What is TTL used in this project?

Projects General Guidance
1

I want to make a project based on arduino leonardo. However I don't need all of the board, just the microcontroller and the pins. I saw this video:

My quesion is, what is the TTL used for? If I want to make a wired version, do I need that? Also, I need to burn the bootloader, can I do as described in the uno tutorial here?
https://support.arduino.cc/hc/en-us/articles/360012048080-How-to-burn-the-bootloader-between-two-Arduino-UNO

Currenly this is where I am, I wired the crystal and the LED (are these good?), the last thing is the PTC and the usb cable:

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2

Not sure what a "TTL" is. TTL is an obsolete logic series which used voltages which are not quite compatible with CMOS which is what these chips employ.

If you are referring to the USB-to-serial interface, that is required to communicate with the ATmega328 chip such as when uploading a "sketch". Nothing whatsoever to do with "TTL". The 32U4 however uses its own USB interface and an entirely different bootloader.

R28 and R29 in your circuit are meaningless; there is no need whatsoever to be critical about the value of the series resistor, 200 Ohms is perfectly fine as is 300 Ohms or in fact, 470 - whatever you have 200 Ohms or above within a couple of "k".

Putting it all together in this fashion can be regarded as "creative head-banging" - makes you feel you are doing something but entirely impractical. :grin:

3

You can get very small boards based on the 32U4, for example

https://de.aliexpress.com/item/32819992328.html

4

I used ATmega8 / 88/128 chips on my own. As a programmer I use Arduino Nano and I do not put a bootloader on the chips but only set fuse bits and then upload the program I need.

5

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