Startup pins status

[HighTechPower]

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.

 

[BradtheRad]

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.

 

[KlausST]

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.