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.

[deadprogram]

I see the LED flickering but it does not seem to be communicating with my MCP3008.

[jespino]

I see the LED flickering but it does not seem to be communicating with my MCP3008.

@deadprogram is this not working in any mode (mode 0 or mode 3) or is not working in mode 3 only? I have the max7218 working properly in mode 1, but is only for output, so it is a very limited test. Other option is to buy myself an mcp3008 and try it here.

[deadprogram]

Works in mode 0, so I think we're good! Thanks @jespino do you want to squash some commits here?

[jespino]

I'm ok if you squash and merge through the GitHub interface 🙂. I already ordered a mcp3008 to test it, so I'll make it work for mode 3 in a subsequent PR.

[deadprogram]

Thank you @jespino now squash/merging.