Datasheet

AD7984
Rev. A | Page 18 of 24
CS MODE, 3-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7984 is connected
to an SPI-compatible digital host having an interrupt input.
The connection diagram is shown in Figure 30, and the
corresponding timing is given in Figure 31.
With SDI tied to VIO, a rising edge on CNV initiates a
conversion, selects the
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
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 reading controlled by the digital host. The AD7984 then
enters the acquisition phase and goes into standby mode. 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 it has
an acceptable hold time. After the optional 19
th
SCK falling edge
or when CNV goes high (whichever occurs first), SDO returns
to high impedance.
If multiple AD7984s are selected at the same time, the SDO
output pin handles this contention without damage or induced
latch-up. Meanwhile, it is recommended to keep this contention
as short as possible to limit extra power dissipation.
AD7984
SDI SDO
CNV
SCK
CONVERT
DATA IN
CLK
DIGITAL HOST
VIO
IRQ
VIO
47kΩ
06973-020
Figure 30.
CS
Mode, 3-Wire with Busy Indicator Connection Diagram (SDI High)
SDO
D17 D16 D1 D0
t
DIS
SCK
123 171819
t
SCK
t
SCKL
t
SCKH
t
HSDO
t
DSDO
CNV
CONVERSIONACQUISITION
t
CONV
t
CYC
ACQUISITION
SDI = 1
t
CNVH
t
ACQ
06973-021
Figure 31.
CS
Mode, 3-Wire with Busy Indicator Serial Interface Timing (SDI High)