AD7713 datasheet(21/29 Pages) AD

21 / 29 page

REV. D

–20–

AD7713

signal must remain valid for the duration of the serial read opera-

tion. With A0 high, data is accessed from either the output

accessed from the control register.

The function of the

DRDY line is dependent on only the output

update rate of the device and the reading of the output data

DRDY goes low when a new data-word is available in

the output data register. It is reset high when the last bit of data

(either 16th bit or 24th bit) is read from the output register. If

data is not read from the output register, the

DRDY line will

remain low. The output register will continue to be updated at

the output update rate, but

DRDY will not indicate this. A read

from the device in this circumstance will access the most recent

word in the output register. If a new data-word becomes avail-

able to the output register while data is being read from the

output register,

DRDY will not indicate this, and the new data-

word will be lost to the user.

DRDY is not affected by reading

from the control register or the calibration register.

Data can be accessed from the output data register only when

DRDY is low. If RFS goes low while DRDY is high, no data

transfer will take place.

DRDY does not have any effect on read-

ing data from the control register or from the calibration registers.

Figures 12a and 12b show timing diagrams for reading from the

AD7713 in the external clocking mode. Figure 12a shows a

situation where all the data is read from the AD7713 in one

read operation. Figure 12b shows a situation where the data is

read from the AD7713 over a number of read operations. Both

read operations show a read from the AD7713’s output data

is similar, but in these cases, the

DRDY line is not related to the

read function. Depending on the output update rate, it can go

low at any stage in the control/calibration register read cycle

without affecting the read and its status should be ignored. A

read operation from either the control or calibration registers

must always read 24 bits of data from the respective register.

Figure 12a shows a read operation from the AD7713 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 12b 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 12a, but Figure 12b 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 AD7713, 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

SDATA (O)

SCLK (I)

THREE-STATE

RFS (I)

A0 (I)

t22

MSB

LSB

DRDY (O)

t21

t20

t23

t26

t29

t28

t27

t25

t24

Figure 12a. 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)

t22

t20

t31

t26

t24

t25

t27

t30

t24

t25

Figure 12b. External Clocking Mode, Output Data Read (

RFS Returns High During Read Operation)

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