Connecting 3.3V out pin of MCU to 3.3V from other component

Thread Starter

jai.whitey

Joined Oct 19, 2021
16
Hello - if I want to connect a 3.3V GPIO PIN from my MCU to another module that accepts 3.3V input voltage on its pin. I understand I can't simply connect them directly as this may cause too much current inrush, so good practice is to put a resistor, say 10K ohms, between the pins. My question is: should I simply put a 10K resistor between the pins with no ground connection, or should it be a pull-down resistor that is connected to ground?

Thanks
 

MrChips

Joined Oct 2, 2009
25,931
Your situation is unclear.

What is the part number of the MCU? Which pin are you trying to connect?
What is the part number of the other module and which pin are you trying to connect?

In general, if one of the two pins is an input pin then it is safe to connect the two directly.

Tell us more and why you think that this presents a problem.
 

Thread Starter

jai.whitey

Joined Oct 19, 2021
16
The MCU is a PIC16F1 MCU and the other module is a UART module. I'm basically connecting the RX/TX pins together between the MCU and UART.

So it seems it's safe to connect the two directly with no resistor.

Thanks for the help.
 

MrChips

Joined Oct 2, 2009
25,931
The MCU is a PIC16F1 MCU and the other module is a UART module. I'm basically connecting the RX/TX pins together between the MCU and UART.

So it seems it's safe to connect the two directly with no resistor.

Thanks for the help.
Not quite safe yet.

We still don't know what you are attempting to do.
Tell us which pin you want to connect to which pin. Be specific.
Give us full part numbers and pin numbers or show us a full circuit schematic.
 

MisterBill2

Joined Jan 23, 2018
11,624
Digital Integrated circuit devices have three classes of pin types: Power pins, control pins, and signal pins. Within the signal pin type are inputs, outputs, and combination input/output pins that can be either, depending on internal instructions.
Within a given logic family, CMOS, TTL, ECL, and what ever else may be selected, if the output voltage levels match the input voltage levels, no current limiting resistor is needed. For using an input as a constant, feeding from the supply power source, many of the higher current logic families did need current limiting resistors. TTL and possibly ECL still do, I think. CMOS never needed such resistors. For connecting a GPIO pin on an MCU to a logic input on a UART you will not need a resistor if they are the same voltages for hi and lo.
 

ErnieM

Joined Apr 24, 2011
8,274
The MCU is a PIC16F1 MCU and the other module is a UART module. I'm basically connecting the RX/TX pins together between the MCU and UART.

So it seems it's safe to connect the two directly with no resistor.
This post make so much more sense than your first one.

I would welcome you to start a new tread with your full schematic and invite comment, some of which will be helpful and relevant, until the thread devolves into a "spirited discussion" about which way electrons go around the circuit in the southern hemisphere.
 
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