If the pin is not A/D but smith-trigger, it is possible perform that reading in a much more complex way, but working with RC circuit timming.
I done with only 3 keys. I´m not sure if works properly with 8.
Works this way :
Each 1 of 3 keys, is connected to a resistor ( R1,R2,R3 ) from VDD bus, and all them are conected to the same capacitor-resistor discharging circuit ( C//R ).
Values of 3 resistors are scaled from ~3x one from other.
Time account of Rise/Fall determines wich key were pressed/released.
Obviouslly, RC timing must be chosen in order to be lower than user typing rate.
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