After reset, it is a very simple QSPI-based, 8-bit DAC (digital to analog converter). You feed it with 4-bit halves, which are then read on each rising edge of CLK. It also breakouts UIO’s pins to test it externally (delays, electrical characteristics, etc).
After power up set nCS high, set RST_N low, then set RST_N high. If you want to set a new DAC value, first set nCS low, then set 4-bits, beginning from the lowest half. Now trigger a rising edge of CLK, set highest 4-bits and trigger another rising edge of CLK. Now you can continue setting next bytes (divided into 4-bit halves) or set nCS high and start from the beginning, by setting nCS low. You can observe effects on the 7seg display or remove it and implement R-2R network to have the acutal DAC. There is also a break out of bi-directional ports to test them externally.