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APPLICATION NOTE
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Configuring the AD7656/AD7657/AD7658 for
Serial and Daisy-Chain Interface Modes of Operation
by Claire Croke
INTRODUCTION
The AD7656/AD7657/AD7658 can be configured to operate in
three interface modes: parallel interface mode, serial interface
mode, or daisy-chain interface mode. The parallel mode of
operation is covered extensively in the AD7656/AD7657/AD7658
data sheet. In addition to the information provided in the data
sheet, this application note further describes how to configure
the AD7656 for the serial and daisy-chain interface modes.
FUNCTIONAL BLOCK DIAGRAM
CONVST A
VDD
CLK
OSC
REF
CONVST B CONVST C AVCC
DVCC
If only one or two DOUT lines are used for the serial interface, the
unused DOUT lines should be left unconnected. These unused
DOUT lines should have their respective SEL pin tied to logic low.
CS
SER/PAR
VDRIVE
STBY
CONTROL
LOGIC
interface, DOUT A should be used. To enable DOUT A, the SEL A
pin should be tied to logic high, and the SEL B and SEL C pins
should be tied to logic low. To configure the serial interface to
have two data output lines, DOUT A and DOUT B should be
used. To enable DOUT A and DOUT B, the SEL A and SEL B
pins should be tied to logic high, and SEL C should be tied to a
logic low. To configure the serial interface to operate with all three
data output lines, DOUT A, DOUT B, and DOUT C should be
enabled. To enable all three DOUT lines, the SEL A, SEL B, and
SEL C lines should be to tied to logic high (see Figure 2, Figure 3,
and Figure 4).
BUF
T/H
16-/14-/12-BIT SAR
V2
T/H
16-/14-/12-BIT SAR
OUTPUT
DRIVERS
VDRIVE
DOUT A
VDRIVE
SCLK
OUTPUT
DRIVERS
BUF
V3
V4
16-/14-/12-BIT SAR
T/H
T/H
OUTPUT
DRIVERS
16-/14-/12-BIT SAR
OUTPUT
DRIVERS
BUF
V5
T/H
16-/14-/12-BIT SAR
V6
T/H
16-/14-/12-BIT SAR
SER/PAR
HS
SEL A DOUT A
DOUT B
AD7656
SEL B
DOUT B
NC
SEL C
DOUT C
NC
DOUT C
DATA/
CONTROL
LINES
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V1
NC = NO CONNECT
RD
Figure 2. AD7656 with One DOUT Line
AD7656/AD7657/AD7658
VSS
AGND
DGND
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WR
VDRIVE
VDRIVE
SER/PAR
HS
SEL A DOUT A
Figure 1.
AD7656
When operating in serial interface mode, the AD7656 must be
configured to select the number of serial DOUT lines required
by the application. SEL A, SEL B, and SEL C are used to enable
data output lines DOUT A, DOUT B, and DOUT C,
respectively. If only one DOUT line is required for the serial
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DOUT B
SEL C
DOUT C
NC = NO CONNECT
NC
Figure 3. AD7656 with Two DOUT Lines
VDRIVE
VDRIVE
SER/PAR
HS
SEL A DOUT A
AD7656
SEL B
DOUT B
SEL C
DOUT C
NC = NO CONNECT
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The SER/PAR/SEL pin on the AD7656 is used to select between
the parallel interface mode and the serial interface mode. To select
the serial interface mode, this pin must be tied to logic high. When
in serial interface mode, the AD7656 should be configured for
the hardware select mode of operation and H/S SEL should be tied
to a logic low. In serial interface mode, the AD7656 cannot be
configured for the software mode of operation.
SEL B
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CONFIGURING THE AD7656, AD7657, AND
AD7658 FOR SERIAL INTERFACE MODE
Figure 4. AD7656 with Three DOUT Lines
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TABLE OF CONTENTS
Introduction ...................................................................................... 1
Configuring the Hardware Programmable Pins ...........................3
Functional Block Diagram .............................................................. 1
Reading Data from the AD7656......................................................3
Configuring the AD7656, AD7657, and AD7658 for Serial
Interface Mode .................................................................................. 1
Daisy-Chain Mode ............................................................................5
Revision History ............................................................................... 2
REVISION HISTORY
3/07—Revision 0: Initial Version
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CONFIGURING THE HARDWARE
PROGRAMMABLE PINS
When operating the AD7656 in serial mode and hardware select
mode, several pins must be configured depending on the required
mode of operation for the AD7656. These pins include, REFEN/DIS,
DCIN C, DCIN B, DCIN A, DCEN, DB11, DB12, DB13, DB15,
REFBUFEN/DIS, RESET, RANGE, STBY, and W/B.
