AD7984

Rev. 0 | Page 18 of 24

CS MODE, 3-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7984 is connected

to an SPI-compatible digital host having an interrupt input.

The connection diagram is shown in Figure 30, and the

corresponding timing is given in Figure 31.

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

conversion, selects the 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

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

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

data reading controlled by the digital host. The AD7984 then

enters the acquisition phase and goes into standby mode. 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 it has

or when CNV goes high (whichever occurs first), SDO returns

to high impedance.

If multiple AD7984s are selected at the same time, the SDO

output pin handles this contention without damage or induced

latch-up. Meanwhile, it is recommended to keep this contention

as short as possible to limit extra power dissipation.

AD7984

SDI

SDO

CNV

SCK

CONVERT

DATA IN

CLK

DIGITAL HOST

VIO

IRQ

VIO

47kΩ

Figure 30. CS Mode, 3-Wire with Busy Indicator Connection Diagram (SDI High)

SDO

D17

D16

D1

D0

SCK

12

3

17

18

19

CNV

CONVERSION

ACQUISITION

ACQUISITION

SDI = 1

Figure 31. CS Mode, 3-Wire with Busy Indicator Serial Interface Timing (SDI High)