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AD7985

Rev. 0 | Page 22 of 28

CS MODE, 4-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7985 is connected

to an SPI-compatible digital host with an interrupt input and

when it is desired to keep CNV, which is used to sample the

analog input, independent of the signal used to select the data

reading. This independence is particularly important in applica-

tions where low jitter on CNV is desired. This mode is available

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

diagram is shown in Figure 32, and the corresponding timing is

given in Figure 33.

With SDI high, a rising edge on CNV initiates a conversion,

selects CS mode, and forces SDO to high impedance. In this

mode, CNV must be held high during the conversion phase and

the subsequent data readback. (If SDI and CNV are low, SDO is

driven low.) Prior to the minimum conversion time, SDI can be

used to select other SPI devices, such as analog multiplexers, but

SDI 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 accept-

able hold time. After the optional 17th SCK falling edge or when

SDI goes high (whichever occurs first), SDO returns to high

impedance.

AD7985

SDI

SDO

CNV

SCK

CONVERT

DATA IN

CLK

DIGITAL HOST

IRQ

VIO

47kΩ

CS1

TURBO

Figure 32. CS Mode, 4-Wire with Busy Indicator Connection Diagram

(I/O QUIET

TIME)

SDO

D15

D14

tDIS

tQUIET

SCK

123

tSCK

tSCKL

tSCKH

tHSDO

tDSDO

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

SDI

CNV

tSSDICNV

tHSDICNV

TURBO = 0

Figure 33. CS Mode, 4-Wire with Busy Indicator Serial Interface Timing

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