AD7621
Rev. 0 | Page 24 of 32
SLAVE SERIAL INTERFACE
External Clock
The AD7621 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 AD7621 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 AD7621 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
recommended 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.
Among the advantages of this method is the fact 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 AD7621 provides a daisy-chain
feature using the RDC/SDIN pin for cascading multiple
converters 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.
04565-042
SCLK
SDOUTRDC/SDIN
AD7621
#1
(DOWNSTREAM)
AD7621
#2
(UPSTREAM)
BUSY
OUT
BUSYBUSY
DATA
OUT
SCLK
RDC/SDIN SDOUT
SCLK IN
CNVST IN
CNVST
CS
CNVST
CS
CS IN
Figure 39. Two AD7621 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 (at least 30 MHz when impulse mode is
used, 60 MHz when normal mode is used, or 80 MHz when
warp mode is used) 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. However, this is not recommended when using the
fastest throughput of any mode since the acquisition times are
only 70 ns, 100 ns, and 50 ns for warp, normal, and impulse
modes.
If the maximum throughput is not used, thus allowing more
acquisition time, then the use of a slower clock speed can be
used to read the data.
