kbd and lcd share the same port

lgeorge123 · Joined: 05 Dec 2004 Posts: 31

Is it possible kbd and lcd share the same port (e.g. port d ) on microchip mcu like 16f877 using ccs c with standard ccs c library ?? If so, please give me the link or diagram. I also search this forum but no result !

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

You either need additional outputs to drive the key matrix or logic to deselect the keyboard with active E signal.
Also decoupling diodes may be required. Generally, you have to care, that the LCD operation isn't blocked with
any combination of pressed keys. Following this idea, you're hopefully able to figure out the details.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

See this thread for a discussion of using an LCD and keypad on the
same port, using CCS example files:
http://www.ccsinfo.com/forum/viewtopic.php?t=26229

John P · Joined: 17 Sep 2003 Posts: 331

I've done this, and in fact I made each pin that sends data to the LCD do 2 other jobs--read a keypad, and drive an LED or a transistor (the transistor controls the LCD backlight).

As FvM says, "Generally, you have to [take] care, that the LCD operation isn't blocked with any combination of pressed keys". That's true, but there may be times when it's acceptable to let the user be careless, and take the consequences. In my design, combinations of keys are allowed if they're in the same row. Otherwise yes, the display is likely to show the wrong thing.

Here's part of the circuit diagram:
http://files.myopera.com/John98wbr/albums/661338/KEYPAD_LCD.png

As you might expect, the processor has to go through a little dance in order to keep the various functions from interfering with each other. But it can be done!

Edited to say sorry, the significance of "using ccs c with standard ccs c library" just hit me. Yes, I used CCS C. But not the standard library.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

You can of course give up the demand that any key combination must not interfere with the display operation.
But sometimes simultaneous keypress is required by the application. Furthermore, if you share all
keyboard pins with the display, you have to do more. Possibly series resistors towards the keyboard can
be a solution. But their dimensioning must consider the display internal pullup resistors and limited current
strength of PIC outputs as well. A more general solution would be in using a tri-state buffer controlled by E.

lgeorge123 · Joined: 05 Dec 2004 Posts: 31

Thanks all of your help !!