Definition
SPI (Serial Peripheral Interface) is a synchronous serial bus that a controller chip uses to exchange data with one or more peripheral chips on the same board. It is full-duplex, meaning data can move in both directions at once, and it is fast and simple to implement, which is why it shows up everywhere inside ASIC miner control boards and the small accessory boards around them.
The four signal lines
A standard SPI connection uses four logic signals. SCLK is the serial clock, driven by the controller so both sides stay in lock-step. MOSI carries data from the controller to the peripheral, and MISO carries data back the other way. The fourth line, chip select (sometimes labelled SS or CS), tells a specific peripheral that the bus is talking to it. Because the controller supplies the clock, there is no agreed baud rate to negotiate the way an asynchronous link needs one; the clock speed is set by the controller and can run from a few hundred kilohertz to tens of megahertz depending on the parts involved.
Where it appears in mining hardware
Inside a typical control board you will find SPI driving NOR or NAND flash that holds firmware, configuration EEPROMs, displays, and sometimes the ADCs that monitor board health. When you are reading a chip with an external programmer during a repair, you are usually clocking it out over SPI. Its main trade-off is wiring: every extra peripheral needs its own chip-select line, so SPI scales in throughput but not in pin count the way a two-wire bus does.
For how the addressing line works in practice, see chip select. For the slower two-wire alternative used for sensors and PMICs, compare the entry on the system bus and related serial buses.
In Simple Terms
SPI (Serial Peripheral Interface) is a synchronous serial bus that a controller chip uses to exchange data with one or more peripheral chips on the…