AD7840

REV. B

–10–

MICROPROCESSOR INTERFACING

The AD7840 logic architecture allows two interfacing options

for interfacing the part to microprocessor systems. It offers a

14-bit wide parallel format and a serial format. Fast pulse

widths and data setup times allow the AD7840 to interface

directly to most microprocessors including the DSP processors.

Suitable interfaces to various microprocessors are shown in

Figures 15 to 23.

Parallel Interfacing

Figures 15 to 17 show interfaces to the DSP processors, the

ADSP-2100, the TMS32010 and TMS32020. An external

timer controls the updating of the AD7840. Data is loaded to

the AD7840 input latch using the following instructions:

ADSP-2100: DM(DAC) = MR0

TMS32010: OUT DAC,D

TMS32020: OUT DAC,D

MR0 = ADSP-2100 MR0 Register

D = Data Memory Address

DAC = AD7840 Address

Figure 15. AD7840–ADSP-2100 Parallel Interface

Figure 16. AD7840–TMS32010 Parallel Interface

Figure 17. AD7840–TMS32020 Parallel Interface

Some applications may require that the updating of the AD7840

DAC latch be controlled by the microprocessor rather than the

external timer. One option (for double-buffered interfacing) is

to decode the AD7840 LDAC from the address bus so that a

write operation to the DAC latch (at a separate address than the

input latch) updates the output. An example of this is shown in

the 8086 interface of Figure 18. Note that connecting the

LDAC

input to the CS input will not load the DAC latch cor-

rectly since both latches cannot he transparent at the same time.

AD7840–8086 Interface

Figure 18 shows an interface between the AD7840 and the 8086

microprocessor. For this interface, the LDAC input is derived

from a decoded address. If the least significant address line, A0,

is decoded then the input latch and the DAC latch can reside at

consecutive addresses. A move instruction loads the input latch

while a second move instruction updates the DAC latch and the

AD7840 output. The move instruction to load a data word

WXYZ to the input latch is as follows:

MOV DAC,#YZWX

DAC = AD7840 Address

Figure 18. AD7840–8086 Parallel Interface