AD7623
Rev. 0 | Page 24 of 28
SLAVE SERIAL INTERFACE
External Clock
The AD7623 is configured to accept an externally supplied
serial data clock on the SCLK pin when the EXT/
INT
pin is
held high. In this mode, several methods can be used to read
the data. The external serial clock is gated by
CS
. When
CS
and
RD
are both low, the data can be read after each conversion or
during the following conversion. The external clock can be
either a continuous or a discontinuous clock. A discontinuous
clock can be either normally high or normally low when
inactive.
Figure 40 and Figure 41 show the detailed timing
diagrams of these methods.
While the AD7623 is performing a bit decision, it is important
that voltage transients be avoided on digital input/output pins,
or degradation of the conversion result could occur. This is
particularly important during the second half of the conversion
phase because the AD7623 provides error correction circuitry
that can correct for an improper bit decision made during the
first half of the conversion phase. For this reason, it is recom-
mended that when an external clock is being provided, it is a
discontinuous clock that is toggling only when BUSY is low or,
more importantly, that it does not transition during the latter
half of BUSY high.
External Discontinuous Clock Data Read After
Conversion
Though the maximum throughput cannot be achieved using
this mode, it is the most recommended of the serial slave
modes.
Figure 40 shows the detailed timing diagrams of this
method. After a conversion is complete, indicated by BUSY
returning low, the conversion result can be read while both
CS
and
RD
are low. Data is shifted out MSB first with 16 clock
pulses and is valid on the rising and falling edges of the clock.
One advantage of this method is that conversion performance
is not degraded because there are no voltage transients on the
digital interface during the conversion process. Another
advantage is the ability to read the data at any speed up to
80 MHz, which accommodates both the slow digital host
interface and the fastest serial reading.
Finally, in this mode only, the AD7623 provides a daisy-chain
feature using the RDC/SDIN pin for cascading multiple con-
verters together. This feature is useful for reducing component
count and wiring connections when desired, as, for instance, in
isolated multiconverter applications.
An example of the concatenation of two devices is shown in
Figure 39. Simultaneous sampling is possible by using a
common
CNVST
signal. It should be noted that the RDC/SDIN
input is latched on the edge of SCLK opposite to the one used to
shift out the data on SDOUT. Hence, the MSB of the upstream
converter just follows the LSB of the downstream converter on
the next SCLK cycle.
00574-039
SCLK
SDOUTRDC/SDIN
AD7623
#1
(DOWNSTREAM)
AD7623
#2
(UPSTREAM)
BUSY
OUT
BUSYBUSY
DATA
OUT
SCLK
RDC/SDIN SDOUT
SCLK IN
CNVST IN
CNVST
CS
CNVST
CS
CS IN
Figure 39. Two AD7623 Devices in a Daisy-Chain Configuration
External Clock Data Read During Previous Conversion
Figure 41 shows the detailed timing diagrams of this method.
During a conversion, while both
CS
and
RD
are low, the result
of the previous conversion can be read. The data is shifted out,
MSB first, with 16 clock pulses, and is valid on both the rising
and falling edge of the clock. The 16 bits have to be read before
the current conversion is complete; otherwise, RDERROR is
pulsed high and can be used to interrupt the host interface to
prevent incomplete data reading. There is no daisy-chain
feature in this mode, and RDC/SDIN input should always be
tied either high or low.
To reduce performance degradation due to digital activity, a fast
discontinuous clock of at least 40 MHz is recommended to
ensure that all the bits are read during the first half of the SAR
conversion phase.
It is also possible to begin to read data after conversion and
continue to read the last bits after a new conversion has been
initiated.
