DAC interfacing with Atmega128
Software Bit bang is possible. Or, Go for SPI with little changes:
Use only MOSI and SCLK of SPI to connect to DAC.
SYNC pin of DAC can be driven low by a GPIO of MCU, while SPI can be configured to transmit data to DAC.
MOSI pin sends out the data in three consecutive writes along with 24 clocks from SCLK.
Assuming CPHASE, CPOL, and LSB First requirements are taken care of in software before initiating the write. Figure 38 in http://www.ti.com/lit/ds/symlink/dac8532.pdf may help.