ADS8661 Datasheet, PDF (35/71 Pages) Texas Instruments – 12-Bit, High-Speed, Single-Supply, SAR ADC Data Acquisition System with Programmable, Bipolar Input Ranges

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ADS8661, ADS8665
SBAS780 – DECEMBER 2016
Device Functional Modes (continued)
7.4.1.3 Multiple Devices: Daisy-Chain Topology
A typical connection diagram showing multiple devices in a daisy-chain topology is shown in Figure 71.
Host Controller
Device 1
Device 2
Device N
Figure 71. Daisy-Chain Connection Schematic
The CONVST/CS and SCLK inputs of all devices are connected together and controlled by a single CONVST/CS
and SCLK pin of the host controller, respectively. The SDI input pin of the first device in the chain (device 1) is
connected to the SDO-x pin of the host controller, the SDO-0 output pin of device 1 is connected to the SDI input
pin of device 2, and so forth. The SDO-0 output pin of the last device in the chain (device N) is connected to the
SDI pin of the host controller.
To operate multiple devices in a daisy-chain topology, the host controller must program the configuration
registers in each device with identical values. The devices must operate with a single SDO-0 output, using the
external clock with any of the legacy, SPI-compatible protocols for data read and data write operations. In the
SDO_CTL_REG register, bits 7-0 must be programmed to 00h.
All devices in the daisy-chain topology sample their analog input signals on the rising edge of the CONVST/CS
signal and the data transfer frame starts with a falling edge of the same signal. At the launch edge of the SCLK
signal, every device in the chain shifts out the MSB to the SDO-0 pin. On every SCLK capture edge, each device
in the chain shifts in data received on its SDI pin as the LSB bit of the unified shift register; see Figure 68.
Therefore, in a daisy-chain configuration, the host controller receives the data of device N, followed by the data
of device N-1, and so forth (in MSB-first fashion). On the rising edge of the CONVST/CS signal, each device
decodes the contents in its unified and takes appropriate action.
For N devices connected in a daisy-chain topology, an optimal data transfer frame must contain 32 × N SCLK
capture edges (see Figure 72). A shorter data transfer frame can result in an erroneous device configuration and
must be avoided. For a data transfer frame with > 32 × N SCLK capture edges, the host controller must
appropriately align the configuration data for each device before bringing CONVST/CS high.
Note that the overall throughput of the system is proportionally reduced with the number of devices connected in
a daisy-chain topology.
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