Switchable pullup/pulldown resistors

[wlamers]

I have a digital input to an embedded system that needs switchable pullup and pulldown resistors. The input has nothing more than ESD protection and a 3.3v clamp diode to prevent high voltage input signals to be saturated. The switching frequency is max 1 Mhz. The pullup and pulldown resistor have to be switched on and off using 2 GPIO pins on a microcontroller. Can I use some sort of a transistor or MOSFET for doing this? What do I have to look out for? Thanks for your input!

 

[ARQuattr]

If you don't need a strong pull-up/down, you could simply attach a resistor to a single GPIO pin, with the other end tied to your input. This is assuming the same value for pull-up and pull-down can be used, otherwise you can use both GPIO with one resistor on each. One GPIO (the pull-up one) would go high while the other is set as high-impedance (an input), or the other way around. Again this assumes the pull-up/down current doesn't exceed the maximum GPIO pin output current. Otherwise would drive external transistors with those two GPIO.

 

[KlausST]

Hi,

switching frequency: you mean signal switching frequency? (not pullup/pulldown switching frequency)

**
i´d do it like ARQuattr posted.

But i wonder why you talk about TWO GPIO pins.
Are there multiple signal IOs, where you want all to pullup or all to pulldown?

If you can´t go the way of ARQuattr then a simple drawing would clarify.

Klaus