Datasheet

AD7986
Rev. B | Page 22 of 28
CS
MODE, 4-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7986 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
applications where low jitter on CNV is desired. This mode is
only available in normal conversion mode (TURBO = low).
The connection diagram is shown in Figure 31, and the
corresponding timing is given in Figure 32.
With SDI high, a rising edge on CNV initiates a conversion,
selects the
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 impedance 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 AD7986
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 19
th
SCK falling
edge or SDI going high (whichever occurs first), SDO returns to
high impedance.
AD7986
SDI SDO
CNV
SCK
CONVERT
DATA IN
CLK
DIGITAL HOST
IRQ
VIO
47kΩ
CS1
TURBO
07956-024
Figure 31.
CS
Mode, 4-Wire with Busy Indicator Connection Diagram
(I/O QUIET
TIME)
SDO
D17 D16 D1 D0
t
DIS
t
QUIET
SCK
1 2 3 17 18 19
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
07956-025
Figure 32.
CS
Mode, 4-Wire with Busy Indicator Serial Interface Timing