usingInterrupt() registers an interrupt with the SPI library to prevent conflicts when performing SPI transactions within an interrupt service routine (ISR).
When an interrupt is registered using this function, SPI.beginTransaction() will automatically disable the specified interrupt before starting the transaction, and SPI.endTransaction() will re-enable it. This prevents data corruption that could occur if an interrupt tries to use the SPI bus while a transaction is in progress.
This function is part of the SPI library. See the SPI main page for more information.
SPI.usingInterrupt(interruptNumber)
interruptNumber: the interrupt number to register. This can be the value returned by digitalPinToInterrupt() or a named interrupt constant. Allowed data types: int.Nothing.