User's Manual

Lab-NB User Manual 4-10 © National Instruments Corporation

A/D FIFO Register

Reading the A/D FIFO Register returns the next A/D conversion value stored in the A/D FIFO.

Whenever the A/D FIFO Register is read, the value read is removed from the A/D FIFO, thereby

freeing space for another A/D conversion value to be stored. Values are stored into the A/D

FIFO Register by the ADC whenever an A/D conversion is complete. Although A/D conversion

values are in 12-bit format, they are automatically sign-extended to 16 bits in the FIFO.

The A/D FIFO is emptied when all values it contains are read. The Status Register should be

read 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 A/D FIFO Register can

be read 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 Register are available in two different binary

formats: straight binary or two's complement binary. The binary format used is selected by the

TWOSCMP bit in the A/D Configuration Register. The bit pattern returned for either format is

given below.

Address: Base address + 0 8010 (hex)

Type: Read-only

Word Size: 16-bit

Bit Map: Straight binary mode

15 14 13 12 11 10 9 8

0 0 0 0 D11 D10 D9 D8

76543210

D7 D6 D5 D4 D3 D2 D1 D0

Bit Name Description

15–0 D<15..0> Data Bit—These bits contain the straight binary result of a 12-bit

A/D conversion. Bits D<15..12> are always 0 in straight binary

mode. Values read, therefore, range from 0 to +4,095 decimal

(0000 to 0FFF hex). Straight binary mode is useful for unipolar

analog input readings because all values read reflect a positive

polarity input signal.