2018-04-01 04:22 AM
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
2018-04-02 01:03 AM
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.
2018-04-02 03:29 AM
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?
2018-04-02 07:20 AM
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..
2018-04-02 08:50 AM
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
2018-04-02 09:01 AM
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 ?
2018-04-02 10:46 AM
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.
2018-04-02 07:41 PM
But look at Note 1 beneath the table ...
2018-04-04 12:01 AM
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
2018-04-04 02:17 AM
So...the only pin that can accept voltage while VCC=0 is BOOT0?