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AD7944

Data Sheet

Rev. C | Page 20 of 28

CS MODE, 3-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7944 is connected

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

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

connection diagram is shown in Figure 28, and the corresponding

timing is given in Figure 29.

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

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 AD7944 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 15th SCK falling edge, SDO returns

to high impedance.

If multiple AD7944 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.

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

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

AD7944

SDI

SDO

CNV

SCK

CONVERT

DATA IN

CLK

DIGITAL HOST

VIO

IRQ

VIO

47kΩ

TURBO

SDO

D13

D12

tDIS

SCK

tSCK

tSCKL

tSCKH

tHSDO

tDSDO

CNV

CONVERSION

ACQUISITION

tCONV

tCYC

ACQUISITION

TURBO = 0

SDI = 1

tCNVH

tACQ

tQUIET

(I/O QUIET

TIME)

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