>I was wondering if you could modify his design to use a 16c84
>and have the dtmf chip and the lcd use some of the same
>data lines and seperate enable lines? Does this seem possible
>or would you potentially lose some dtmf tones comming in?
>(I'm new at this stuff....).
>
If the DTMF chip has an enable pin, then it can be used to share a bus with
the LCD, serial EEPROM, and other chips assuming they have enables as well.
Each chip needs a dedicated line back to the PIC to select it.  In a larger
system a decoder like a 74HCT138 could be used (unfortunately, the LCD,
93CXX, and most DTMF chips have active high enables, so an active-low
decoder wouldn't work well).  Data will not be lost since it only takes a
few microseconds to access each chip and go to the next one.  Also, many
DTMF chips latch the last tone code received until another one is detected.
I usually use the LCDs in the 4-bit mode to save PIC pins as well.

Using a bus like this, with none of the peripheral chips selected the PIC
pins can be used as inputs by driving them through resistors.  This could be
used for the 'tone detected' function from the DTMF chip as well as any user
controls that the project has.

Another note about this project, the HD44780-based LCD units have 80 bytes
of display RAM and 64 bytes of custom-character RAM, regardless of the size
of the screen.  Especially if the display is smaller than 80 characters, the
extra 128 bytes or so of RAM could be used for (volatile) scroll-back
storage instead of a serial EEPROM.  The specifications I have don't say how
much power the display units use with the display disabled though, it may
not be practical to 'sleep' them and keep the RAM alive if the
decoder/display is expected to retain the data for a long time.

-Mike