ATtiny84 enable pullup resistors

• If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated.

  pinMode(PB0,INPUT);     // set PB0 as input
  digitalWrite(PB0,HIGH); // enable the internal pull-up resistor

• If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven high (one).