The Serial Peripheral Interface minimizes the number of host interface signals resulting in a small footprint device that can be interfaced to a wide range of industry-standard microcontrollers supporting SPI. Alternatively, the SPI signals may be controlled using just four general purpose I/O port pins from a microcontroller or custom FPGA. The SPI and all control signals are CMOS and TTL compatible and support 3.3V or 5V operation. The HI-3585 applies the ARINC 429 protocol to the receiver and transmitter. ARINC 429 databus timing comes from a 1 MHz clock input, or an internal counter can derive it from higher clock frequencies having certain fixed values, possibly the external host processor clock.