Electronics > Projects, Designs, and Technical Stuff
control pin state at power up
ToBeFrank:
I'm using an Arm Cortex M0 micro. The reset state of all gpio pins is input with pull-up enabled. I need one pin to be low on power up because it is connected to a bus where high is considered to be active on the bus. What can I do to make sure this pin is low at reset? I tried various values of pulldowns (100k down to .5k) and none of them worked.
Bored@Work:
Measure.
If a 500 Ohm resistor can't pull down the bus it could be
- that you have connected another pin to the bus, i.e. you are messing with the wrong pin
- that something else is pulling the bus up
Therefore measure what is really going on on the bus, e.g. how much current is going through your pull-down resistor.
And if you are willing to risk some magic smoke, use a stiff wire to pull the pin to ground. The component that starts to get hot or releases the magic smoke is likely the one trying to pull the bus up.
A Hellene:
Well, how about using a Reset Supervisor chip (in a TO3/SOT23/etc. 3-pin package), to be sure?
-George
Bored@Work:
It doesn't make sense to throw more hardware onto the problem if you don't know what the problem is. So let me repeat: Measure.
A Hellene:
--- Quote from: ToBeFrank on August 03, 2011, 07:29:46 am ---[...] (100k down to .5k) [...]
--- End quote ---
Oops... I thought I read that the pull-down range resistors were 100K .. 5K...
Sorry for that. If a 500 Ohm resistor failed to pull the line down, YES, measure and investigate what the strong pull-up source is.
-George
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