For systems that contain several DACx1001 devices, the SDO pin is used to daisy-chain the devices together. The daisy-chain feature is useful in reducing the number of serial interface lines. The first falling edge on the SYNC pin starts the operation cycle, as shown in Figure 48. SCLK is continuously applied to the input shift register while the SYNC pin is kept low. The DAC is updated with the data on rising edge of SYNC pin.
If more than 32 clock pulses are applied, the data ripple out of the shift register and appear on the SDO line. These data are clocked out on the rising edge of SCLK and are valid on the falling edge. By connecting the SDO output of the first device to the SDI input of the next device in the chain, a multiple-device interface is constructed. Each device in the system requires 32 clock pulses.
As a result, the total number of clock cycles must be equal to 32 × N, where N is the total number of devices in the daisy-chain. When the serial transfer to all devices is complete the SYNC signal is taken high. This action transfers the data from the SPI shift registers to the internal register of each device in the daisy-chain and prevents any further data from being clocked into the input shift register. The DACx1001 implement a bit that enables higher speeds for clocking out data from the SDO pin. Enable this feature by setting FSDO (bit 13, address 02h) to 1. See Table 4 and Table 3 for more information.