[![the output of my GPIO is 1.8V which is pulled up to 3.3V using a 4.7K resistance and that gpio is connected to the gate terminal of a P-channel mosfet, when i measure voltage across the gate terminal it results in 1.8V and i got 1.8325V by nodal analysis at gate terminal but i'm not getting why so, can anyone provide me what things should i learn to solve these kind of problems][1]][1]


closed as unclear what you're asking by TonyM, Finbarr, Chris Stratton, DKNguyen, Elliot Alderson Oct 19 at 12:32

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Please add a schematic of your circuit, and datasheets of both the MCU and the transistor. \$\endgroup\$ – Sorenp Oct 18 at 12:38
  • \$\begingroup\$ Welcome to the site. Please realise this is not a free design house, homework-answering service or an on-line technical encyclopedia, copied out to you on demand. People will help you take the next step if your question shows you've done as much as you possibly could on your own - which yours doesn't, I'm afraid. Please edit your question and greatly improve it. Show your work and findings so far in considerable detail with a schematic. The schematic tool here is easy to use. The better the quality of question, the better the quality of the answers you will attract. Again, a warm welcome. \$\endgroup\$ – TonyM Oct 18 at 12:39
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    \$\begingroup\$ Probably your ESD protection diodes towards the 1v8 GPIO supply rail start conducting. \$\endgroup\$ – Oldfart Oct 18 at 13:12
  • \$\begingroup\$ @TonyM thanks for all the above comments, i'm a fresher i don't have any experience so when i face a problem i'm always out of solutions i hope by experience the quality and clarity of my questions will increase. \$\endgroup\$ – Vignesh C Oct 19 at 6:06
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    \$\begingroup\$ @VigneshC ignoring the specific requests for critically missing information will not get you far \$\endgroup\$ – Chris Stratton Oct 19 at 6:27

The output is very likely a push pull stage which has a (relatively) low output resistance.

What you effectively have is this (assuming you are using a CMOS device)


simulate this circuit – Schematic created using CircuitLab

The output when high will have M2 on and M1 off.

The output of this stage when high will have a typical resistance to 1.8V of perhaps a few tens (sometimes higher) of ohms; the voltage divider formed of M2 and the pullup resistor will maintain the output voltage at close to the GPIO drive voltage of 1.8V

You could use an open drain output but ensure it is safe to use with a 3.3V pullup (that will be in the datasheet).

The effective circuit in the high output state is this:


simulate this circuit

From the voltage divider principle, the voltage at Vout will be 1.83125V (assuming the on resistance of M2 is 100 ohms).

You should not normally pull a push pull / totem pole output beyond its supply voltages whether that be positive or negative.

  • \$\begingroup\$ can you please elaborate how the divider formed will maintain 1.8 V at output and can you suggest me any articles to gain knowledge regarding these subjects. \$\endgroup\$ – Vignesh C Oct 18 at 12:57
  • \$\begingroup\$ @VigneshC Answer updated with equivalent circuit. \$\endgroup\$ – Peter Smith Oct 18 at 13:07

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