machine/attiny85: add USI-based SPI support

[jespino]

Summary

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

• 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
• Software-based frequency control via delay loops
• All 4 SPI modes supported
• LSB-first bit order supported via software bit reversal

The implementation uses the USI control register (USICR) to toggle the clock pin, which triggers automatic bit shifting in hardware.

Frequency Configuration

The ATtiny85 USI lacks hardware prescalers, so frequency is controlled via software delay loops between clock toggles:

// Configure SPI at 1 MHz
machine.SPI0.Configure(machine.SPIConfig{
    Frequency: 1000000,
})

// Configure SPI at 400 kHz (useful for SD card initialization)
machine.SPI0.Configure(machine.SPIConfig{
    Frequency: 400000,
})

// Configure SPI at maximum speed (no delay)
machine.SPI0.Configure(machine.SPIConfig{
    Frequency: 0,
})

SPI Mode Configuration

All 4 SPI modes are supported:

Mode	CPOL	CPHA	Clock Idle	Data Sampled
0	0	0	Low	Rising edge
1	0	1	Low	Falling edge
2	1	0	High	Falling edge
3	1	1	High	Rising edge

// Configure SPI in Mode 0 (default)
machine.SPI0.Configure(machine.SPIConfig{
    Mode: machine.Mode0,
})

// Configure SPI in Mode 3
machine.SPI0.Configure(machine.SPIConfig{
    Mode: machine.Mode3,
})

Bit Order Configuration

Both MSB-first (default) and LSB-first bit orders are supported:

// Configure SPI with LSB-first bit order
machine.SPI0.Configure(machine.SPIConfig{
    LSBFirst: true,
})

LSB-first is implemented via software bit reversal since the USI hardware only supports MSB-first.

References

• tinySPI library - reference implementation
• ATtiny85 datasheet USI section

[jespino]

I have a problem about the Modes here, I can try to implement them but I don't have SPI devices with other modes, also, I don't have an SPI device for reading. Maybe I need somebody helping me with that.