micronucleus VS ATTinyCore

You can actually bit-bang low speed USB on Arduino [0]. There is even a bootloader [1] for ATtiny. It is obviously not compatible with Arduino, but those tiny boards look too cute to ignore [2]. Also, you can use USB port both for downloading firmware and your application.

[0] https://github.com/obdev/v-usb

[1] https://github.com/micronucleus/micronucleus

[2] https://cpldcpu.wordpress.com/2014/04/25/the-nanite-85/

The Digispark[1] and its clones use the ATTINY85 and offer flashing over USB via the Micronucleus bootloader[2] using V-USB[3]. It comes at the cost of a significant chunk of flash storage, but the convenience is excellent. Plus, V-USB support means you can emulate all sorts of USB devices. I've used it to build a DIY NES controller adapter, but it's commonly used as a cheap Rubber Ducky[4]. For projects where USB interfacing makes sense, it can be a good fit.

1: http://digistump.com/products/1

2: https://github.com/micronucleus/micronucleus

3: https://www.obdev.at/products/vusb/index.html

4: https://shop.hak5.org/products/usb-rubber-ducky-deluxe

So it looks like they're using the micronucleus bootloader for the linked example. That bootloader not only does the normal bootloader job, but also acts as a bitbanging USB device in order to communicate with the computer over USB. The USB bitbanging is a huge endeavor to undertake on top of writing a bootloader.

Resources I used: - How to Burn ATTiny85 Using Arduino-Mega As ISP : 5 Steps - Instructables - To program Arduino as an ISP. This was for my Arduino mega. There are a more than a few links for "normal" Arduinos if you google a bit. - GitHub - SpenceKonde/ATTinyCore: Arduino core for ATtiny 1634, 828, x313, x4, x41, x5, x61, x7 and x8 - arduino support for attiny microcontrollers.

You can code in Arduino using the ATTinyCore - https://github.com/SpenceKonde/ATTinyCore

Using this ATtiny85 board support building the 20 LED variant of the blink sketch in the Readme I get:

Install ATTinyCore package by Space Konde (author in video is using it). More info on ATTiny core github.

The Arduino core (i.e. the Arduino libraries) for the ATTiny85 (and a couple more ATTinys) is here and you can find the link for the ATTiny85 specific information a bit down the main page which, inside, includes this excelent refence picture.

After that go check the analogRead implementation in the ATTinyCore and the Arduino Samd core (it's in wiring_analog.c)