I've been maintaining AVR cores for Arduino since early 2015, and started taking it seriously in 2016. Now, there are 3 cores - covering all the best AVR parts
ATTinyCore. Arduino core for ATtiny 1634, 828, x313, x4, x41, x5, x61, x7 and x8
1.8kmegaTinyCore. Arduino core for the tinyAVR 0/1/2-series - Ones's digit 2,4,5,7 (pincount, 8,14,20,24), tens digit 0, 1, or 2 (featureset), preceded by flash in kb. Library maintainers: porting help available!
676DxCore. Arduino core for AVR DA, DB, DD, EA and future DU-series parts - Microchip's latest and greatest AVRs. Library maintainers: Porting help and adviccee is available.
230AVR-Guidance. A guide to best practices when using the Arduino IDE to program AVR microcontrollers
191jtag2updi. UPDI programmer software for Arduino (targets ATtiny 417 / 814 / 816 / 817 / 1614 / 1616 / 1617 and similar MCUs)
81AzzyAI. AzzyAI for Ragnarok Online
47arduino-tiny-841. Arduino core for ATtny841, 828, 1634 and 441
29AzzyRF. 433mhz OOK RF stuff for Arduino
13TinyWireM. I2C library for USI-using chips with support for 1634 added - code ripped off from Adafruit, who in turn took it from BroHogan's code on Arduino Playground
11AVR_Research. "Late to bed and early to rise, work like hell and advertise." Hey, 2/3! Wait, it's supposed to be *early* to bed? Oh dear... nevermind
10UsefulArduinoPosts. In
10tinyNeoPixel. tinyNeoPixel library - automatically included in core releases.
7Adafruit_NeoPixel_Static. Adafruit w/out pinMode or malloc()/free() to save flash.
6LEDCTRLA. Simple Neopixel animation sketch for private event. Suitable as demonstration of Neopixel_Static library use.
5AVRProg. Objective: AVRProg.h library to include in Arduino sketch which will be able to program another AVR via ISP or UPDI
4ProductInfo. Information on products from Azduino (formerly known as Azzy's Electronics)
4tinySPI. Arduino hardware SPI library for ATtiny microcontrollers
4echo. For controlling local devices with the Amazon Echo
3OneWire. Library for Dallas/Maxim 1-Wire Chips
3ArduinoCore-avr. The Official Arduino AVR core
2Adafruit_NeoPixel_Universal. Adafruit NeoPixel library with support for port A added.
2LarduinoISP. make arduino board to be LGT8FX8D/P programmer
2ADCTouch. touch sensing library for Arduino
2RTClib_Tiny. A fork of Jeelab's fantastic RTC library
2Adafruit_NeoPixel_PortA. Neopixel library for port A of AVR microcontrollers
2TinierGPS. TinyGPS, but with only GGA support, to save flash.
2LiquidCrystal_I2C_Tiny. Library for the LiquidCrystal LCD display connected to an Arduino board.
2arduino-volume1. Arduino tone() just got 8-bit volume control - with no extra components!
1avrdude-build-script. avrdude build scripts for Arduino/Genuino
1SD. SD Library for Arduino
1ReleaseScripts. Board manager JSON + associated scripts
1toolchain-avr. The AVR toolchain used by the Arduino IDE
1micronucleus-firmware. Maintained fork of the firmware part of the micronucleus repository
1DigistumpArduino. DigistumpArduino
1tiny-mega-i2c. Minimal I2C master routines for the new AVR microcontrollers.
1MicronucleusArduino. Shell
1SoftI2CMaster. Software I2C / TWI library for Arduino allows any two pins to be SDA & SCL
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