Startup pins status

Hi. I want to know what happens immediately after microcontroller is given power. Does all bits goes high at startup for a very short period of time? If yes then how to keep them low even at very startup.

To pull a terminal down briefly at powerup...
Attach a capacitor to ground. Attach a resistor to V+.
A sensible R value is 1-100 kohm. Size the C value to create your desired time delay. As the cap charges, the resistor pulls the terminal high.

Hi,

In detail the datasheet will give answers.
There may be pins that are "floating" after power up, there may be pins that are pulled to a known level (dedicated I2C pins may be internally pulled up).
To be sure you need to read the datasheets - like all of us.

So at most microcontrollers I've seen the port pins go "high impedance" and not "HIGH" as you assumed.
The usual method to get a known state at power up is to use external pull up or pull down resistors. Whether an additional capacitor is useful depends on your requirements.

Klaus

Note: at most of my microcontroller projects there are some pins externally pulled HIGH or LOW. Like /SS, /CS, /RD, /WR...
This avoids short circuit that happens when multiple bus partners actively drive data lines at the same time.