Hello.
I can't find out the truth! Reading a lot of pages about Pico GPIO current limits I am still confused about limiting current when the GPIO is set to input. Why do I need to? I get that the GPIO has a current limit as an output, and I presume the term source or sink current refers to an GPIO sinking current when set as an output and driven low. But does the Pico sink significant current when set as an input? I would have thought it would be very high impedance as an input. And yet there are web pages that state the pin should be current limited when set as an input. And there are pages (including Adafruit) that show no current limiting resistor on the input and using the weak pullups. See an example below, 3.3v out directly connected to an input pin:
The reason I am asking is that I am looking into whether I can attach encoders over long ish cables and use fairly beefy pullup resistors. The 3.3v out is limited to 300ma so surely that is the factor that determines what pullup resistors I can use? When I say long cables I am talking 7m maximum and I am trying to avoid having to use optos.
I get the feeling that because the limit refers to GPIO output my searches return ambiguous results because it doesn't apply to input and so my question gets lost in the mass or results pertaining to "GPIO" and "current limit"
I have tried to find the answer myself many times and I am none the wiser.
I can't find out the truth! Reading a lot of pages about Pico GPIO current limits I am still confused about limiting current when the GPIO is set to input. Why do I need to? I get that the GPIO has a current limit as an output, and I presume the term source or sink current refers to an GPIO sinking current when set as an output and driven low. But does the Pico sink significant current when set as an input? I would have thought it would be very high impedance as an input. And yet there are web pages that state the pin should be current limited when set as an input. And there are pages (including Adafruit) that show no current limiting resistor on the input and using the weak pullups. See an example below, 3.3v out directly connected to an input pin:
The reason I am asking is that I am looking into whether I can attach encoders over long ish cables and use fairly beefy pullup resistors. The 3.3v out is limited to 300ma so surely that is the factor that determines what pullup resistors I can use? When I say long cables I am talking 7m maximum and I am trying to avoid having to use optos.
I get the feeling that because the limit refers to GPIO output my searches return ambiguous results because it doesn't apply to input and so my question gets lost in the mass or results pertaining to "GPIO" and "current limit"
I have tried to find the answer myself many times and I am none the wiser.
Statistics: Posted by jimseng — Sun Jun 29, 2025 2:12 pm