Datasheet

AD7949 Data Sheet
Rev. D | Page 22 of 32
DIGITAL INTERFACE
The AD7949 uses a simple 4-wire interface and is compatible
with SPI, MICROWIRE™, QSPI™, digital hosts, and DSPs, for
example, Blackfin® ADSP-BF53x, SHARC®, ADSP-219x, and
ADSP-218x.
The interface uses the CNV, DIN, SCK, and SDO signals and
allows CNV, which initiates the conversion, to be independent
of the readback timing. This is useful in low jitter sampling or
simultaneous sampling applications.
A 14-bit register, CFG[13:0], is used to configure the ADC for
the channel to be converted, the reference selection, and other
components, which are detailed in the Configuration Register,
CFG, section.
When CNV is low, reading/writing can occur during conversion,
acquisition, and spanning conversion (acquisition plus conver-
sion), as detailed in the following sections. The CFG word is
updated on the first 14 SCK rising edges, and conversion results
are output on the first 13 (or 14 if busy mode is selected) SCK
falling edges. If the CFG readback is enabled, an additional
14 SCK falling edges are required to output the CFG word
associated with the conversion results with the CFG MSB
following the LSB of the conversion result.
A discontinuous SCK is recommended because the part is
selected with CNV low, and SCK activity begins to write a new
configuration word and clock out data.
Note that in the following sections, the timing diagrams indicate
digital activity (SCK, CNV, DIN, SDO) during the conversion.
However, due to the possibility of performance degradation,
digital activity should occur only prior to the safe data reading/
writing time, t
DATA
, because the AD7949 provides error correc-
tion circuitry that can correct for an incorrect bit during this
time. From t
DATA
to t
CONV
, there is no error correction and conver-
sion results may be corrupted. The user should configure the
AD7949 and initiate the busy indicator (if desired) prior to
t
DATA
. It is also possible to corrupt the sample by having SCK or
DIN transitions near the sampling instant. Therefore, it is
recommended to keep the digital pins quiet for approximately
20 ns before and 10 ns after the rising edge of CNV, using a
discontinuous SCK whenever possible to avoid any potential
performance degradation.
READING/WRITING DURING CONVERSION, FAST
HOSTS
When reading/writing during conversion (n), conversion
results are for the previous (n − 1) conversion, and writing the
CFG register is for the next (n + 1) acquisition and conversion.
After the CNV is brought high to initiate conversion, it must be
brought low again to allow reading/writing during conversion.
Reading/writing should only occur up to t
DATA
and, because this
time is limited, the host must use a fast SCK.
The SCK frequency required is calculated by
DATA
SCK
t
EdgesSCKNumber
f
__
≥
The time between t
DATA
and t
CONV
is a safe time when digital
activity should not occur, or sensitive bit decisions may be
corrupted.
READING/WRITING AFTER CONVERSION, ANY
SPEED HOSTS
When reading/writing after conversion, or during acquisition
(n), conversion results are for the previous (n − 1) conversion,
and writing is for the (n + 1) acquisition.
For the maximum throughput, the only time restriction is that
the reading/writing take place during the t
ACQ
(minimum) time.
For slow throughputs, the time restriction is dictated by the
throughput required by the user, and the host is free to run at
any speed. Thus for slow hosts, data access must take place
during the acquisition phase.