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Jesús Espino
d88634fb03
Implement SPI communication for ATTiny85 using the USI (Universal Serial Interface) hardware in three-wire mode. The ATTiny85 lacks dedicated SPI hardware but can emulate SPI using the USI module with software clock strobing. Implementation details: - Configure USI in three-wire mode for SPI operation - Use clock strobing technique to shift data in/out - Pin mapping: PB2 (SCK), PB1 (MOSI/DO), PB0 (MISO/DI) - Support both Transfer() and Tx() methods The implementation uses the USI control register (USICR) to toggle the clock pin, which triggers automatic bit shifting in hardware. This is more efficient than pure software bit-banging. Current limitations: - Frequency configuration not yet implemented (runs at max software speed) - Only SPI Mode 0 (CPOL=0, CPHA=0) supported - Only MSB-first bit order supported Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com> Co-authored-by: Ona <no-reply@ona.com>