ADC0803 datasheet(31/41 Pages) NSC

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Functional Description (Continued)

The following schematic and sample subroutine (DATA IN)

may be used to interface (up to) 8 ADC0801’s directly to the

MC6800 CPU. This scheme can easily be extended to allow

the interface of more converters. In this configuration the

converters are (arbitrarily) located at HEX address 5000 in

the MC6800 memory space. To save components, the clock

signal is derived from just one RC pair on the first converter.

This output drives the other A/Ds.

All the converters are started simultaneously with a STORE

instruction at HEX address 5000. Note that any other HEX

address of the form 5XXX will be decoded by the circuit,

pulling all the CS inputs low. This can easily be avoided by

using a more definitive address decoding scheme. All the

interrupts are ORed together to insure that all A/Ds have

completed their conversion before the microprocessor is

interrupted.

The subroutine, DATA IN, may be called from anywhere in

the user’s program. Once called, this routine initializes the

CPU, starts all the converters simultaneously and waits for

the interrupt signal. Upon receiving the interrupt, it reads the

converters (from HEX addresses 5000 through 5007) and

stores the data successively at (arbitrarily chosen) HEX

addresses 0200 to 0207, before returning to the user’s pro-

gram. All CPU registers then recover the original data they

had before servicing DATA IN.

5.2 Auto-Zeroed Differential Transducer Amplifier

and A/D Converter

The differential inputs of the ADC0801 series eliminate the

need to perform a differential to single ended conversion for

a differential transducer. Thus, one op amp can be elimi-

nated since the differential to single ended conversion is

provided by the differential input of the ADC0801 series. In

general, a transducer preamp is required to take advantage

of the full A/D converter input dynamic range.

SAMPLE PROGRAM FOR

Figure 16 ADC0801–MC6820 PIA INTERFACE

DS005671-A2

ADC0803 Datasheet(PDF) 31 Page - National Semiconductor (TI)