AD7721

REV. A

–14–

Parallel Interface

In parallel mode, the

DRDY signal is still available. This signal

can be used to generate an interrupt in the DSP as

DRDY goes

high for two clock cycles when a conversion is complete. Data

is available from the AD7721 every 32 CLK cycles. The ADC

outputs the 12-bit digital word automatically. Hence, latches are

needed into which the 12-bit parallel word can be transferred.

Because

RD and CS are permanently tied to DGND when the

ADC is performing A-to-D conversions, some further glue logic

is needed to interface the AD7721 to a DSP in parallel mode.

When a digital word is available from the AD7721, it will be

automatically transferred to the latches. The

DRDY signal

informs the DSP that a new word is available to be read. The

DSP then reads the word from the latches. By using the

latches, the microprocessor is free to perform other tasks be-

tween reads from the AD7721.

When using the parallel mode,

CS and RD should be permanently

tied to DGND,

RD being taken high only when a control word

is being written to the AD7721.

CS and RD should not be

pulsed, as is the procedure with other ADCs, as the specifications

for the device will degrade and the part may become unstable.

CS

DSP

RD

WR

INTERRUPT

AD7721

DRDY

WR

RD

CS

1Y1

1Y2

1Y3

2Y1

2Y2

2Y3

2Y4

HC244

1G 2G 1A1

1A2

1A3

1A4

2A1

2A2

2A3

2A4

1Y4

DECODE

DB11

DB10

DB9

DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

DB11

DB10

DB9

DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

1Y1

1Y3

2A1

2A2

2A3

2A4

HC244

1G 2G 1A1

1A2

1A3

1A4

2Y1

2Y2

2Y3

2Y4

1Y4

1Y2

Figure 16. Interfacing the AD7721 to a Microprocessor in

Parallel Mode

AD7721 to ADSP-21xx Interface

Figure 17 shows the AD7721 to ADSP-21xx interface.

DRDY

is used to interrupt the DSP when a conversion is complete and

the HC244 latches contain a new word. The

WR signal from

the DSP is used to drive both the

RD and WR inputs of the

AD7721 since

RD will be tied low at all times except when the

control register of the device is being written to. The

RD signal

of the DSP is used to enable the outputs of the latches so that

the 12 bit word can be read into the DSP. Two 8-bit latches

are used. Twelve of the latches are used to hold the 12-bit

conversion from the AD7721. The remaining four latches are

used to hold the control information being transferred from the

DSP to the AD7721. When a control word is being written to

the AD7721, Bits 4 to 6 and Bits 9 to 10, which are test bits,

need to be loaded with zeros. Therefore, pull-down resistors

are used so that Pins 4 to 6 and 9 to 10 are tied to ground when

the control register is being loaded.

CS

AD7721

DRDY

WR

RD

CS

1Y1

1Y2

1Y3

2Y1

2Y2

2Y3

2Y4

HC244

1G 2G 1A1

1A2

1A3

1A4

2A1

2A2

2A3

2A4

1Y4

1Y1

1Y2

1Y3

2A1

2A2

2A3

2A4

HC244

1G 2G 1A1

1A2

1A3

1A4

2Y1

2Y2

2Y3

2Y4

1Y4

DB11

DB10

DB9

DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

RD

WR

ADSP-21xx

DMD11–DMD

DMA13–DMA0

IRQ

DMS

EN ADDR

DECODE

Figure 17. AD7721 to ADSP-21xx Interface

AD7721 to DSP56002 Interface

Figure 18 shows the AD7721 to DSP56002 interface. The

connections for the DSP56002 are similar to those for the

ADSP-21xx family. The diagram shows the connections for

the DSP56002, but the connections for the DSP56000 and

DSP56001 are similar.

CS

AD7721

DRDY

WR

RD

CS

1Y1

1Y2

1Y3

2Y1

2Y2

2Y3

2Y4

HC244

1G 2G 1A1

1A2

1A3

1A4

2A1

2A2

2A3

2A4

1Y4

1Y1

1Y2

1Y3

2A1

2A2

2A3

2A4

HC244

1G 2G 1A1

1A2

1A3

1A4

2Y1

2Y2

2Y3

2Y4

1Y4

DB11

DB10

DB9

DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

RD

WR

IRQ

A15–A0

D11–D0

DS

DSP56002

EN ADDR

DECODE

Figure 18. AD7721 to DSP56002 Interface

AD7721 to TMS320C20/C25/C5x Interface

Figure 19 shows the AD7721 to TMS320C20/C25 interface

while Figure 20 shows the AD7721 to TMS320C5x interface.

Again, the interface is similar to that of the ADSP-21xx. However,

the TMS320C20/C25 has a common RD/

W pin. This output

is decoded using the

STRB pin. The TMS320C5x has a RD/W

pin also so external glue logic can be used to decode the RD/

W

pin as done for the C20 and C25. An alternative is to use the

RD and WE pins of the C5x. Using these outputs, WE oper-

ates as the

WR signal while RD functions as the RD signal.

Also, additional glue logic is not required.