Serial Peripheral Interface (SPI) is a full duplex two-way master-slave communications protocol which allows for multiple devices to share the same bus. In this protocol, all devices share a clock signal, a Master-Out-Slave-In (MOSI) signal, and a Master-In-Slave-Out (MISO) signal. Each device gets a unique Chip Select signal controlled by the bus host. Devices on this bus wait until their specific chip select line is asserted by the master. The master sends out data on the MOSI line, while toggling the serial clock line (SCLK) at the point where the data should be captured by the slave. This is what makes the communication synchronous, which allows for greater design flexibility as the host is not constrained by any specific frequency. In SPI, the data is transferred most commonly in full bytes. The illustration above shows a two byte transmission. Full duplex means that the slave device responds at the same time as the host is sending data. For the AMT22 absolute encoder series, this means incredibly fast position responses. Observing the illustration above users can see that “TCLK” is the maximum time from when the Chip Select line is asserted to when position data is ready to be received from the encoder. To obtain the position of the encoder the host sends blank commands because the encoder is always ready to respond with position, but if extended commands need to be issued to the encoder, the encoder is always listening and is ready to execute those commands after sending the position up front.