AD7985 datasheet(20/28 Pages) AD

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AD7985

Rev. 0 | Page 20 of 28

CS MODE, 3-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7985 is connected

to an SPI-compatible digital host that has an interrupt input. It

is available only in normal conversion mode (TURBO is low).

The connection diagram is shown in Figure 28, and the corre-

sponding timing is given in Figure 29.

With SDI tied to VIO, a rising edge on CNV initiates a con-

version, selects CS mode, and forces SDO to high impedance.

SDO is maintained in high impedance until the completion of

the conversion, irrespective of the state of CNV. Prior to the

minimum conversion time, CNV can be used to select other SPI

devices, such as analog multiplexers, but CNV must be returned

low before the minimum conversion time elapses and then held

low for the maximum possible conversion time to guarantee the

generation of the busy signal indicator.

When the conversion is complete, SDO goes from high imped-

ance to low impedance. With a pull-up on the SDO line, this

transition can be used as an interrupt signal to initiate the data

readback controlled by the digital host. The AD7985 then enters

the acquisition phase and powers down. The data bits are then

clocked out, MSB first, by subsequent SCK falling edges. The

data is valid on both SCK edges. Although the rising edge can

be used to capture the data, a digital host using the SCK falling

edge allows a faster reading rate, provided that it has an acceptable

hold time. After the optional 17th SCK falling edge, SDO returns

to high impedance.

If multiple AD7985 devices are selected at the same time, the

SDO output pin handles this contention without damage or

induced latch-up. Meanwhile, it is recommended that this

contention be kept as short as possible to limit extra power

dissipation.

AD7985

SDI

SDO

CNV

SCK

CONVERT

DATA IN

CLK

DIGITAL HOST

VIO

IRQ

VIO

47kΩ

TURBO

Figure 28. CS Mode, 3-Wire with Busy Indicator Connection Diagram (SDI High)

SDO

D15

D14

tDIS

SCK

tSCK

tSCKL

tSCKH

tHSDO

tDSDO

CNV

CONVERSION

ACQUISITION

tCONV

tCYC

ACQUISITION

TURBO = 0

SDI = 1

tCNVH

tACQ

tQUIET

(I/O QUIET

TIME)

Figure 29. CS Mode, 3-Wire with Busy Indicator Serial Interface Timing (SDI High)

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