Pins RB0, RB1 and RB2 are used for controlling AND driving text to the LCD display.
Most of the time the PIC's are sufficient enough for most applications, except when
it comes to more I/O. This simply cannot be expanded, except when you go to the BIG
GUYS like the PIC16C74 etc. where I have done some applications with too.
Observe the schematics: An PIC16C54 is the heart of the whole thing. It drives the
HEF4094 CMOS serial2parallel converter. This gives us the databus towards the LCD
display. Since the HEF4094 strobe is activated at the rising edge, and the LCD
display on the falling edge, these can be shared. So, on the rising edge the 4094
spits out it's new byte, and on the falling edge the LCD reads it in. By the way,
this concept cannot read out info from the LCD display. (Personal opinion: It is
useless anyway!) Now the hard part comes: How to derive 'text' from 'commands'??
The LCD has a pin for it: The RS-pin. When it is clear, commands are accepted. when
set, text is accepted. How is it solved?
Before I spit out a character to the HEF4094, I set the clock for 500uSec. Resistor
R1 will load capacitor C5. Then, I spit the text character towards the 4094 as soon as
possible. Therefore the capacitor simply does not have the time to discharge: The LCD
will accept it as text.
For commands it is the same, however, of course the other way around: The capacitor
must be discharged. T1 forms an emitter follower to buffer the R/C network. The reason
for this is that the LCD RS input is an TTL input, so without proper buffering it will
The code contains a few basic routines to handle the LCD display. The switch
that I added is purely for fun: To be able to toggle rotation of the text. I used an
16 characters / 2 lines LCD display from an old security keypad. (Go to a surplus
electronics store, they always have some!)
P.S. Any suggestions for good code from YOUR side are always welcome!
Best Regards from msimons@IAEhv.nl, your PIC Scueezer Weezel!
MSIMONS.ZIP includes the schematics, source code and include file needed for this example.