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asrock70
asrock70
Associate III
November 25, 2012
Question

STM32 5V tolerant I/O ?

  • November 25, 2012
  • 4 replies
  • 7526 views
Posted on November 25, 2012 at 11:22

I'm sorry, but I do not quite 5v tolerant ports for STM32

.

I do not unterstand concept ''

Total injected current (sum of all I/O and control pin)''

Example

STM32F0 and on 8 5V tolerant I/O connected LED with resistor powered from +5V (resistor limit current of every LED on max 5mA).

After reset is I/O configured as input. What current will flow through the I/O

port from 5V over

LED

+ R? I

t

injection current and

exceed this design Total injected current 25mA?

In

In

RM0091

Reference manual page 121 read protection diode on FT ''

V

DD_FT is a potential specific to five-volt tolerant I/Os and different from VDD''. W

here the

potential

at the cathode

takes?

I/O switched, on Output with open drain (No pull-up, pull-down), if port is set on 0 LED light and current to I/O is 8x5mA.

    This topic has been closed for replies.

    4 replies

    Andrew Neil
    Andrew Neil
    Super User
    November 25, 2012
    Posted on November 25, 2012 at 16:28

    ''LED with resistor powered from +5V''

     

    Why create trouble for yourself?!

    Why not just power the LEDs from 3.3V??

    A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
    asrock70
    asrock70Author
    Associate III
    November 25, 2012
    Posted on November 25, 2012 at 22:52

    While this was just an example, but I see a few reasons why that might be.

    1. LED is blue and need 3,6V

    2. All circuit is 5V and STM32 is powered over little LDO in SOT23

    3. Optotriac with second signaling LED in series

    4. 40 mA LED flash will certainly affect the accuracy of the power and ST decided to use as a reference for the ADC supply voltage VDA. LED power from different sources may affect the accuracy of the ADC

    However, as I have already said this is an example where I would like to understand things around a 5 volt tolerant I/O

    waclawek.jan
    waclawek.jan
    Super User
    November 26, 2012
    Posted on November 26, 2012 at 11:36

    C'mon gentlemen... 5V-tolerant IO means that you can apply 0-5V on an input pin, and drive into 5V-connected load (within the allowed current limit) safely.

    What is meant by injection is discussed in the datasheets quite extensively, e.g. in STM32F051x datasheet in chapter 6.3.12. Simply speaking, in the 3V-only pins (and some other too) it's the maximum allowable current into the protection diodes, so it flows if the input voltage exceeds 0V (in the negative direction) or the supply voltage.

    The 5V-tolerant pins can't be protected by a simple diode into the positive supply rail, so there is a level shifter there. Don't think of it as a power supply of higher voltage, it's not like that - there are several different techniques out there but regardless of what is the true implementation you can safely think of it as a Zener diode - in this case a 4V into the VDD rail (there is one consequence of this, namely that you should not apply 5V into the 5V-tolerant pin of unpowered device). That's why the datasheet states, that there's no positive injection into the 5V-tolerant pins until the VDD+4.0V is reached (sadly they don't state what would be the allowed positive injection above this).

    Regardless of injection, even if staying within VSS and VDD, you should still count with *leakage* current, which is in the order of microamps and is specified elsewhere in the datasheet.

    I hope this helps to understand this issue a bit better.

    JW

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