www.nutsvolts.com/index.php?/magazine/article/november2010_DesignCycle
DESIGN CYCLE
■ SCREENSHOT 1. This is the initial component layout. The
silkscreen has been disabled to give you an unobstructed
view of the top-side traces.
microcontroller, external devices must be attached to the
microcontroller as the design dictates. Since the universal
design is built up on its own PCB, one or more peripheral
device PCBs may be required. In reality, this turns out to
be a good thing as most any design targeting the universal
design’s microcontroller can be implemented.
The PIC18F46J50’s 44-pin TQFP package allows us to
start our universal design small. The daughterboard
captured in Screenshot 1 measures in at 2. 15 x 1.675
inches. The silkscreen has been disabled in Screenshot 1
to give you an unobstructed view of the traces and the
component locations. Only the PIC18F46J50 and 16
supporting components are mounted on the
daughterboard. As you can see, the design is not at all
complex and lends itself to an inexpensive ExpressPCB
double-sided PCB. Screenshot 2 shows just how simple
■ SCREENSHOT 3. This screenshot reveals a 3. 3 volt
voltage regulator that can be powered by the USB portal
or an external power source. Powering from an external
source while powering from USB is a NO-NO.
the PIC18F46J50 daughterboard design really is. Only four
traces are laid in on the bottom side of the fiberglass.
It took a few iterations to come to the finality of
Screenshot 3. With all of the components in their final
positions, I silkscreened in the PIC18F46J50 I/O pin labels.
To keep the silkscreen chaos to a minimum, I suppressed
the silkscreen part identifiers. You can get all of the part’s
scoop by simply clicking on the part. I’ve provided the
original ExpressPCB files in the article downloads for this
purpose. All of the RPxx silkscreen legends call out pins
that support the PIC18F46J50’s peripheral pin select
feature.
Note that the TC1262-3.3 voltage regulator can be
powered from an outside source, as well as the VBUS
voltage from the USB portal. The power sources are
mutually exclusive as the USB specification states that the
■ SCREENSHOT 2. Pretty simple, huh? The bottom-side
trace pattern is minimal because the plane we will add
later will make all of the ground connections on this side of
the printed circuit board.
■ SCREENSHOT 4. Everything is in place and the ground
planes have been applied. Note that the ground planes are
simultaneously used as ground traces and shields.
November 2010 55
