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..

[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.

[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

[Andrew Neil]

Posted on April 04, 2018 at 11:12

It means that the STM32 must be powered.

So the answer to your original question:

Can I connect the output pin directly to the secondary device

is, 'No'.

[Nir Alon]

Posted on April 04, 2018 at 11:17

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

[Andrew Neil]

Posted on April 04, 2018 at 11:37

 ,

 ,

No.

See: ,

https://community.st.com/0D70X000006SsgsSAC

 ,

[LMI2]

Posted on April 04, 2018 at 18:11

Where should that link point to? Nothing happens

[Dvorak.Peter]

Posted on April 04, 2018 at 20:12

Simple 

Change the polarity of logic and use input pulldown. For extra safety use 2K series resistor.

Peter

[T J]

Posted on April 05, 2018 at 00:37

then I think the Output will be always on while the processor is asleep,

that's why my original post had a transistor, relieving all issues..

[Simon Peacock]

Posted on April 06, 2018 at 04:23

1/ When the STM32F411 powers down, its input diodes will try to power its internal PSU circuits.  This shouldn't be an issue as the 411 will take more current than is available.  You may however get hick-ups... the resistor charges the capacitors around the 411 to a point where the 411 turns on.. it then starts, drains the capacitors are stops again.. only to charge and start again (this mostly happens at higher temperatures).

2/ It might be wise to use an external pull-up due to internal semiconductor resisters being highly variable.  Even better, when the 411 is powered down, turn the pull up off to avoid any issues.

3/ Option C... external resistor pulled up to the 411's power rail, 3 V will still turn on the 3.3 V micros logic

4/ Option D... no resistor, totem-pole the output 3V into 3.3V is cool, pull down for safety (if required)

Simon