AD7712_17 datasheet(23/29 Pages) AD

23 / 29 page

REV. F

–22–

AD7712

Figure 13a shows a read operation from the AD7712 where

RFS remains low for the duration of the data word transmis-

sion. With

DRDY low, the RFS input is brought low. The input

SCLK signal should be low between read and write operations.

RFS going low places the MSB of the word to be read on the

serial data line. All subsequent data bits are clocked out on a

high to low transition of the serial clock and are valid prior to

the following rising edge of this clock. The penultimate falling

edge of SCLK clocks out the LSB and the final falling edge

resets the

DRDY line high. This rising edge of DRDY turns off

the serial data output.

Figure 13b shows a timing diagram for a read operation where

RFS returns high during the transmission of the word and

returns low again to access the rest of the data-word. Timing

parameters and functions are very similar to that outlined for

Figure 13a, but Figure 13b has a number of additional times to

show timing relationships when

RFS returns high in the middle

of transferring a word.

RFS should return high during a low time of SCLK. On the

rising edge of

RFS, the SDATA output is turned off. DRDY

remains low and will remain low until all bits of the data-word

are read from the AD7712, regardless of the number of times

RFS changes state during the read operation. Depending on the

time between the falling edge of SCLK and the rising edge of

RFS, the next bit (BIT N + 1) may appear on the data bus

before

RFS goes high. When RFS returns low again, it activates

the SDATA output. When the entire word is transmitted, the

DRDY line will go high, turning off the SDATA output as per

Figure 13a.

RFS (I)

SCLK (I)

SDATA (O)

LSB

MSB

THREE-STATE

A0 (I)

DRDY (O)

t20

t21

t22

t23

t24

t25

t26

t27

t28

t29

Figure 13a. External Clocking Mode, Output Data Read Operation

THREE-STATE

MSB

BIT N

BIT N+1

SDATA (O)

SCLK (I)

RFS (I)

A0 (I)

DRDY (O)

t20

t22

t26

t24

t25

t27

t31

t24

t25

t30

Figure 13b. External Clocking Mode, Output Data Read Operation (

RFS Returns High during Read Operation)

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