* machine/attiny85: add USI-based SPI support
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>
* machine/attiny85: add SPI frequency configuration support
Add software-based frequency control for USI SPI. The ATtiny85 USI lacks
hardware prescalers, so frequency is controlled via delay loops between
clock toggles.
- Calculate delay cycles based on requested frequency and CPU clock
- Fast path (no delay) when frequency is 0 or max speed requested
- Delay loop uses nop instructions for timing control
Co-authored-by: Ona <no-reply@ona.com>
* machine/attiny85: add SPI mode configuration support
Add support for all 4 SPI modes (Mode 0-3) using USI hardware:
- Mode 0 (CPOL=0, CPHA=0): Clock idle low, sample on rising edge
- Mode 1 (CPOL=0, CPHA=1): Clock idle low, sample on falling edge
- Mode 2 (CPOL=1, CPHA=0): Clock idle high, sample on falling edge
- Mode 3 (CPOL=1, CPHA=1): Clock idle high, sample on rising edge
CPOL is controlled by setting the clock pin idle state.
CPHA is controlled via the USICS0 bit in USICR.
Co-authored-by: Ona <no-reply@ona.com>
* machine/attiny85: add LSB-first bit order support
Add software-based LSB-first support for USI SPI. The USI hardware only
supports MSB-first, so bit reversal is done in software before sending
and after receiving.
Uses an efficient parallel bit swap algorithm (3 operations) to reverse
the byte.
Co-authored-by: Ona <no-reply@ona.com>
* GNUmakefile: add mcp3008 SPI example to digispark smoketest
Test the USI-based SPI implementation for ATtiny85/digispark.
Co-authored-by: Ona <no-reply@ona.com>
* machine/attiny85: minimize SPI RAM footprint
Reduce SPI struct from ~14 bytes to 1 byte to fit in ATtiny85's limited
512 bytes of RAM.
Changes:
- Remove register pointers (use avr.USIDR/USISR/USICR directly)
- Remove pin fields (USI pins are fixed: PB0/PB1/PB2)
- Remove CS pin management (user must handle CS)
- Remove frequency control (runs at max speed)
- Remove LSBFirst support
The SPI struct now only stores the USICR configuration byte.
Co-authored-by: Ona <no-reply@ona.com>
* Revert "machine/attiny85: minimize SPI RAM footprint"
This reverts commit 387ccad494.
Co-authored-by: Ona <no-reply@ona.com>
* machine/attiny85: reduce SPI RAM usage by 10 bytes
Remove unnecessary fields from SPI struct while keeping all functionality:
- Remove register pointers (use avr.USIDR/USISR/USICR directly)
- Remove pin fields (USI pins are fixed: PB0/PB1/PB2)
- Remove CS pin (user must manage it, standard practice)
Kept functional fields:
- delayCycles for frequency control
- usicrValue for SPI mode support
- lsbFirst for bit order support
SPI struct reduced from 14 bytes to 4 bytes.
Co-authored-by: Ona <no-reply@ona.com>
---------
Co-authored-by: Ona <no-reply@ona.com>
This is much, _much_ faster than __tinygo_spi_transfer which can only
transfer a single byte at a time. This is especially important for
simulated displays.
I've already implemented the browser side of this on the playground and
have used this patch for local testing where it massively speeds up
display operations.
This makes the code a bit cleaner because ErrTxInvalidSliceSize isn't
redefined in every file that uses SPI and Mode0/Mode1/Mode2/Mode3 is
defined for every target that uses SPI.
Jesús Espino
deadprogram
dkegel-fastly
Ayke van Laethem
Yurii Soldak