8-bit Microcontrollersのトピックoutput pin state at power up

output pin state at power up

irob — Thu, 07 Sep 2006 03:42:20 GMT

Hey gang,

Does anyone know how to set the level of output pins during power up? For instance, I have a few outputs which crucially must remain low during power up until I set them as outputs, and start to use them.

If memory serves, back in the 68HC11 days one had to set the pins in question to the desired state before setting their direction registers.

I've done this (setting pins low before their DDRs) and somewhat arbitrarily one of my outputs wants to float high while another floats low.

Any ideas?

Re: output pin state at power up

rocco — Thu, 07 Sep 2006 05:20:10 GMT

Hi, irob:

From the time reset asserts (or power is applied) until you write the DDR, the pins are inputs, and might float wherever they please.

The only way that I know to make them behave during reset is to use either a pull-up or pull-down resistor. 10k is my favorite value, but it depends on what is being driven by the pin.

My experience is that it is usually the not-yet-driven input that determines in which direction the net floats, due to leakage current.