* 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>
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Co-authored-by: Ona <no-reply@ona.com>
* machine/attiny85: add PWM support for Timer0 and Timer1
Add complete PWM implementation for ATtiny85, supporting both Timer0
and Timer1 with their respective output channels:
- Timer0: 8-bit timer for pins PB0 (OC0A) and PB1 (OC0B)
- Timer1: 8-bit high-speed timer for pins PB1 (OC1A) and PB4 (OC1B)
Timer1 provides more flexible period control with configurable top value
(OCR1C) and extended prescaler options (1-16384), making it well-suited
for LED PWM control and other applications requiring variable frequencies.
Implements full PWM interface including Configure, SetPeriod, Channel,
Set, SetInverting, Top, Counter, and Period methods.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
* machine/digispark: document PWM support on pins
Add documentation to the Digispark board file indicating which pins
support PWM output:
- P0 (PB0): Timer0 channel A
- P1 (PB1): Timer0 channel B or Timer1 channel A
- P4 (PB4): Timer1 channel B
Includes package comment explaining Timer0 vs Timer1 capabilities,
with Timer1 recommended for more flexible frequency control.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
* machine/attiny85: optimize PWM prescaler lookups
Replace verbose switch statements with more efficient implementations:
- SetPeriod: Use bit shift (top >>= prescaler-1) instead of 15-case
switch for dividing uint64 by power-of-2 prescaler values
- Period: Replace switch statements with compact uint16 lookup tables
for both Timer0 and Timer1, casting to uint64 only when needed
This addresses review feedback about inefficient switch-based lookups.
On AVR, this approach is significantly smaller:
- Bit shifts for uint64 division: ~34 bytes vs ~140 bytes
- uint16 tables: 22 bytes code + 32/16 bytes data vs ~140 bytes
- Total savings: ~190 bytes (68% reduction)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
* examples/pwm: add digispark support and smoketest
Add digispark.go configuration for PWM example using Timer1 with pins P1 (LED) and P4. Also add digispark PWM example to GNUmakefile smoketests.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>
Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
All the AVRs that I've looked at had the same pin/port structure, with
the possible states being input/floating, input/pullup, low, and high
(with the same PORT/DDR registers). The main difference is the number of
available ports and pins. To reduce the amount of code and avoid
duplication (and thus errors) I decided to centralize this, following
the design used by the atmega2560 but while using a trick to save
tracking a few registers.
In the process, I noticed that the Pin.Get() function was incorrect on
the atmega2560 implementation. It is now fixed in the unified code.
Jesús Espino
Yurii Soldak
Ayke van Laethem