Data Sheet

AD7980

Rev. F | Page 19 of 26

3-WIRE CS MODE WITH BUSY INDICATOR

This mode is usually used when a single AD7980 is connected

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

The connection diagram is shown in Figure 33, and the

corresponding timing is given in Figure 34.

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 conversion time to

guarantee the generation of the busy signal indicator. When the

conversion is complete, SDO goes from high impedance to low.

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 AD7980 then enters the acquisition phase and

powers down. The data bits are 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 an acceptable hold time. After the optional

17th SCK falling edge or when CNV goes high, whichever is

earlier, SDO returns to high impedance.

If multiple AD7980 devices 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.

Figure 33. 3-Wire CS Mode with Busy Indicator

Connection Diagram (SDI High)

Figure 34. 3-Wire CS Mode with Busy Indicator Serial Interface Timing (SDI High)

SDI

DATA IN

IRQ

DIGITAL HOST

CONVERT

CLK

VIO

VIO

47kΩ

CNV

SCK

AQUISITION

AQUISITION

CONVERSION

SCK

CNV

SDI = 1

SDO

D15

D14

D1

D0

1

2

3

15

16

17