AVR Freaks

With an external pull down you would have:

VCC--->Int. Pull-up resistor---->PIN--->Ext. Pull-down resistor---->GND

So the internal and external resistors would form a voltage divider and that voltage would be input to the pin. If this voltage is neither >>HIGH<< nor >>LOW<<, you will have even more power consumption (besides the one flowing through the resistors) because CMOS transistors from the input buffer present in every pin will be in an intermediate state.

Another problem related to pull up resistors.
I have written a small program to make PB0 low on my Xplained Mini board with a switch to light the LED.

But this is unstable, sometimes the LED stays on constantly or randomly goes out.
It is also sensitive when touched with a finger.

If I connect an external resistor (100kOhm) from PB0 to Vcc it works well, because I can then turn the LED on and off with the switch.

How could I solve this?

regards,
Bert

The source for testing I got from "Micrsoft Makes" on Youtube and works with  the Xplained Mini board.

Here is the source

#define F_CPU  8000000UL

#include <avr/io.h>
#include <avr/interrupt.h>

#define LED_ON  PORTB |= (1<<PORTB5)
#define LED_OFF PORTB &= ~(1<<PORTB5)
#define LED_TOGGLE  PINB |= (1<<PINB5)
#define SWITCH_PRESSED  !(PINB & (1<<PINB0))

ISR(PCINT0_vect)
{
       if(SWITCH_PRESSED)  // if PINB0 is low
       {
           LED_ON;
       }
       else
       {
           LED_OFF;
       }
}

int main(void)
{
    DDRB |= (1<<DDB5);
    DDRB &= ~(1<<DDB0);

    PCMSK0 |=(1<<PCINT0);
    PCICR |=(1<<PCIE0);

    sei();

    while (1)
    {

    }
}

I only changed it for using PB0 and according to the ATmega 328 datasheet there are internal pull up resistors on PORT B.