AVR-Guidance VS megaTinyCore

Check out the newer attiny chips......cheap, available, and LOW POWER. No external crystal or other components required. Programmable via Arduino IDE.,,see: https://github.com/SpenceKonde/megaTinyCore You can use any arduino as a programmer (JTAGTOUPDI) or make one with a cheap serial adapter: https://github.com/SpenceKonde/AVR-Guidance/blob/master/UPDI/jtag2updi.md

For Serial UPDI attach either a diode or resistor between RX and TX, then connect RX to UPDI PIN, along with and VCC and GND. https://github.com/SpenceKonde/AVR-Guidance/blob/master/UPDI/jtag2updi.md

I own a PICKIT4, so normally I would just breakout Vcc, Gnd, and UPDI pins and directly connect my PICKIT to it. This time though my friend recommended me to also include a SerialUPDI circuit on board to accommodate Arduino users (and ones without PICKIT in general). The part related to SerialUPDI are JP3, JP4, D35, and R5.

Yup, that should do the trick. You might want to give this guide a good read-through. It has some chip-specific tips (search for FTDI) and general recommendations for wiring and removing the Rx LED if it gives you troubles.

Choice of hardware

You can use RX/TX with the UPDI interface directly, see https://github.com/SpenceKonde/AVR-Guidance/blob/master/UPDI/jtag2updi.md. You should at least add a resistor on the line. Then you'll be able to program it with pymcuprog in serial mode, without a bootloader.

ISP looks a lot like spi. Updi is asynchronous uart. It's a lot different and I would recommend looking at serial updi.

They're compatible with Arduino by using the megaTinyCore and programmed with an UPDI programmer. A homemade UPDI programmer can be made with a USB-Serial adaptor and a Schottky diode (instructions on that site).

Programming is done via the Arduino IDE, although NOT all libraries work. See: https://github.com/SpenceKonde/megaTinyCore/blob/master/megaavr/extras/LibraryCompatibility.md

Go check the site for the Arduino megaTinyCore which includes amongst other things instructions on how to make a programmer for that kind of ATTiny microcontroller (the so-called "modern" attinys).

So this megaTinyCore GitHub page talks a lot about the programmers and various ways you can flash to these chips. I basically used 3 male header pins (vcc, gnd, updi) connected to a modified serial programmer that I insert into pads on the board. Crude but honestly pretty painless and effective.

You can program them with Arduino using the MegaTinyCore and you upload programs to it using the UPDI protocol which uses just one wire. You can make a programmer for it with with a Serial-USB Adaptor, a schottky diode and some wires - all the instructions in the link above.

Yeah, do a quick search for PIN1_bm in this file https://github.com/SpenceKonde/megaTinyCore/blob/master/megaavr/extras/ioheaders/iotn412.h

The modern line up of ATtiny micro controllers is pretty sweet. They go for under a dollar a piece on digikey (best place to get them). They are mostly surface mount so an ebay smd adapter board kit makes them easy to use. I picked the attiny1604 but there are many variations. This is the arduino core for them and if you scroll down has all the info and variation comparison. https://github.com/SpenceKonde/megaTinyCore

I am programming the Tiny via UPDI through a Arduino Nano using the Arduino IDE with the MegaTinyCore. It does verify the code after upload. I can use a 1306 display (I2C), and manipulate all the GPIO's with no issue. I can use the same GPIO that I have hooked the 18B20 up to for anything else, just not 1-Wire. I have tried other GPIO with the same response. I am using the example for the Dallas Temperature and update the pin assignment. The exact same code works fine on a 328P with the same sensor, so I dont doubt it is a library issue but I'm not experienced enough to figure out exactly where the issue could be.