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]

[...] (100k down to .5k) [...]

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

[ToBeFrank]

Thanks guys. I thought I must be doing something wrong when the pull downs didn't work. I'll measure the current tonight. Fortunately, I don't have to worry about the magic smoke because the bus is guaranteed by it's specification to handle short to ground.