For serial interface mode, the W/B, RD, DB11, DB12, DB13, and
DB15 pins should be tied to logic low.
Similar to operating the AD7656 in parallel interface mode, the
REFEN/DIS, REFBUFEN/DIS, RESET, RANGE, and STBY pins
should be configured as required by the application.
When operating the AD7656 in serial interface mode but not in
daisy-chain mode, the DCEN, DCIN A, DCIN B, and DCIN C
pins should be tied to DGND.
READING DATA FROM THE AD7656
When operating the AD7656 in serial interface mode, the
conversion control is the same as that outlined in the AD7656
data sheet for parallel interface mode. The CONVST pins are
used to initiate conversions on the AD7656 channel pairs.
When the BUSY pin returns low to indicate that the conversion
is complete, the user can read the data from the AD7656.
In serial interface mode, the CS, SCLK, and DOUT signals are used
to access the conversion data from the AD7656. Figure 5 shows the
typical read sequence using all three data output DOUT lines.
When using three DOUT lines to read the conversion data of
the six channels, the AD7656 channel data is presented on the
DOUT lines as described in Table 1.
Table 1.
DOUT LINE
Channel Data
DOUT A
V1, V2
DOUT B
V3, V4
DOUT C
V5, V6
When using two DOUT lines to read the conversion data of the
six channels, the conversion data is presented on the DOUT
lines as described in Table 2.
Table 2.
DOUT LINE
Channel Data
DOUT A
V1, V2, V5
DOUT B
V3, V4, V6
When converting on four channels (V1, V2, V3, and V4) of
the AD7656, the conversion data can be read using one or two
DOUT lines. In the case of one DOUT line (DOUT A), the data
is output in ascending order. In the case of using two DOUT lines
(DOUT A and DOUT B), the data is output as described in Table 3.
Table 3.
DOUT LINE
Channel Data
DOUT A
V1, V2
DOUT B
V3, V4
The timing for the CONVST and BUSY signals in serial
interface mode is the same as outlined in AD7656 data sheet for
the parallel interface mode. As mentioned previously, the RD
signal should be tied to logic low for serial interface mode.
When the BUSY signal returns low to indicate the end of the
conversion, the CS signal can be brought low immediately, as
indicated by t2 in the timing specifications of the AD7656 data
sheet. The falling edge of CS is used to clock out the MSB of the
first conversion result and to take the bus out of three-state (see
Figure 5). The access time t17 indicates how long after the CS edge
the MSB will become valid. Subsequent data bits are clocked out
of the AD7656 on the SCLK rising edge. Again, t17 is the time
required after the SCLK rising edge for the data to become valid.
Data should be clocked into the processor on the SCLK falling
edge. If data must be clocked into the processor on the SCLK
rising edge, ensure that the MSB is clocked in after the CS signal
is brought low. Sixteen SCLKs are required to read the complete
conversion result from each channel on the AD7656.
If all six channels are performing conversions, the minimum
SCLK pulses required to read the six channels depends on the
number of DOUT lines used: 32 SCLKs are required for three
DOUT lines, 48 SCLKs for two DOUT lines, and 96 SCLKs for
one DOUT line.
When the CS signal is brought low to clock out the MSB of the
first conversion, it can remain low for the remainder of the read
sequence. However, if the user wishes, the CS signal can be
pulsed between the individual channel reads (see Figure 6).
At the end of the serial read, the CS signal should return high,
and the user should ensure that the tQUIET time elapses before
initiating the next conversion.
When using one DOUT line to read the conversion data of the
six channels, the conversion data is presented on DOUT A in
ascending order.
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CONVST A,
CONVST B,
CONVST C
BUSY
t10
t1
tCONV
tACQ
t2
t17
tQUIET
CS
DOUT A,
DOUT B,
DOUT C
t18
t17
t16
DB15
t20
DB14
DB13
t21
DB1
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t19
SCLK
DB0
Figure 5. Typical Serial Read Timing Using Three DOUT Lines
CONVST A,
CONVST B,
CONVST C
tACQ
tCONV
BUSY
CS
32
16
SCLK
V1
V2
DOUT B
V3
V4
V5
V6
DOUT C
Figure 6. AD7656 Serial Read Timing Diagram
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tQUIET
DOUT A
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DAISY-CHAIN MODE
CONVERT
The DCIN pins should be connected to the corresponding
DOUT pins of the AD7656 device upstream in the daisy chain.
Figure 7 to Figure 9 show the various daisy-chain configurations.
