Texas Instruments ADS868 Series Manual Download Page 34

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SCLK

SDO

Sample

N + 1

Data from sample N

D15

D14

B15

B14

X X X X X X X X X

SDI

B10

ADS8684, ADS8688

SBAS582C – JULY 2014 – REVISED APRIL 2015

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8.4.1.2 Data Acquisition Example

This section provides an example of how a host processor can use the device interface to configure the device
internal registers as well as convert and acquire data for sampling a particular input channel. The timing diagram
shown in

Figure 72

provides further details.

Figure 72. Device Operation Using the Serial Interface Timing Diagram

There are four events shown in

Figure 72

. These events are described below:

•

Event 1:

The host initiates a data conversion frame through a falling edge of the CS signal. The analog input

signal at the instant of the CS falling edge is sampled by the ADC and conversion is performed using an
internal oscillator clock. The analog input channel converted during this frame is selected in the previous data
frame. The internal register settings of the device for the next conversion can be input during this data frame
using the SDI and SCLK inputs. Initiate SCLK at this instant and latch data on the SDI line into the device on
every SCLK falling edge for the next 16 SCLK cycles. At this instant, SDO goes low because the device does
not output internal conversion data on the SDO line during the first 16 SCLK cycles.

•

Event 2:

During the first 16 SCLK cycles, the device completes the internal conversion process and data are

now ready within the converter. However, the device does not output data bits on SDO until the 16th falling
edge appears on the SCLK input. Because the ADC conversion time is fixed (the maximum value is given in
the

Electrical Characteristics

table), the 16th SCLK falling edge must appear after the internal conversion is

over, otherwise data output from the device is incorrect. Therefore, the SCLK frequency cannot exceed a
maximum value, as provided in the

Timing Requirements: Serial Interface

table.

•

Event 3:

At the 16th falling edge of the SCLK signal, the device reads the LSB of the input word on the SDI

line. The device does not read anything from the SDI line for the remaining data frame. On the same edge,
the MSB of the conversion data is output on the SDO line and can be read by the host processor on the
subsequent falling edge of the SCLK signal. For 16 bits of output data, the LSB can be read on the 32

SCLK falling edge. The SDO outputs 0 on subsequent SCLK falling edges until the next conversion is
initiated.

•

Event 4:

When the internal data from the device is received, the host terminates the data frame by

deactivating the CS signal to high. The SDO output goes into a Hi-Z state until the next data frame is initiated,
as explained in Event 1.

8.4.1.3 Host-to-Device Connection Topologies

The digital interface of the ADS8684 and ADS8688 offers a lot of flexibility in the ways that a host controller can
exchange data or commands with the device. A typical connection between a host controller and a stand-alone
device is illustrated in

Figure 70

. However, there are applications that require multiple ADCs but the host

controller has limited interfacing capability. This section describes two connection topologies that can be used to
address the requirements of such applications.

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