EVAL-AD7985FMCZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7985FMCZ' PDF : 28 Pages View PDF
AD7985
Data Sheet
CHAIN MODE WITH BUSY INDICATOR
This mode can daisy-chain multiple AD7985 devices on a 3-wire
serial interface while providing a busy indicator. It is available
only in normal conversion mode (TURBO is 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 AD7985 devices is shown in Figure 36, and the cor-
responding 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 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 AD7985
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 AD7985 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 the data MSB first, and 16 × N + 1 clocks are
required to read back the N ADCs. Although the rising edge can
capture the data, a digital host using the SCK falling edge allows
a faster reading rate and consequently more AD7985 devices in the
chain, provided that the digital host has an acceptable hold time.
CNV
SDI AD7985 SDO
A
SCK TURBO
CNV
SDI AD7985 SDO
B
SCK TURBO
CNV
SDI AD7985 SDO
C
SCK TURBO
Figure 36. Chain Mode with Busy Indicator Connection Diagram
CONVERT
DIGITAL HOST
DATA IN
IRQ
CLK
TURBO = 0
CNV = SDIA
tCONV
ACQUISITION CONVERSION
SCK
tHSCKCNV
SDOA = SDIB
tSSCKCNV
tEN
SDOB = SDIC
SDOC
tDSDOSDI
tDSDOSDI
tCYC
tACQ
ACQUISITION
tSCKH
tSCK
1
2
3
4
15 16 17 18 19
tSSDISCK
tHSDISCK
tSCKL
31 32 33 34 35
DA15 DA14 DA13
tHSDO
tDSDO
DA1 DA0
DB15 DB14 DB13
DB1 DB0 DA15 DA14
DA1 DA0
DC15 DC14 DC13
DC1 DC0 DB15 DB14
DB1 DB0 DA15 DA14
Figure 37. Chain Mode with Busy Indicator Serial Interface Timing
47 48 49
tDSDOSDI
tDSDOSDI
tDSDOSDI
DA1 DA0
Rev. C | Page 24 of 28
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