MODE[1:0]
Pins
ADS1274/78
Mode
New Mode
New Mode
Valid Data Ready
DRDY
SPI
Protocol
Frame-Sync
Protocol
t
NDR-SPI
DOUT
New Mode
Valid Data on DOUT
t
NDR-FS
Previous
Mode
SBAS367F
–
JUNE 2007
–
REVISED FEBRUARY 2011
Figure 72. Mode Change Timing
Table 10. New Data After Mode Change
SYMBOL
DESCRIPTION
MIN
TYP
MAX
UNITS
t
NDR-SPI
Time for new data to be ready (SPI)
129
Conversions (1/f
DATA
)
t
NDR-FS
Time for new data to be ready (Frame-Sync)
(1)
127
128
Conversions (1/f
DATA
)
(1)
If mode change is asynchronous to the FSYNC clock, t
NDR-FS
varies from 127 to 128 conversions. If the mode change is made
synchronous to FSYNC, t
NDR-FS
is stable.
SYNCHRONIZATION (SYNC)
See
for the Frame-Sync format timing
requirement.
The ADS1274/78 can be synchronized by pulsing the
SYNC pin low and then returning the pin high. When
After
synchronization,
indication
of
valid
data
the pin goes low, the conversion process stops, and
depends on whether SPI or Frame-Sync format was
the internal counters used by the digital filter are
used.
reset.
When
the
SYNC
pin
returns
high,
the
conversion process restarts. Synchronization allows
In the SPI format, DRDY goes high as soon as SYNC
the conversion to be aligned with an external event,
is taken low; see
. After SYNC is returned
such as the changing of an external multiplexer on
high, DRDY stays high while the digital filter is
the analog inputs, or by a reference timing pulse.
settling. Once valid data are ready for retrieval,
DRDY goes low.
Because the ADS1274/78 converters operate in
parallel from the same master clock and use the
In the Frame-Sync format, DOUT goes low as soon
same SYNC input control, they are always in
as SYNC is taken low; see
. After SYNC is
synchronization with each other. The aperture match
returned high, DOUT stays low while the digital filter
among internal channels is typically less than 500ps.
is settling. Once valid data are ready for retrieval,
However, the synchronization of multiple devices is
DOUT begins to output valid data. For proper
somewhat different. At device power-on, variations in
synchronization, FSYNC, SCLK, and CLK must be
internal reset thresholds from device to device may
established before taking SYNC high, and must then
result in uncertainty in conversion timing.
remain running. If the clock inputs (CLK, FSYNC or
SCLK)
are
subsequently
interrupted
or
reset,
The SYNC pin can be used to synchronize multiple
re-assert the SYNC pin.
devices to within the same CLK cycle.
illustrates the timing requirement of SYNC and CLK
For consistent performance, re-assert SYNC after
in SPI format.
device power-on when data first appear.
©
2007
–
2011, Texas Instruments Incorporated
27
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