National Instruments PC-LPM-16/PnP User Manual

Page 87

Appendix D Register-Level Programming

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National Instruments Corporation

D-15

PC-LPM-16/PnP User Manual

A/D FIFO Low-Byte Register and A/D FIFO High-Byte Register

The 13-bit A/D conversion results are sign-extended to 16-bit data in two’s complement
format and are stored in a 16-word deep A/D FIFO buffer. Two 8-bit registers, the A/D
FIFO Low-Byte Register and the A/D FIFO High-Byte Register, must be read to return
an A/D conversion value stored in the A/D FIFO. The A/D FIFO Low-Byte Register,
which must be read first, contains the low byte of the 16-bit value, and the A/D FIFO
High-Byte Register contains the high byte of the 16-bit value.

Note:

The A/D FIFO Low-Byte Register

MUST

be read first.

The value read is removed from the A/D FIFO, thereby freeing space to store another A/D
conversion value.

The A/D FIFO is empty after reading all the values it contains. The Status Register should
be checked before the A/D FIFO Register is read. If the A/D FIFO contains one or more
A/D conversion values, the DAVAIL bit is set in the Status Register, and the external
device can read the A/D FIFO Register to retrieve a value. If the DAVAIL bit is cleared,
the A/D FIFO is empty, in which case reading the A/D FIFO Register returns
meaningless information.

The values returned by reading the A/D FIFO Registers are available in two’s
complement binary format. When the analog input range is unipolar, any small negative
value returned from FIFO should be explained as zero.

Address:

A/D FIFO Low-Byte Register

Base address + 02 (hex)

A/D FIFO High-Byte Register

Base address + 03 (hex)

Type:

Read-only

Word Size:

8-bit

Bit Map:

Two’s complement binary mode

High Byte

{---Sign and Sign Extension Bits---}

Low Byte

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a.Book : l.Appendix D Page 15 Wednesday, November 20, 1996 6:36 PM