Texas Instruments ADS868 Series, Manual

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CS
SDO
SCLK
SDI
CS
SDO
SCLK
SDI
CS
SDO
SCLK
SDI
SDO
SCLK
ADC
1
ADC
2
ADC
N
SDI
CS
1
CS
2
CS
N
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ADS8684, ADS8688
SBAS582C – JULY 2014 – REVISED APRIL 2015 www.ti.com
The following points must be noted about the daisy-chain configuration illustrated in Figure 73 :
• The SDI pins for all devices are connected together so each device operates with the same internal
configuration. This limitation can be overcome by spending additional host controller resources to control the
SDI input of devices with unique configurations.
• If the number of devices connected in daisy-chain is more than four, loading increases on the shared output
lines from the host controller (CS, SDO, and SCLK). This increased loading may lead to digital timing errors.
This limitation can be overcome by using digital buffers on the shared outputs from the host controller before
feeding the shared digital lines into additional devices.
8.4.1.3.2 Star Topology
A typical connection diagram showing multiple devices in the star topology is shown in Figure 75 . The SDI and
SCLK inputs of all devices are connected together and are controlled by a single SDO and SCLK pin of the host
controller, respectively. Similarly, the SDO outputs of all devices are tied together and connected to the SDI input
pin of the host controller. The CS input pin of each device is individually controlled by separate CS control lines
from the host controller.
Figure 75. Star Topology Connection Schematic
The timing diagram for a typical data frame in the star topology is the same as in a stand-alone device operation,
as illustrated in
Figure 72 . The data frame for a particular device starts with the falling edge of the CS signal and
ends when the CS signal goes high. Because the host controller provides separate CS control signals for each
device in this topology, the user can select the devices in any order and initiate a conversion by bringing down
the CS signal for that particular device. As explained in
Figure 72 , when CS goes high at the end of each data
frame, the SDO output of the device is placed into a Hi-Z state. Therefore, the shared SDO line in the star
topology is controlled only by the device with an active data frame (CS is low). In order to avoid any conflict
related to multiple devices driving the SDO line at the same time, ensure that the host controller pulls down the
CS signal for only one device at any particular time.
TI recommends connecting a maximum of four devices in the star topology. Beyond that, loading may increase
on the shared output lines from the host controller (SDO and SCLK). This loading may lead to digital timing
errors. This limitation can be overcome by using digital buffers on the shared outputs from the host controller
before being fed into additional devices.
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