STM32F411 Output pin connection while no VCC is applied

[Nir Alon]

Posted on April 01, 2018 at 13:22

Hello all,

In my application, I'm controlling a

secondary 

device using enable pin which has an internal pull-up resistor.

The output pin is set to be ''0'' or open drain type output, and the STM32F411 VCC1 can be powered down while the

secondary 

device's VCC2 is still active.

Can I connect the output pin directly to the secondary device enable pin or should I place a Tri-state buffer between them?

Thanks!

Nir

[T J]

Posted on April 02, 2018 at 10:03

easy enough with a transistor or Fet,

do you want it to operate only when you are powered up ?

then you need to signal high to indicate ON

use a BC817 with a 1k from the processor to the base.

this way the trigger can only operate when the processor is up.

[Nir Alon]

Posted on April 02, 2018 at 10:29

I will rephrase the question - can the stm32f411 output handle (In terms of damage) the 3.3V VCC2 voltage (coming through the pull-up resistor) while VCC1 is 0V?

[T J]

Posted on April 02, 2018 at 14:20

It use to be, that a pin will draw current if it gets to 1.2V (ish), and appear as a low voltage drive pin.

Please correct me guys, I know you will...

but I think you will power the processor 3V rail from the protection diodes within the pin IO structure.

thereby sinking 20mA or more which may take out the pin after a short period.

other than that, I think you will be ok..

[Nir Alon]

Posted on April 02, 2018 at 15:50

Hi

I just came across this figures (page 60 - DocID026289 Rev 7):

 

If I understand correctly, than if VDD=0 I can apply 4.0V at any pin safely without any damage to the pin.

(In my case it would be 3.3V through a 100K pull-up resistor)

Am I right?

Nir

[T J]

Posted on April 02, 2018 at 16:01

with a 100k. that's 10uA per volt.

that's 20uA flowing into the pin, charging it to where the protection diodes will start to conduct

clamping it down to roughly 1.2V

should be ok, I guess.

did you try it ?

what will your drive do at 1.2V ?

[LMI2]

Posted on April 02, 2018 at 19:46

CPUs have a common earth I suppose? Some chips may boot up badly if there is already current flowing into one pin when VCC is rising.

[Andrew Neil]

Posted on April 03, 2018 at 02:41

But look at Note 1 beneath the table ...

[Nir Alon]

Posted on April 04, 2018 at 07:01

1. CPUs have a common ground

2. Yes, I build it and and while the VCC1 MCU is on, the output changes between 0 - 3.3V as expected, and when the VCC1 MCU is off, I measured 3.3V coming from the secondary MCU. Powering up the VCC1 MCU again was just fine.

3. Andrew, did I missed something in note 1? isn't it obvious that VDD, VSS should work in the permitted range?

Thanks again guys,

Nir

[Nir Alon]

Posted on April 04, 2018 at 11:17

So...the only pin that can accept voltage while VCC=0 is BOOT0?