Datasheet

AD7986
Rev. B | Page 24 of 28
CHAIN MODE WITH BUSY INDICATOR
This mode can also be used to daisy-chain multiple AD7986
devices on a 3-wire serial interface while providing a busy
indicator. This feature is useful for reducing component count
and wiring connections, for example, in isolated multiconverter
applications or for systems with a limited interfacing capacity.
Data readback is analogous to clocking a shift register. A
connection diagram example using three AD7986 devices is
shown in Figure 35, and the corresponding timing is given in
Figure 36.
When SDI and CNV are low, SDO is driven low. With SCK
high, a rising edge on CNV initiates a conversion, selects the
chain mode, and enables the busy indicator feature. In this
mode, CNV is held high during the conversion phase and the
subsequent data readback. When all ADCs in the chain have
completed their conversions, the SDO pin of the ADC closest to
the digital host (see the AD7986 ADC labeled C in Figure 35) is
driven high. This transition on SDO can be used as a busy indicator
to trigger the data readback controlled by the digital host. The
AD7986 then enters the acquisition phase and powers down.
The data bits stored in the internal shift register are clocked out,
MSB first, by subsequent SCK falling edges. For each ADC, SDI
feeds the input of the internal shift register and is clocked by the
SCK falling edge. Each ADC in the chain outputs its data MSB
first, and 18 × N + 1 clocks are required to read back the N ADCs.
Although the rising edge can be used to capture the data, a digital
host using the SCK falling edge allows a faster reading rate and
consequently more AD7986 devices in the chain, provided that
the digital host has an acceptable hold time.
CONVERT
DATA IN
CLK
DIGITAL HOST
AD7986
SDI SDO
CNV
C
SCK
AD7986
SDI SDO
CNV
A
SCK
IRQ
AD7986
SDI SDO
CNV
B
SCK
TURBOTURBOTURBO
07956-028
Figure 35. Chain Mode with Busy Indicator Connection Diagram
SDO
A
= SDI
B
D
A
17 D
A
16 D
A
15
SCK
1 2 3 39 53
54
t
EN
CONVERSION
ACQUISITION
t
CONV
t
CYC
t
ACQ
ACQUISITION
CNV = SDI
A
D
A
1
4 17
t
SCK
t
SCKH
t
SCKL
D
A
0
19 3818
SDO
B
= SDI
C
D
B
17 D
B
16 D
B
15 D
A
1D
B
1 D
B
0 D
A
17 D
A
16
55
t
SSDISCK
t
HSDISCK
t
HSDO
t
DSDO
SDO
C
D
C
17 D
C
16 D
C
15 D
A
1 D
A
0D
C
1 D
C
0 D
A
16
21 35 3620
37
D
B
1 D
B
0 D
A
17D
B
17 D
B
16
t
DSDOSDI
t
HSCKCNV
D
A
0
t
DSDOSDI
t
DSDOSDI
t
DSDOSDI
t
DSDOSDI
TURBO = 0
t
QUIET
07956-029
Figure 36. Chain Mode with Busy Indicator Serial Interface Timing