Datasheet

AD7944
Rev. A | Page 24 of 28
CHAIN MODE WITH BUSY INDICATOR
This mode can be used to daisy-chain multiple AD7944 devices
on a 3-wire serial interface while providing a busy indicator. It
is available only in normal conversion mode (TURBO low).
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 AD7944 devices is shown in Figure 36, and
the corresponding timing is given in Figure 37.
When SDI and CNV are low, SDO is driven low. With SCK high,
a rising edge on CNV initiates a conversion, selects chain mode,
and enables the busy indicator. 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 AD7944
ADC labeled C in Figure 36) is driven high. This transition on
SDO can be used as a busy indicator to trigger the data read-
back controlled by the digital host. The AD7944 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 14 × 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
AD7944 devices in the chain, provided that the digital host has
an acceptable hold time.
CONVERT
DATA IN
CLK
DIGITAL HOST
AD7944
SDI SDO
CNV
C
SCK
AD7944
SDI SDO
CNV
A
SCK
IRQ
AD7944
SDI SDO
CNV
B
SCK
TURBOTURBOTURBO
04658-019
Figure 36. Chain Mode with Busy Indicator Connection Diagram
SDO
A
= SDI
B
D
A
13 D
A
12 D
A
11
SCK
1 2 3 31 41 42
t
EN
CONVERSION
ACQUISITION
t
CONV
t
CYC
t
ACQ
ACQUISITION
CNV = SDI
A
D
A
1
4 13
t
SCK
t
SCKH
t
SCKL
D
A
0
15 3014
SDO
B
= SDI
C
D
B
13 D
B
12 D
B
11 D
A
1D
B
1 D
B
0 D
A
13 D
A
12
43
t
SSDISCK
t
HSDISCK
t
HSDO
t
DSDO
SDO
C
D
C
13 D
C
12 D
C
11 D
A
1 D
A
0D
C
1 D
C
0 D
A
12
17 27 2816
29
D
B
1 D
B
0 D
A
13D
B
13 D
B
12
t
DSDOSDI
t
HSCKCNV
D
A
0
t
DSDOSDI
t
DSDOSDI
t
DSDOSDI
t
DSDOSDI
TURBO = 0
t
SSCKCNV
04658-020
Figure 37. Chain Mode with Busy Indicator Serial Interface Timing