PullNone input mode not working as expected?

Electronics and hardware
1

Hi all - thanks in advance for any help or suggestion.

I’m referring to the schematics below, used for level conversion of a Nucleo L432KC board 3.3v logic to 5v, for an external old CMOS circuitry sharing in and out on a single line (CONV_OUT). Please don’t ask why conversion is done like that. Let me just say it’s for a project now on PCB, so relatively hard to change…

Setting the input pin mode to “pull none”:

DigitalIn             in_D_OUT    (PC_1); 
DigitalOut        out_D_OUT   (D11);     
...
in_D_OUT.mode(PullNone);

Then, setting the output pin values, I measure on CONV_OUT:

out_D_OUT = 0 --> 0.00 V
out_D_OUT = 1 --> 3.76 V

I expected the high level to be about 4.3 V (considering the 0.7 drop over the diode), while it’s actually much lower than that. This unfortunately is not enough, for a reliable behavior on the external device.

Irrespective of how I set in_D_OUT.mode ( PullNone, PullUp, PullDown ), I always get out 3.76 V high level.

Only if I physically UNPLUG the in_D_OUT pin from Nucleo, I get the expected levels:

out_D_OUT = 0 --> 0.0V
out_D_OUT = 1 --> 4.33V

I thought “PullNone” mode to be equivalent to virtually unplugging the GPIO pins, but that does not seems to be the case.

Am I doing, or evaluating, something wrong?

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2

I noticed an (old) answer to a somewhat related question:

https://os.mbed.com/questions/80894/DigitalIn-ve-DigitalOut-modes/

saying possible pin modes are:

typedef enum {
    PullNone          = 0,
    PullUp            = 1,
    PullDown          = 2,
    OpenDrainPullUp   = 3,
    OpenDrainNoPull   = 4,
    OpenDrainPullDown = 5,
    PushPullNoPull    = PullNone,
    PushPullPullUp    = PullUp,
    PushPullPullDown  = PullDown,
    OpenDrain         = OpenDrainPullUp,
    PullDefault       = PullNone
} PinMode;

In that case, I’d try

OpenDrainNoPull

My doubt is whether the pin mode settings apply to both the output and input parts of the GPIO port, even if I set it for an pin defined as DigitalIn?

thanks
Fabio

3

“PullNone” just means it floats about as nothing to pull it to ground or nothing pulling it up to Vcc.

4

Exactly, so, with a PullNone, I expected in_D_OUT not to influence the behaviour of the level converter on out_D_OUT.
That is not the case, looks like.