ATtiny85 (PDIP-8)

Fig. 1. ATtiny85 (PDIP-8), PDIP-8. Hover or tap a signal to light up every pin that carries it.

• Arduino-style numbering under ATTinyCore / Digispark is PB0-PB5 = D0-D5, which is not the physical pin order. ADC channels are different again: ADC0 is on PB5, ADC1 on PB2, ADC2 on PB4, ADC3 on PB3.
• PB5 is RESET by default, so you get four free I/O until you burn the RSTDISBL fuse. Doing that gives you a fifth pin but locks out ISP programming. After that you need a high-voltage programmer to recover the chip.
• No hardware SPI or I2C. The USI block does the clocking, and the Arduino libraries (TinyWireM, USIWire) are built on it.
• There is no hardware UART. Serial on an ATtiny85 is bit-banged software serial.
• The USB bootloader boards (Digispark, micronucleus) take PB3 and PB4 for D- and D+, leaving PB0, PB1 and PB2.
• Default fuses give 8 MHz internal with CKDIV8, so a fresh chip runs at 1 MHz. 16.5 MHz from the internal PLL is available and is what USB bootloaders use.
• The speed grade tracks the supply, as on the ATmega328P: 20 MHz needs 4.5 V, 10 MHz needs 2.7 V.

• Drive a pin on the only port there is

  avr-libc

  #include <avr/io.h>
  
  /* Everything lives on port B. PB0 to PB4 are free; PB5 is RESET
     until you burn a fuse you cannot easily undo. */
  
  DDRB  |=  (1 << PB1);          /* PB1 = pin 6, the LED on most boards */
  PORTB |=  (1 << PB1);
  PORTB &= ~(1 << PB1);
  
  DDRB  &= ~(1 << PB2);          /* PB2 = pin 7, input */
  PORTB |=  (1 << PB2);          /* internal pull-up */
  
  if (!(PINB & (1 << PB2))) {
  }

  Arduino numbering under ATTinyCore is PB0 to PB5 = D0 to D5, which is not the physical pin order. PB1 is physical pin 6.

Table 2. Pin assignments.