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MNapi
MNapi
Senior II
April 18, 2023
Question

Is internal pull up the same as external pull up resistor.

  • April 18, 2023
  • 4 replies
  • 7187 views

for example IC2 needs pull up resistor on the board but there is internal pull up options for pins.

I wonder if this is the same. So by setting internal pull up you can skip the pull up resistors on the board. I know that NRST pin has internal pull up resistor.


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    4 replies

    KnarfB
    KnarfB
    Super User
    April 18, 2023

    No. internal pull-up has a much higher resistance(~40 kOhm? check data sheet) than a typical I2C pull-up (4,7 kOhm, depending on your setup).

    hth

    KnarfB

    MNapi
    MNapiAuthor
    Senior II
    April 24, 2023

    I followed example from CubeMX to connect 2 Nucleo boards F446, using I2C and they use internal pull up and it does work. I wonder if I can use for example internal pull up for example for SDMMC.

    DTomi.1
    DTomi.1
    Senior
    April 24, 2023

    What was the clock speed used in this example?

    Keep in mind that this 40 kΩ weak pull-up in combination with line and input capacitances is forming a low-pass filter.

    So it depends how long and wide your PCB traces are (rule of thumb is 1 pF/cm length for 0.2mm trace width and 0.2mm height above plane - prepreg thickness) and how many slave devices you have on the bus (I/O pad capacitance - around 5 pF per pad).

    For example a 10 cm PCB trace (10 pF) + input capacitance (5 pF) = 15 pF

    which gives a rise-time of 1.3 us so not good at all for anything above around 500 kHz.

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