■ FIGURE 1. The active-low RESET line can
be tied logically high or controlled via a
microcontroller I/O pin. Otherwise, all of the
standard eight-bit controls and data lines are
offered to the user.
deep-sleeping, 128K program Flash version
of the PIC18F46J50.
If you had the opportunity to join in last
month’s Design Cycle discussion, you
already know that USCM is short for
Universal Storage Control Module. In our
current situation, the storage comes in the
form of a microSD card and the Control Module is based
on the PIC18F47J53. Although the PIC18F46J50 and
PIC18F47J53 are kissing cousins, the USCM-47J53 differs
from last month’s USCM-46J50 in a number of ways. Let’s
analyze the USCM-47J53 hardware suite that lies under
the lens in Photo 1, beginning with a very different type
THE EA DOGM162L-A
I’ve got a box full of standard LCD modules that I pull
from for specific Design Cycle projects. Despite the
differing number of rows and columns associated with
each of my LCD modules, every module in the box has
one common factor: They all require a 5.0 volt power
They still sell 5.0 volt voltage regulators. So, the good
old TTL logic levels most of us grew up with are still valid.
However, the PIC18F47J53 and many other modern low
power microcontrollers feed on 3. 3 volt power. We get
away with driving the inputs of a 5.0 volt LCD with a 3. 3
volt microcontroller because + 3. 3 volts is good enough to
rate as a logical TTL high.
Another common trait of my box full of LCD modules
is the way they are driven. If you have plenty
of microcontroller I/O pins available to you,
you can lash up all eight of the LCD module’s
data I/O pins, plus the three control inputs.
The alternate method of driving a standard
LCD module is to push four bits at a time into
the LCD controller. The four-bit method
requires only seven of the host
microcontroller’s I/O pins. If you opt not to
read the LCD’s status or internal display
memory, you can save one more
logically low which will only allow writes to the LCD
module. Fred Eady’s First Law of Embedded Computing —
which states that “Nothing is Free” — applies to each pin-saving LCD configuration. Extra data entry cycles are
required to use four-bit mode and you lose the ability to
read the LCD status and resources by tying the LCD
module’s R/W pin logically low. In addition, the loss of the
ability to read the LCD module’s Busy Flag forces the
designer/programmer to time the LCD accesses.
■ FIGURE 2. In that the EA DOGM162L-A is
being powered by a 3. 3 volt power source, the
boost capacitors are present here, as well. The
four-bit data path is identical to that of an
HD44780-based LCD module.
December 2010 55