“Smaller microcontrollers like Attiny84 microcontrollers dont have UART interface and in many cases you may not need it. But if you will want to have an USART option, you will need to user software USART library or write your own routines. You can find many great software USART libraries for that purpose. As a rule you will have to use two pins to establish communication. But if you are tight on I/Os then you can cheat a little and make it work from single pin. Ralph has been experimenting with simple but smart circuit which allows to perform half duplex UART communications with other systems.”