Data Sheet

AD7902

Rev. B | Page 19 of 28

CS Mode, 3-Wire Interface with Busy Indicator

CS mode, using a 3-wire interface with a busy indicator, is usually

used when a single AD7902 is connected to an SPI-compatible

digital host having an interrupt input.

The connection diagram is shown in Figure 40, and the

corresponding timing is shown in Figure 41.

With SDIx tied to VIOx, a rising edge on CNVx initiates

a conversion, selects CS mode, and forces SDOx to high

impedance. SDOx is maintained in high impedance until the

completion of the conversion, regardless of the state of CNVx.

Prior to the minimum conversion time, CNVx can be used to

select other SPI devices, such as analog multiplexers, but CNVx

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, SDOx goes from high

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

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

data reading controlled by the digital host. The AD7902 then

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

then clocked out, MSB first, by subsequent SCKx falling edges.

The data is valid on both SCKx edges. Although the rising edge

can be used to capture the data, a digital host using the SCKx

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

or when CNVx goes high (whichever occurs first), SDOx

returns to high impedance.

If multiple ADCs are selected at the same time, the SDOx 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.

Figure 40. CS Mode, 3-Wire Interface with a Busy Indicator Connection Diagram (SDIx High)

Figure 41. CS Mode, 3-Wire Interface with a Busy Indicator Serial Interface Timing (SDIx High)

AD7902

SDIx

SDOx

CNVx

SCKx

CONVERT

DATA IN

CLK

DIGITAL HOST

VIOx

IRQ

VIOx

47kΩ

ACQUISITION

ACQUISITION

CONVERSION

SCKx

CNVx

SDIx = 1

SDOx

D15

D14

D1

D0

1

2

3

15

16

17