8.3.17 Daisy-Chaining

Multiple ADS1278-SPs can be daisy-chained together to output data on a single pin. The DOUT1 data output pin of one device is connected to the DIN of the next device. As shown in Figure 77, the DOUT1 pin of device 1 provides the output data to a controller, and the DIN of device 2 is grounded. Figure 78 shows the data format when reading back data.

The maximum number of channels that may be daisy-chained in this way is limited by the frequency of f_SCLK, the mode selection, and the CLKDIV input. The frequency of f_SCLK must be high enough to completely shift the data out from all channels within one f_DATA period. Table 12 lists the maximum number of daisy-chained channels when f_SCLK = f_CLK.

To increase the number of data channels possible in a chain, a segmented DOUT scheme may be used, producing two data streams. Figure 79 illustrates four ADS1278-SPs, with pairs of ADS1278-SPs daisy-chained together. The channel data of each daisy-chained pair are shifted out in parallel and received by the processor through independent data channels.

Whether the interface protocol is SPI or Frame-Sync, it is recommended to synchronize all devices by tying the SYNC inputs together. When synchronized in SPI protocol, it is only necessary to monitor the DRDY output of one ADS1278-SP.

In Frame-Sync interface protocol, the data from all devices are ready after the rising edge of FSYNC.

Since DOUT1 and DIN are both shifted on the falling edge of SCLK, the propagation delay on DOUT1 creates a setup time on DIN. Minimize the skew in SCLK to avoid timing violations.

NOTE:

The number of chained devices is limited by the SCLK rate and device mode.

Figure 77. Daisy-Chaining of Two Devices, SPI Protocol (Format[2:0] = 000 or 001)

Figure 78. Daisy-Chain Data Format of Figure 77

NOTE:

The number of chained devices is limited by the SCLK rate and device mode.

Figure 79. Segmented DOUT Daisy-Chain, Frame-Sync Protocol (Format[2:0] = 011 or 100)