Datasheet

AD7985
Rev. A | 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 17
th
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
07947-015
Figure 32.
CS
Mode, 4-Wire with Busy Indicator Connection Diagram
(I/O QUIET
TIME)
SDO
D15 D14 D1 D0
t
DIS
t
QUIET
SCK
1 2 3 15 16 17
t
SCK
t
SCKL
t
SCKH
t
HSDO
t
DSDO
t
EN
CONVERSIONACQUISITION
t
CONV
t
CYC
t
ACQ
ACQUISITION
SDI
CNV
t
SSDICNV
t
HSDICNV
TURBO = 0
07947-016
Figure 33.
CS
Mode, 4-Wire with Busy Indicator Serial Interface Timing