mosis

MOSI stands for Master Out Slave In. It is the data line in the Serial Peripheral Interface (SPI) bus that carries data from the master device to the slave devices. In typical configurations, there is one MOSI line shared by all devices on the bus, while each slave has its own chip select (CS) or slave select (SS) input to enable or disable it. Data transferred on MOSI is clocked in by the slaves on a specific clock edge defined by the SPI mode (a combination of CPOL and CPHA).

MOSI is driven by the master’s SPI peripheral; slaves that are not selected generally listen but do not drive the line. The line is usually push-pull and driven at logic levels compatible with the devices (for example 3.3 V or 5 V). Transmission is synchronous with SCK, which is generated by the master. Some devices label this line as SDI (Serial Data In) rather than MOSI in their datasheets. The term MOSIs can refer to MOSI lines on different chips or different SPI buses in a system; in most boards there is a single MOSI net shared by all slaves.

Because all slaves share MOSI, care must be taken to avoid bus contention and to ensure that the master is the only device driving the line when communications occur. On multi-master SPI, proper arbitration and tri-stating is required to prevent conflicts.

Electrical considerations: typical speeds range from a few kHz to tens of MHz; signal integrity may require short traces, proper decoupling, and level shifting when devices operate at different voltages.