The Mega only sees High or Low.An SPDT is usually 3 pins, with Common in the middle. Is your plan to connect the Common to Gnd, and the two sides to two Input pins on the Mega (with internal pullup resistors enabled) and have the inputs read as High unless the switch is thrown to one side or the other to take one of the two pins Low?(...)Alternately, you could use parallel input/serial output shift registers and read the High/Low states in from the shift registers. The only 3 pins are needed - Latch, Clock, and DataIn.
Every switch generates three states.
Naming a switch (in English) turns out to be rocketscience...
One more question please about the PCF8575.I'm looking at the datasheet and I'm confused how to hook up the switches. In the datasheet there is a pic with a few examples. The example shows a 100K pull-up on port 05 and a switch, but there is a second resistor just beside the switch. What's its function and what value should it be?