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I'm trying to get a keypad to work, writing my own library (just because I want to learn it, and later use the keypad).

For the columns I need pullup resistors, and I initially selected the internal pull-up of the GPIOs of the STM32F103C86.

After quite some time debugging I found out it worked much better with 1K resistors, instead of the 50K internal pull-up resistors.

So since it matters much what the pull-up value is (at least for the keypad), I was wondering, why MCUs are just having one resistor value for pull up (or pull down) ?

Is it because the keypad is an exception (i.e. for most components a 'generic single value' pull-up is enough?

I think a resistor is one of the most simple components to add in a complicated MCU, so why not have more of them?

Just wondering, what the generic reason is (this question is similar for Arduino's and probably more MCUs).

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  • \$\begingroup\$ Ok clear, I was thinking a resistor would be 'easier' than a transistor (where millions can be put in a small space), but if the area of a resistor is bigger, than it indeed is a valid design decision not to add 'too many'. \$\endgroup\$ Feb 22, 2020 at 23:33
  • \$\begingroup\$ I think the weak pullups on typical micrcontrollers are actually implemented with a transistor. Some analog IC's have actual on-die resistors and they do take up a lot of room. \$\endgroup\$
    – user57037
    Feb 22, 2020 at 23:53

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You not only need a resistor but also a device to enable/disable it, a register to hold its configuration as well as circuitry to write/read that register.

Also although resistors are simple they actually can take up a lot of die area and so usually a very small transistor is used instead - the actual value will vary greatly from device to device and over temperature (maybe +/-35% from nominal).

The pull-up/pull downs in a GPIO port are not normally used as part of an active circuit where the value is critical but as a way to set the default value when the pin is not driven. I would not recommend using them as I2C pull-ups for example.

Overall the additional functionality by adding a choice of pull-up resistor values is almost certainly not worth the die cost, especially compared to other I/O features such as a pull-down, or selectable drive strength.

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  • \$\begingroup\$ Thanks, and clear. \$\endgroup\$ Feb 23, 2020 at 0:25

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