Op codes in assembler

Each op code has an explanation via the help screen

You can also go to the Atmel AVR assembler site and look at instruction like SER.

Memory on AVR microprocessors

arduino schematic

A good introduction to reading a schematic

The SPI bus

Fastest of the serial I/O on the Arduino.

A good tutorial on spi using a digital potentiometer chip here

A tutorial on the spi bus, more technical here

Two Arduino's connected via spi here. Code via link to github.

AVR assembler delays

Using delays in AVR studio

Write a variation on the above program to produce any delay required for your project.

AVR assembler example

You'll need to download AVR Studio 4.19 for further work on AVR assembly language.

The I2C bus

Most popular of the serial buses. Uses one wire for clock and one wire for bidirectional data. Based on Phillips' iic network. Lots of chips use the i2c protocol.

Good short introduction here. Not Arduino based.

Using i2c to connect arduino to eeprom chip that holds 256k bits, here.

Interesting use of i2c OLED display.

SPI or i2c. Which one is ""better"?

This video is about the IIC bus and gives a brief overview.

and this one is more of an ad but has some essential information.

This You Tube video is a bit more comprehensive than the above two.

A good example is here.