attiny 84 & 85 serial overview. has comparison of 84 v 85 code: https://www.instructables.com/id/ATtiny85-ATtiny84-Analog-Pins-Serial-Communication/

attiny 84 small uart TX blog post and code on gh: https://marcelmg.github.io/software_uart/ https://github.com/MarcelMG/AVR8_BitBang_UART_TX/blob/master/main.c

attiny 85 datasheet: https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-2586-AVR-8-bit-Microcontroller-ATtiny25-ATtiny45-ATtiny85_Datasheet.pdf

84 datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/doc8126.pdf

attiny port manipulation: https://www.instructables.com/id/ATTiny-Port-Manipulation/

Attiny85 arduino quick reference: https://cdn.sparkfun.com/assets/2/8/b/a/a/Tiny_QuickRef_v2_2.pdf