AD7927 datasheet(15/29 Pages) AD

15 / 29 page

AD7927

Data Sheet

Rev. D | Page 14 of 28

SHADOW REGISTER

Table 10. Shadow Register Bit Functions

MSB

LSB

VIN0

VIN1

VIN2

VIN3

VIN4

VIN5

VIN6

VIN7

VIN0

VIN1

VIN2

VIN3

VIN4

VIN5

VIN6

VIN7

--------------------------------SEQUENCE ONE--------------------------------

--------------------------------SEQUENCE TWO--------------------------------

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

which the AD7927 cycles with each consecutive conversion

after the write to the shadow register. Sequence One is per-

formed first and then Sequence Two. If the user does not

wish to perform 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 continuously cycles through

the selected channels in ascending order, beginning with the

lowest channel, until a write operation occurs (that is, the WRITE

bit is set to 1) with the SEQ and SHADOW bits configured in

any way except 1, 0 (see Table 9). The bit functions are outlined

in Table 10.

DOUT: CONVERSION RESULT FROM

PREVIOUSLY SELECTED CHANNEL

A2 TO CHANNEL A0.

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT CHANNEL A2 TO CHANNEL A0

FOR CONVERSION.

SEQ = SHADOW = 0

WRITE BIT = 1,

SEQ = SHADOW = 0

DUMMY CONVERSION

DIN = ALL 1s

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT CHANNEL A2 TO CHANNEL A0

FOR CONVERSION.

SEQ = SHADOW = 0

POWER-ON

Figure 11. SEQ Bit = 0, SHADOW Bit = 0 Flowchart

Figure 11 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 12 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 opera-

tion of a multichannel ADC (as outlined in Figure 11), ensure

that the WRITE bit = 1 and the SEQ = SHADOW = 0 on the

next serial transfer. Figure 13 shows how a sequence of consecu-

tive channels 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 11), ensure the WRITE bit = 1 and the

SEQ = SHADOW = 0 on the next serial transfer.

WRITE BIT = 0

WRITE BIT = 1,

SEQ = 1 SHADOW = 0

WRITE BIT = 0

CONTINUOUSLY

CONVERTS ON THE

SELECTED

SEQUENCE OF

CHANNELS BUT

ALLOWS RANGE,

CODING, AND SO ON,

TO CHANGE IN THE

CONTROL REGISTER

WITHOUT INTERRUPT-

ING THE SEQUENCE,

PROVIDED SEQ = 1,

SHADOW = 0

WRITE BIT = 1,

SEQ = 1,

SHADOW = 0

CONTINUOUSLY

CONVERTS ON THE

SELECTED

SEQUENCE OF

CHANNELS

DOUT: CONVERSION RESULT FROM PREVIOUSLY

SELECTED CHANNEL A2 TO CHANNEL A0.

DIN: WRITE TO SHADOW REGISTER, SELECTING

WHICH CHANNELS TO CONVERT ON; CHANNELS

SELECTED NEED NOT BE CONSECUTIVE CHANNELS

DIN: WRITE TO CONTROL REGISTER,

WRITE BIT = 1,

SELECT CODING, RANGE, AND POWER MODE.

SELECT CHANNEL A2 TO CHANNEL A0

FOR CONVERSION.

SEQ = 0, SHADOW = 1

DUMMY CONVERSION

DIN = ALL 1s

POWER-ON

Figure 12. SEQ and SHADOW Conversion Flowchart to Continuously Convert

a Sequence of Channels

AD7927 датащи(PDF) 15 Page - Analog Devices

AD7927 датащи(HTML) 15 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

датащи скачать

ссылки URL