Datasheet

ADS1212, 1213
26
SBAS064A
Synchronizing Multiple Converters
A negative going pulse on DSYNC can be used to synchro-
nize multiple ADS1212/13s. This assumes that each
ADS1212/13 is driven from the same master clock and is set
to the same Decimation Ratio and Turbo Mode Rate. The
affect that this signal has on data output timing in general is
discussed in the Serial Interface section.
The concern here is what happens if the DSYNC input is
completely asynchronous to this master clock. If the DSYNC
input rises at a critical point in relation to the master clock
input, then some ADS1212/13s may start-up one X
IN
clock
cycle before the others. Thus, the output data will be syn-
chronized, but only to within one X
IN
clock cycle.
For many applications, this will be more than adequate. In
these cases, the timing symbols which relate the DSYNC
signal to the X
IN
signal can be ignored. For other multiple-
converter applications, this one X
IN
clock cycle difference
could be a problem. These types of applications would
include using the DRDY and/or the SCLK output from one
ADS1212/13 as the “master” signal for all converters.
To ensure exact synchronization to the same X
IN
edge, the
timing relationship between the DSYNC and X
IN
signals,
as shown in Figure 22, must be observed. Figure 23 shows
a simple circuit which can be used to clock multiple
ADS1212/13s from one ADS1212/13, as well as to ensure
that an asynchronous DSYNC signal will exactly synchro-
nize all the converters.
FIGURE 22. DSYNC to X
IN
Timing for Synchronizing
Mutliple ADS1212/13s.
SERIAL INTERFACE
The ADS1212/13 includes a flexible serial interface which
can be connected to microcontrollers and digital signal
processors in a variety of ways. Along with this flexibility,
there is also a good deal of complexity. This section de-
scribes the trade-offs between the different types of interfac-
ing methods in a top-down approach—starting with the
overall flow and control of serial data, moving to specific
interface examples, and then providing information on vari-
ous issues related to the serial interface.
Multiple Instructions
The general timing diagrams which appear throughout this
data sheet show serial communication to and from the
ADS1212/13 occurring during the DRDY LOW period (see
Figures 4 through 10 and Figure 36). This communication
represents one instruction that is executed by the ADS1212/
13, resulting in a single read or write of register data.
However, more than one instruction can be executed by the
ADS1212/13 during any given conversion period (see Fig-
ure 24). Note that DRDY remains HIGH during the subse-
quent instructions. There are several important restrictions
on how and when multiple instructions can be issued during
any one conversion period.
DRDY
Serial
I/O
24 t
XIN
Internal
Update of DOR
FIGURE 24. Timing of Data Output Register Update.
The first restriction is that the converter must be in the Slave
Mode. There is no provision for multiple instructions when
the ADS1212/13 is operating in the Master Mode. The
second is that some instructions will produce invalid results
if started at the end of one conversion period and carried into
the start of the next conversion period.
FIGURE 23. Exactly Synchronizing Multiple ADS1212/13s to an Asynchronous DSYNC Signal.
C
2
6pF
C
1
6pF
XTAL
DSYNC
X
IN
X
OUT
DGND
ADS1212/13
SDOUT
SDIO
SCLK
DV
DD
D
CLK
1/2 74HC74
1/6 74HC04
Q
Q
DSYNC
X
IN
X
OUT
DGND
ADS1212/13
SDOUT
SDIO
SCLK
DV
DD
DSYNC
X
IN
X
OUT
DGND
ADS1212/13
SDOUT
SDIO
SCLK
DV
DD
Asynchronous
DSYNC
Strobe
DGND
t
33
t
34
t
35
X
IN
DSYNC