SSD1331 RGB OLED SPI interface.
The serial interface consists of serial clock CLK, serial data DIN, DC and CS. DIN is shifted into an 8-bit shift register on every rising edge of CLK in the order of Bit7, Bit6… Bit0. DC is sampled on every eighth clock and the data byte in the shift register is written to the Display Data RAM or command register in the same clock.
Under serial mode, only write operations are allowed.