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REV. F
–22–
AD7711
Figures 12a and 12b show timing diagrams for reading from the
AD7711 in the external clocking mode. Figure 12a shows a
situation where all the data is read from the AD7711 in one read
operation. Figure 12b shows a situation where the data is read
from the AD7711 over a number of read operations. Both read
operations show a read from the AD7711’s output data register.
A read from the control register or calibration registers is similar
but in these cases the
DRDY
line is not related to the read func-
tion. Depending on the output update rate, it can go low at any
stage in the control/calibration register read cycle without affect-
ing 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 AD7711 where
RFS
remains low for the duration of the data word transmission.
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 AD7711, 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 databus 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 12a.
t
20
SDATA (O)
SCLK (I)
RFS
(I)
A0 (I)
DRDY
(O)
t
22
t
24
t
25
t
26
t
27
t
29
t
28
t
23
t
21
THREE-STATE
MSB
LSB
Figure 12a. External-Clocking Mode, Output Data Read Operation
t
20
SDATA (O)
SCLK (I)
RFS
(I)
A0 (I)
DRDY
(O)
t
22
t
24
t
25
t
26
t
27
t
24
t
25
THREE-STATE
MSB
t
30
t
31
BIT N
BIT N+1
Figure 12b. External-Clocking Mode, Output Data Read Operation (
RFS
Returns High During Read Operation)