PIC 12F508 Logic Level Calculations

Most microcontroller datasheets have a section "(DC) electrical characteristics", usually after the functional description of the device. Electrical characteristics usually start with "Absolute Maximum Ratings" (AMR), which you may want to look at too, and especially the note below the table.

You'll see that there are different tables for "Industrial" and "Extended" parts. This refers to their operation temperature range; Industrial is up to 85 °C, extended up to 125 °C. You'll probably want Industrial, which is the standard range. Then on page 67 (Acrobat Reader page 69) you'll find \$V_{IL}\$ and \$V_{IH}\$, which is the maximum input voltage which will be read as a low level and the minimum which will be read as a high level. Most microcontrollers only have one type of input, and then there's just one value for each, usually as a fraction of the supply voltage.

For the standard type I/Os you have

\$V_{IL}\$ is 0.15 \$V_{DD}\$ maximum, so for a 5 V supply that's 0.75 V.
\$V_{IH}\$ is 0.25 \$V_{DD}\$ + 0.8 V minimum, for a 5 V supply voltage that's 2.05 V.

Notes:
1. At the top of the table the conditions for these parameters are mentioned. Sometimes that includes power supply voltage range, but here it's only temperature range.
2. The equation for \$V_{IH}\$ with the constant factor of 0.8 V is a bit unusual. You'll more often find a value like 0.7 \$V_{DD}\$ = 3.5 V.

MCLR is simply the !RESET line on the PIC processors - that is, any time MCLR becomes low, the processor will reset (it is possible to disable this functionality in the processor configuration). Even though it's tempting to avoid the (unlabeled pullup) resistor altogether and just tie MCLR to VDD, it will cause problems if you try to connect to the ICD/PICKit programmer/debugger, since the ICD will no longer be able to pull MCLR low to enter debug mode and program. Having MCLR floating without disabling the reset functionality is of course a bad idea, as your processor will randomly reset depending on RF noise, your hand touching it, etc.

The capacitor is actually there with the series resistor (R1) to form a R-C circuit. This is to delay the start-up of the PIC so that the power supply has some time to become stable before code execution starts. Typically this is not needed if the power is already from a stable supply (such as a battery), but might be required if the power source is something that has significant ramp-up.