Today I got my first comment on this blog, and I appreciate it a lot. Hopefully more comments follow.
The switchpad of the calculator came from Jameco, and was basically 16 switches in a grid, as displayed in this schematic. To connect them to the PIC16F877A was simple: on the read side the voltage is pulled up to a known voltage (5V makes perfect sense) using a resistor network. The values of these resistors are arbitrary. I like 100k Ohm because this reduces power consumption. Speed does not really matter as they are switches anyway, and their fastest response time is 10ms. On the scan side you just connect the pins to the PIC16F877A. To read it is a matter of setting a particular line to 0V and see if any of the receiving ports detect it. If this is the case, that particular switch was pressed.
The software is slightly more complicated, because switches tend to "bounce" a bit, and it needs to be debounced, preferably over a 20ms range. So if there are changes faster than 20ms, these should be ignored, and the system should wait until the signal is stable for over 20ms. This is relatively easy to code as well, though.
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