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you might get some problems if you really want to do that.
You can reuse RW RS and the DATA pins of your PIC while E is inactive (L) but you can not easily connect keys (which switch to GND) because you never know when a key is pressed and this will give you severe problems if not done right.
But if you are interested I can show you how to scan 12 keys with only 4 pins of your PIC (you will also need 4 diodes and a little bit of software).
C-Man is correct about the problem with Key press disturbing the LCD write operation. However, you may still be able to control this by adjusting the pull ups (and diodes) in the keypad matrix.
But then, you said no additional hardware so I suppose resistors and diodes are out? Or do you already have the resistors (for pull up on the keypad matrix)? If so, then adjusting values may help.
Best way is to wire the keypad in 2x8 matrix (since you already are using 8 pin bus the LCD) and tri state the 2 source lines for the keypad. If you can tri state these lines, a key press would NOT affect the LCD write operation.
I am not sure if I am clear in my description. There used to be an application note in the Microchip PIC databook where the keypad lines were used with the LED bus using resistors and diodes.
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