AD7927 查看數據表（PDF） - Unspecified
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AD7927 Datasheet PDF : 20 Pages
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AD7927
SHADOW REGISTER
The Shadow Register on the AD7927 is a 16-bit, write-only
register. Data is loaded from the DIN pin of the AD7927 on the
falling edge of SCLK. The data is transferred on the DIN line at
the same time that a conversion result is read from the part. This
requires 16 serial clock falling edges for the data transfer. The
information is clocked into the Shadow Register, provided that the
SEQ and SHADOW bits were set to 0,1, respectively, in the
previous write to the Control Register. MSB denotes the first bit
in the data stream. Each bit represents an analog input from
Channel 0 to Channel 7. Through programming the Shadow
Register, two sequences of channels may be selected, through
which the AD7927 will cycle with each consecutive conversion
after the write to the Shadow Register. Sequence One will be
performed first and then Sequence Two. If the user does not
wish to preform a second sequence option, then all 0s must be
written to the last eight LSBs of the Shadow Register. To select
a sequence of channels, the associated channel bit must be set for
each analog input. The AD7927 will continuously cycle through
the selected channels in ascending order, beginning with the
lowest channel, until a write operation occurs (i.e., the WRITE bit
is set to 1) with the SEQ and SHADOW bits configured in any
way except 1,0. (See Table IV.) The bit functions are outlined
in Table V.
MSB
VIN0 VIN1 VIN2
VIN3
VIN4
Table V. Shadow Register Bit Functions
VIN5 VIN6 VIN7 VIN0 VIN1 VIN2
VIN3
VIN4
VIN5
LSB
VIN6 VIN7
------------------SEQUENCE ONE-------------------------------------------------------SEQUENCE TWO-----------------------
POWER-ON
DUMMY CONVERSION
DIN = ALL 1s
POWER-ON
DUMMY CONVERSION
DIN = ALL 1s
DIN: WRITE TO CONTROL REGISTER,
CS
WRITE BIT = 1,
SELECT CODING, RANGE, AND POWER MODE.
SELECT CHANNEL A2–A0 FOR CONVERSION.
SEQ = SHADOW = 0
DIN: WRITE TO CONTROL REGISTER,
CS
WRITE BIT = 1,
SELECT CODING, RANGE, AND POWER MODE.
SELECT CHANNEL A2–A0 FOR CONVERSION.
SEQ = 0 SHADOW = 1
DOUT: CONVERSION RESULT FROM PREVIOUSLY
SELECTED CHANNEL A2–A0.
CS
DIN: WRITE TO CONTROL REGISTER,
WRITE BIT = 1,
SELECT CODING, RANGE, AND POWER MODE.
WRITE BIT = 1,
SEQ = SHADOW = 0
SELECT A2–A0 FOR CONVERSION.
SEQ = SHADOW = 0
Figure 2. SEQ Bit = 0, SHADOW Bit = 0 Flowchart
Figure 2 reflects the traditional operation of a multichannel ADC,
where each serial transfer selects the next channel for conversion.
In this mode of operation, the sequencer function is not used.
Figure 3 shows how to program the AD7927 to continuously
convert on a particular sequence of channels. To exit this mode
of operation and revert back to the traditional mode of operation
of a multichannel ADC (as outlined in Figure 2), ensure that the
WRITE bit = 1 and the SEQ = SHADOW = 0 on the next serial
transfer. Figure 4 shows how a sequence of consecutive chan-
nels can be converted on without having to program the Shadow
Register or write to the part on each serial transfer. Again, to exit
this mode of operation and revert back to the traditional mode
of operation of a multichannel ADC (as outlined in Figure 2),
ensure the WRITE bit = 1 and the SEQ = SHADOW = 0 on
the next serial transfer.
DOUT: CONVERSION RESULT FROM PREVIOUSLY
CS
SELECTED CHANNEL A2–A0.
DIN: WRITE TO SHADOW REGISTER, SELECTING
WHICH CHANNELS TO CONVERT ON; CHANNELS
SELECTED NEED NOT BE CONSECUTIVE CHANNELS
WRITE BIT = 0
WRITE BIT = 1
SEQ = 1 SHADOW = 0
CONTINUOUSLY
CONTINUOUSLY
CS
CONVERTS ON THE
SELECTED
CONVERTS ON THE
SELECTED
SEQUENCE OF
SEQUENCE OF
CHANNELS
CHANNELS BUT
WILL ALLOW RANGE,
CODING, AND SO ON,
TO CHANGE IN THE
WRITE BIT = 0
CONTROL REGISTER
WITHOUT INTERRUPT-
WRITE BIT = 0
ING THE SEQUENCE,
PROVIDED SEQ = 1
SHADOW = 0
WRITE BIT = 1,
SEQ = 1,
SHADOW = 0
Figure 3. SEQ Bit = 0, SHADOW Bit = 1 Flowchart
REV. 0
–11–
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