REV. 0

AD7927

–11–

POWER-ON

DUMMY CONVERSION

DIN = ALL 1s

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT CHANNEL A2–A0 FOR CONVERSION.

SEQ = SHADOW = 0

DOUT: CONVERSION RESULT FROM PREVIOUSLY

SELECTED CHANNEL A2–A0.

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT A2–A0 FOR CONVERSION.

SEQ = SHADOW = 0

WRITE BIT = 1,

SEQ = SHADOW = 0

CS

CS

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.

CS

POWER-ON

DUMMY CONVERSION

DIN = ALL 1s

DIN: WRITE TO CONTROL REGISTER,

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.

DIN: WRITE TO SHADOW REGISTER, SELECTING

WHICH CHANNELS TO CONVERT ON; CHANNELS

SELECTED NEED NOT BE CONSECUTIVE CHANNELS

WRITE BIT = 1,

SEQ = 1,

SHADOW = 0

CS

CS

WRITE BIT = 0

WRITE BIT = 1

SEQ = 1 SHADOW = 0

CONTINUOUSLY

CONVERTS ON THE

SELECTED

SEQUENCE OF

CHANNELS

CONTINUOUSLY

CONVERTS ON THE

SELECTED

SEQUENCE OF

CHANNELS BUT

WILL ALLOW RANGE,

CODING, AND SO ON,

TO CHANGE IN THE

CONTROL REGISTER

WITHOUT INTERRUPT-

ING THE SEQUENCE,

PROVIDED SEQ = 1

SHADOW = 0

WRITE BIT = 0

WRITE BIT = 0

Figure 3. SEQ Bit = 0, SHADOW Bit = 1 Flowchart

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.

Table V. Shadow Register Bit Functions

MSB

LSB

------------------SEQUENCE ONE-------------------------------------------------------SEQUENCE TWO-----------------------