teensy 4.0 input pullup in parallel with external pullup..... == voltage divider?

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zumdar

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Hey yall!

I just now realized the Teensy 4.0 is only supposed to handle 3.3 volts....

I did something probably dumb and made a pcb that takes a bunch of normally open push buttons, pull them to 5V through a 10k resistor and go into the teensy. I also used the INPUT_PULLUP in the software.

when i measure the voltage at the teensy input it is like 3.9 volts!! soo. maybe i'm not totally screwed ? Is it possible that the internal pullup of about 33k tp 3.3V and my external pull up of 10k to 5v are making a kind of voltage divider and I am in the clear? or do i need to kludge some wires and change that 5v to a 3.3v..?
i layed the PCB out in the worst way so changing the 5v going to all the resistors is going to be a real pain. everything actually works fine, but this is going in a permanent installation and I won't be there to monitor it and fix things when it goes away from me in a few days.

heres my schematic so you can see clearly what I'm doing. all the labels are going to different pins of the teensy.
Tz87Ucz.png

THANK YOU for any guidance! i just realized this might be bad and it has to leave my shop in a few days!
 
If you are lucky the current was limited enough not to damage the protection diodes on the chip. 10k is
a fairly large value and limits the current to 1.7/10k = 170uA.
 
With active INPUT_PULLUP coming from the T_4.0 - could the 5V just be cut? Remove the resistors or stop the source 5V feeding them?

That would leave the T_4.0 holding 3.3V until the NO button is closed.
 
ohhhhh shooooooot! you are so right.... i could just cut them! keep it simple stupid hahaha. ill try that later with just one :)
thank you. that is definitely the perspective i needed
 
I took out all the external pull ups and it works great. I measured voltage at the teensy pin before taking out the 10ks and it was 3.78V. So 1.22 volts dropped across the 10k. If you do the math with a 5v supply, 10k resistor, 33k resistor and 3.3v supply, the expected voltage there in the middle would be 4.6volts! Maybe the internal pullup is only 3.3k? That would make my pin voltage around 3.7V. Idk just musing here. I dont know what all is going on inside the T4!6-B071736-FE7-B-4716-9-E47-5136-F8754685.jpg
 
Great that was an easy solution for the situation to get your system in spec and ready to ship and install.
 
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