Arsthbt

I am constructing a circuit where 2 microcontrollers will communicate with a high or low state on their IO pin. Basicly a state pin for Bluetooth connected, or not. One microcontroller will have an IO pin as an output and the other an IO pin as an input. I know my microcontroller has an internal pull up (also pull down) resistor, but how does this circuit look like? Below is how I want to connect it, for sure I shouldn't need to have resistors when there are internal ones, right?

^{simulate this circuit – Schematic created using CircuitLab}

So my real question is how does the internal pull up/down resistors look in the microcontroller? Is it like this?

^{simulate this circuit}

In your example, R1 is a pull-down and R2 is a pull-up resistor. Depending on the MCU and the pin involved there may be one or the other or both or neither available. That information will be in the datasheet. There's also another possibility, a "hold" where there is a resistor internally from a buffer output back to the input.

The purpose of a pull-up or pull-down is to put the input line in a known state if the connection to it is high-impedance. On an MCU that can happen if the wire gets disconnected or if the driver is deliberately tristated or during startup before it is configured. If the line is being driven push-pull it does little but waste power.

Whether a pull-up or pull-down is required is dependent on your requirements. As to whether the internal resistor is sufficient, again that depends on the requirements. The IC makers tend to choose rather high values which may not be desirable in certain circumstances where EMI or leakage is present. There might be cases where the values are too low (very low power systems, for example). The on-chip resistors (or equivalent) also have quite a loose tolerance typically. So there are many cases where a pull-up or pull-down is available on the chip, but the designer chooses to use an external resistor.

Pull-ups and pull-downs are usefull for setting the "default" logic level when the input pin may be left unconnected or at a high impedance state. These pull (virtual) resistors can be configured by your code but usually default to being disabled when you do nothing about them.

If you're worried about frying you micro because there are no resistors between the Bluetooth IC output and the micro input to limit current, then don't worry. When set as inputs microcontroller pins have high impedance, which means they draw (almost) no current.

The normal way of doing this is to disable the pullups, at both ends, and drive in both directions.

A feature you may want to add if you're worried about damage and not about signal speed is a series resistor between the two microcontrollers. Size this so that the current flowing if one end drives high and the other drives low is limited to a safe value for both. This is usually about 20ma. That suggests a resistor in the 150-200 Ohm range, although for your purpose you could have any value from 150R - 10k without noticing any adverse effects.

Yes. I think you answered the question by your example. But just to be on the safe side-
Pull down/up resistors are supposed to determine the logic level at startup. Pull down will always be connected to the lowest logic level (e.g GND in your case) and pull up to the highest logic level (e.g VCC of the micro-controller in your case)
So they will be connected internally to the GND/VCC ...