ATtiny84/85 Receive-only Serial UART using the USI hardware
This is a compact and efficient receive-only Serial UART for the ATtiny84 and ATtiny85, based on the work by David Johnson-Davies and Edgar Bonet. See the original work at http://www.technoblogy.com/show?VSX. It has been tested with Arduino-Tiny, an ATtiny core for Arduino1, on ATtiny84 and ATtiny85 microcontrollers. It may work on other ATtinys with little to no modification, but that hasn't been tested2.
Compared to the original version, the following improvements have been made:
- The ATtiny84 and 85 code base is combined.
- For use with Arduino, it checks that
TIMER_TO_USE_FOR_MILLIS != 0, because the USI needs to use Timer01. - After reading a byte, it checks that the UART Stop bit is high before proceeding.
- Received bytes are written to a buffer that can be polled and read from, similar to regular Serial libraries.
This is a Receive-only UART for ATtiny84/85 that uses the hardware USI in three-wire (SPI) mode to receive bytes. The UART MOSI line should be connected to the ATtiny's MOSI/DI pin. MISO/DO and SCK/SCL pins are not used. Timer/Counter0 is used by the USI to read in bytes.1
1: For use with ATtiny84, you must first edit Arduino\hardware\tiny\cores\tiny\core_build_options.h
so that TIMER_TO_USE_FOR_MILLIS == 1 for __AVR_ATtinyX4__ processors. By default, it is set to 0, but the USI
(which this code uses) requires Timer0 to clock against. If you are programming in pure C (no Arduino libraries),
just be aware that Timer0 must be reserved for this code.
2: If you can verify that this code works on any microcontroller other than the ones listed, please let me know, or submit a Pull Request if any code changes were required.