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Alphanumeric LCD displays have become
very popular for microcontroller applications because
they can add a lot to a project in a variety of
different ways. A text message giving the user
instructions as well as feedback can make the
application seem much more "professional" and easy to
use. I like to use LCD's to help debug applications,
with breakpoints set to display variable and I/O
conditions and they are a lot cheaper than using a
microcontroller emulator. To top it off, surplus LCD's
can be found for a dollar or less.
The most popular LCD interface is the
Hitachi 44780 based LCD controller chip which provides a
fairly easy to work with interface and low power
consumption. The major drawback of the interface is the
perceived complexity of working with the interface. This
perception has been promoted by the lack of good (i.e.
well translated) and accurate datasheets and web site
information.
Often the biggest stumbling block to
using alphanumeric LCD displays is the number of pins
required to control them. For the Hitachi 44780, twelve
pins are required from the microcontroller to interface
to the display for it to work in eight bit mode. For
many smaller microcontrollers, twelve pins are not
available or will be better served in the application.
To be fair, this can be reduced to six by using the
44780's "Four Bit" mode, but this can still be more than
acceptable for most applications.
A popular solution that only requires
one pin from the microcontroller to the LCD is the use
of "Serial LCD Interfaces" to the data and signals
necessary for the Hitachi 44780 controllers.
Many of these products (such as the SLI-OEM)
are excellent and can provide useful product interface
and debugging information. The only drawback to them is
the need for properly timed NRZ serial data which may be
difficult or even impossible to guarantee in some
applications.
In this case, different approaches have
to be made. The most popular one is to use synchronous
serial data (requiring a "clock" and "data") pin to load
a serial-in/parallel-out shift register with the data
bits and "R/S" pin information. The "E" Strobe Pin is
driven directly by the microcontroller to latch in the
data from the LCD. This is shown in the diagram below:
The project presented in this article is
an enhancement of this circuit. By combining the shift
register's "Data Line" with the most significant bit of
the shift register, the "E" Strobe can be implemented
without resorting to a separate line for the function.
The 1 K resistor and diode act as an "AND" gate. A
schematic of the circuit is shown below.
The operation of the resistor/diode
"AND" gate may not be immediately obvious. When the
shift register bit is low, the diode pulls the
connection to the "E" pin low. When the shift register
bit is high, the diode will not cause any current flow
from the connection at the "E" pin to the shift
register. The resistor from "Data" to the "E" pin is a
current limiting resistor. When the shift register bit
is low and the data bit is high, then the current
through the resistor will be limited to 5 mA (for a 5
Volt logic application). At the "Data" side of the
resistor, the voltage will still be high, even though
the diode is pulling the "E" pin low.
When both the "Data" line and the shift
register bit are high, the "E" pin will be high. The
"AND" circuit could be a TTL two input AND gate (such as
a 7408), if you have an extra one available for your
application. When I originally created this circuit, I
used the same two transistor and two resistor circuit
that I used for the 89C2051 emulator in "Programming and
Customizing the 8051 Microcontroller". I saw this "AND"
equivalent circuit in an old copy of "Electronics Now"
and found that it worked well in this application.
To load the shift register, it first has
to be cleared to ensure that the "E" will not be strobed
to the LCD inadvertently. This is done by first shifting
in six "0"s to make sure that while the correct data is
being loaded into the shift register, no "high" voltage
level is passed to the "E" pin of the LCD.
Once this is done, the data can be
shifted in. The diagram below shows how the shift
register is initially cleared and then loaded with the
data to be strobed (using "E") into the LCD:
The application code, "2wirelcd.asm" is
an assembler source file written for the PIC16C84. The
file is written to be used with the "MPASM" assembler
built into Microchip's "MPLAB". I wrote the code with
the idea that it should be easily portable to any
low-end or mid-range PICMicro without modification.
For the two I/O pins ("Data" and
"Clock"), I "defined" them to allow you to use virtually
any pins in your PICMicro application. I say "virtually
any" because PORTA pin 4 (also known as "RA4") is of
"open drain" configuration and cannot source a positive
voltage. The code itself is a very straightforward
example of writing a 4-bit LCD application which
displays the string "Hello" on the LCD display. The
important difference between this code and a straight
4-bit LCD output is the "NybbleOut" subroutine, which is
called twice by each of the "SendCHAR" and "SendINS"
subroutines (which send characters and instructions,
respectively, to the LCD).
"NybbleOut" first drops the "Data" line
and then strobes the "Clock" bit six times to clear the
shift register. Next, a "1" is strobed in, followed by
the "R/S" pin value, which is stored in the PICMicro's
"Carry" flag. I used Carry for this purpose because in
this application I used RA0 and RA1 as the output pins
and to simplify the operation of the code, I shift PORTA
with Carry loaded with the "Data" Pin Value.
The only point to notice with this code
is that the "E" strobe will become active on the last
bit if the least significant data bit is high. This lack
of "settling time" before "E" is active does violate the
"true" 44780 specification, but I haven't found it to be
a problem when I've built this circuit
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