used to represent
each bit. Each bit is labeled with a name inside its square. An asterisk
(*) after the bit name indicates that the bit is inverted (negative logic).
In many of the registers, several bits are labeled with an X, indicating
don’t care bits. When a register is read, these bi ts may appear s et or
cleared but should be ignored because they have no significa nc e.
The bit map field for some registers states not applicable, no bits used.
Accessing these registers generates a strobe in the AT-MIO-16X. These
strobes are used to initiate some onboard event to occur. For example,
they can be used to clear the analog input circuitry or to start a data
acquisition operation. The data is ignored when writing to these
registers; therefore, any bit pattern suffices. Likewise, data returned
from a strobe register read access is meaningless.