can I use a board that has resistor ladder buttons on raspberry pi?
I'd suggest that the schematic you show uses a 'time to discharge' algorithm to decide which switch is pressed (or even if multiple switches are pressed). 

On the R'Pi you could connect 'key' line to a GPIO pin using a 1k Ohm resistor (needed because of SW7) then do something like this:


Set the GPIO to output a "1", this will charge up the capacitor (there might be switches pressed so the high voltage depends on which one(s) are pressed)
Setup an interrupt on the GPIO going low
Read the system timer (like this)
Set the GPIO to input with no internal pullup
When you get an interrupt read the system timer again and store it (the difference value is related to the discharge time of the RC network to the GPIO low level). Then you would execute a RET
Back in your mainline code you have both the initial and second system timer value ….subtract the two to get the difference.


It's not very accurate, but good enough to find out which switch is pressed. It's going to be fun to catalog all the variations if multiple switches are pressed, but should be doable.