SPI-based four-wire serial mode. The 3. 3 volt trick that the
EA DOGM162L-A performs is made possible by a pair of
capacitors tied to an internal voltage boost circuit.
The EA DOGM162L-A displays dark green on
yellow/green and is not capable of being backlit. Other
LCD modules in the EA DOGMxxxx-x family require a
backlight or can optionally be backlit. Like other aspects
of the EA DOGMxxx-x LCD family, the method of
backlighting the LCD modules is very clever. Instead of
building in a backlight, the backlight simply clips to the
LCD module and the backlight/LCD assembly is mounted
as a unit. Six different backlight colors can be selected,
depending on the LCD module that is in use.
The EA DOGM162L-A mounted on our USCM-47J53
is a 2 x 16 character LCD. Character height is 5. 7 mm.
Figure 1 is a schematic representation of the EA
DOGM162L-A configured for eight-bit parallel operation.
The active-low CSB (Chip Select) is tied low as there is
no need to “select” the EA DOGM162L-A when it owns
all of the data and control resources. The same chip select
logic holds true for the four-bit EA DOGM162L-A
configuration depicted in Figure 2. The SPI configuration
is capacitor boosted just like the parallel configurations.
However, note that the EA DOGM162L-A’s PSB (Interface
Selection) pin is tied low which forces its internals into
serial mode. Now that the EA DOGM162L-A is an SPI
slave device, it must be able to be selected. Recall that the
microSD card is also a SPI slave device. In addition to
being selectable, the active-low CSB signal resets the EA
DOGM162L-A’s internal shift register and counter on its
falling edge.
Most all of you that have used standard HD44780-
based LCD modules are familiar with the eight-bit and
four-bit parallel modes. So, let’s strike out and configure
our EA DOGM162L-A for SPI operation. To successfully
use the EA DOGM162L-A in our USCM-47J53’s 3. 3 volt
system, we have an engineering decision to make. In
Figures 1, 2, and 3, the capacitor value for the
CAP1P/CAP1N pin pair is specified as 0.1 µF to 1.0 µF.
The ST7036 datasheet tells us that the range for the
CAP1P/CAP1N pin pair is 0.1 µF to 4. 7 µF. As a starting
point, we’ll install 1.0 µF ceramic capacitors at both the
VIN/VOUT and CAP1P/CAP1N positions.
THE USCM-47J53 MAIN HARDWARE
The PIC18F47J53 is a very robust eight-bit
microcontroller. The intent of the USCM-47J53 is to
provide you with a simple yet powerful microcontroller
platform for your particular application. Everything the
PIC18F47J53 needs for basic operation is mounted on the
USCM-47J53 printed circuit board shown in Photo 2.
The USCM-47J53 is ideal for use as a timed-operation-high-storage-capacity embedded computing device. The
PIC18F47J53 supports the Microchip microSD card library
routines which provide standard FAT file system operations
that can be used with the USCM-47J53’s microSD card
hardware.
A hardware real time clock calendar (RTCC) provides
precise time of day data with optional alarm and interrupt
events. The USCM-47J53 is equipped with a 32.768 kHz
clock crystal mounted across the TIMER1 oscillator pins.
The 32.768 kHz crystal in conjunction with TIMER1
provides a precise time base for the PIC18F47J53’s RTCC
hardware.
USB is the primary medium used by the USCM-47J53
to communicate with the outside world. The USB portal
also provides raw power for the PIC18F47J53, the EDTP
microSD card interface, and EA DOGM162L-A LCD.
However, you are not limited to using USB resources as all
of the PIC18F47J53’s SPI, I2C, parallel, and serial
communications modules are available to you via the I/O
A DMP2123L P-channel MOSFET has been
added to the USCM-47J53 design. By connecting
the DMP2123L’s gate to a PIC18F47J53 I/O pin
of your choice, the DMP2123L MOSFET can be
used to switch + 3. 3 volts to peripheral devices
under program control. If the RTCC is not one of
your design points, the 32.768 kHz crystal and its
associated load capacitors can be eliminated from
the design which frees up the PWM output
available at I/O pin RC0. The PWM signal can be
used to drive the DMP2123L and provide a soft-start function that will limit the inrush current
drawn from the host USB portal. The idea behind
limiting inrush current is to keep the initial USB
■ PHOTO 2. Thanks to the Microchip Application
Libraries, the USCM-47J53 is as easy to program
as it is to build.
56
December 2010
