Page 157 - Making PIC Microcontroller Instruments and Controllers

sErrtr{G out oun bEstclt ||{tEltt rae

I The E line, which needs to be toggled to transfer data to the LCD
I The RS line, which selects the rcgister
r The (R/W), which sets the read/write status ofthe operations

Using eight lines for data allows us to generate allthe codes and all the characters
that the chip has in its memory, but more impoftandy, it allows the data [ansfer to be
performed in one step. We can also use a 4-bit protocol and transfer half a byte at a
time. Using four lines for our control scheme means the LCD can be controlled from
just one port (seven lines will be used, leaving us one line to spare). The datasheets
tell us that whether we use four lines or eight, they all must be part of one port, and if
we are using fourlines, they must be the cortiguous fourhigh, or the contiguous four
low, bits of a po -meaning we cannot use just any random lines for the data buss.
The data transfer for the 4-bit prctocol must use fhe I high bits on the LCD, and \\te
must send the datatom tlre PIC to the tlisplal with the high tu a nibble frst and the
low data ibble last. This little gem is not speUed out in the instructions, btlt it's wha't
must be done,
The datasheet also tells us that t}Ie display initializes itself on power-up. We can
reinitialize it under ourcontrol, butil is done automatically on starlup and we cannot
"not
inhibit that. All we have to do is do anything" for about halfa second for the self-
initialization to complete. The busy llag is set high during startup and initialization,
but is indeterminate immediately after initialization starts and for 16.4 milliseconds
after the supply voltage reaches 4.5 volfs, so we cannot determine how long we have
to wait afler powering up to start doing what we want. We will set a o.s-second
wairpause in our progmm at startup. If that is not lorg enough, we will come back
and increase the waiting time. Wait tine is a must. Ifyou do not wait, t}Ie system will
not start up properly.
Automatic initialization sets the foliowing conditions for the display:
I Display cleared
r Set for 8 bit interface
r Set for 1 line ofdisplay
I Set for 5 x 7 dot matrix display
I Display is tumed off
I Cursor is tumed olT
I Blink is tumed off
I Increment between chamcters is set to I (culsor moves over one space automatically)
a Shift is off

The preceding may not be exactly what we want for o r purposes, so we will go
tbrough an initialization sequence as specified by the instrnctions. We do not have to
go through all the steps, rar we rrill, sowe have a completerecord ofwhatneeds to be
done for future projects.
The instructions tell us that the follo\'r'ing six instructions must be sent to the display
during an initialization sequence. The lirst three insfuctions are identical but require
diifercnt waits after each is sent to the display.