The device labeled Device 2, or the device furthest up-stream in
the chain, should not be configured for daisy-chain operation
(DCEN should be set to logic low). Device 1 should be
configured for daisy-chain operation (DCEN should be set to
logic high). When the device is configured for daisy-chain
mode, Pins 12, 13, and 14 are configured as DCIN pins. As with
the serial interface mode, SEL A, SEL B, and SEL C must be
configured to select the number of DOUT interface lines
required.
AD7656
SCLK
CS
CONVST x
DCIN A DOUT A
AD7656
SCLK
CS
DATA IN1
DIGITAL
HOST
CS
SCLK
DCEN = 0
DEVICE NO. 2
DCEN = 1
DEVICE NO. 1
Figure 7. AD7656 Daisy-Chain mode with One DOUT Line
CONVERT
CONVST x
DOUT A
DOUT B
AD7656
SCLK
CS
CONVST x
DCIN A DOUT A
DCIN B DOUT B
AD7656
SCLK
CS
DATA IN1
DATA IN2
DIGITAL
HOST
CS
SCLK
DCEN = 0
DEVICE NO. 2
DCEN = 1
DEVICE NO. 1
Figure 8. AD7656 Daisy-Chain Mode with Two DOUT Lines
CONVERT
CONVST x
DOUT A
DOUT B
DOUT C
AD7656
SCLK
CS
CONVST x
DCIN A DOUT A
DCIN B DOUT B
DCIN C DOUT C
AD7656
SCLK
CS
DATA IN1
DATA IN2
DATA IN3
DIGITAL
HOST
CS
SCLK
DCEN = 0
DEVICE NO. 2
DCEN = 1
DEVICE NO. 1
Figure 9. AD7656 Daisy-Chain Mode with Three DOUT Lines
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06509-009
DOUT A
06509-008
When the devices are configured to operate in daisy-chain
mode (DCEN is logic high), the user can select the number of
DOUT lines by using the SEL A, SEL B, and SEL C pins. These
SEL pins also determine the number of DCIN pins for each
AD7656 device.
CONVST x
06509-007
With serial interface mode, the user has the option of using the
DCEN pin to configure the devices in daisy-chain mode and
cascade multiple devices in a daisy-chain configuration. Daisychain mode has the advantage of using a single serial interface
to control multiple devices. As with the serial interface mode,
the daisy-chain mode allows the user to select the number of
DOUT lines used.
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(Channels 3, 4, and 6) into Device 1. This data is clocked into
DCIN A and DCIN B, respectively, of Device 1. During the
second 48 SCLKs of the serial transfer, Device 1 clocks the data
that was previously clocked in by Device 2 into the digital host, and
Device 2 clocks out 0s into Device 1.
Figure 10 shows the serial read operation with the devices
configured as shown in Figure 8. A single CONVST signal is
sent to both AD7656 devices in the chain. When the BUSY
signal returns low to indicate that the conversion is complete,
the CS signal can be brought low to start the serial transfer.
The CS signal can remain low for the full serial transfer, or it can
be pulsed after each channel read (every 16 SCLKs). During
the first 48 SCLKs, Device 1 transfers its conversion data to
the digital host through DOUT A (Channels 1, 2, and 5) and
DOUT B (Channels 3, 4, and 6), and Device 2 clocks its conversion data out on DOUT A (Channels 1, 2, and 5) and DOUT B
Similar principles apply when configuring the AD7656 in daisychain mode using three or one DOUT lines.
The number of SCLK pulses required for a serial transfer in
daisy-chain mode depends on the number of devices in the
chain and the number of DOUT lines used in the interface.
CONVST A,
CONVST B,
CONVST C
BUSY
CS
1
2
3
15
16
17
31
32
33
47
48
49
63
64
65
94
95
96
DEVICE 1, DOUT A
MSB V1
LSB V1 MSB V2
LSB V2 MSB V5
LSB V5 MSB V1
LSB V1 MSB V2
LSB V5
DEVICE 1, DOUT B
MSB V3
LSB V3 MSB V4
LSB V4 MSB V6
LSB V6 MSB V3
LSB V3 MSB V4
LSB V6
DEVICE 2, DOUT A
MSB V1
LSB V1 MSB V2
LSB V2 MSB V5
LSB V5
DEVICE 2, DOUT B
MSB V3
LSB V3 MSB V4
LSB V4 MSB V6
LSB V6
Figure 10. Daisy-Chain Serial Read Operation
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SCLK
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NOTES
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NOTES
©2007 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN06509-0-3/07(0)
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