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AD7880

LC2MOS Single +5 V Supply, Low Power, 12-Bit Sampling ADC

Analog Devices 

AD7880

a write operation starts a conversion. Data is read at the end of

the conversion sequence as before. Figure 19 shows an example

of initiating conversion using this method. A similar implemen-

tation can be used for DSPs. Note that for all interfaces, a read

operation should not be attempted during conversion.

AD7880–MC68000 Interface

An interface between the AD7880 and the MC68000 is shown

in Figure 18. As before, conversion is initiated using an external

timer. The AD7880 BUSY line can be used to interrupt the

processor or, alternatively, software delays can ensure that con-

version has been completed before a read to the AD7880 is at-

tempted. Because of the nature of its interrupts, the 68000

requires additional logic (not shown in Figure 18) to allow it to

be interrupted correctly. For further information on 68000 in-

terrupts, consult the 68000 users manual.

The MC68000 AS and R/W outputs are used to generate a

separate RD input signal for the AD7880. CS is used to drive

the 68000 DTACK input to allow the processor to execute a

normal read operation to the AD7880. The conversion results

are read using the following 68000 instruction:

MOVE.W ADC, D0

where D0 is the 68000 D0 register

where ADC is the AD7880 address

A15

A0

MC68000

AS

DTACK

ADDRESS BUS

ADDR

DECODE

EN

TIMER

CONVST

CS

AD7880*

RD

R/W

DB11

DB0

D15

D0

DATA BUS

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 18. AD7880–MC68000 Interface

AD7880–8086 Interface

Figure 19 shows an interface between the AD7880 and the

8086 microprocessor. Unlike the previous interface examples,

the microprocessor initiates conversion. This is achieved by gat-

ing the 8086 WR signal with a decoded address output (differ-

ent to the AD7880 CS address). Conversion is initiated and the

result is read from the AD7880 using the following instruction:

MOV AX, ADC

where AX is the 8086 accumulator and

where ADC is the AD7880 address

ADDRESS BUS

8086

ADDR

DECODE

CS

ALE

WR

RD

LATCH

AD7880*

CONVST

RD

DB11

DB0

AD15

AD0

ADDRESS/DATA BUS

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 19. AD7880–8086 Interface

AD7880–6809 Interface

The AD7880 can also interface quite easily with 8-bit micro-

processors. The 12-bit parallel data output from the AD7880

can be read into the microprocessor as an 8+4 byte structure.

Figure 20 shows an interface to the MC6809 8-bit microproces-

sor. As in previous cases, conversion is initiated using an exter-

nal timer. At the end of conversion, BUSY triggers a one-shot

which drives the IRQ interrupt input of the microprocessor. A

double read is then performed to two unique addresses. The

first read fetches the lower 8 bits (DB0–DB7) and loads the

74HC374 latch with the upper 4 bits (DB8–DB11). The sec-

ond read fetches these upper 4 bits.

A15

ADDRESS BUS

A0

TIMER

MC6809

ADDR

DECODE

R/W

E

IRQ

ONE

SHOT

CONVST

CS

AD7880*

RD

BUSY

OE CLK

D3

Q3 D0

Q0

74HC374

DB11

DB8

DB7

DB0

D7

DATA BUS

D0

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 20. AD7880–6809 Interface

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