debug module PCB design here fits all of the electronic
and mechanical components on a single side of the PCB.
The bottom layer copper, and top and bottom filled
copper planes are filtered out in Screenshot 1. You can
easily associate each of the electronic components with
their graphical counterparts shown in Schematic 1.
Components R8 and LED2 are not identified by silkscreen
legends due to space restrictions. Our PCB is a product of
the ExpressPCB generation process. So, you can identify
each electronic component by simply clicking on it which
will display the component’s property window. Within this
property window, you will find the size and value of the
component, and if the component is not identified with a
silkscreen legend component ID.
Between Schematic 1 and the ExpressPCB layout, you
can easily identify and correctly mount the electronic
components and mechanical connectors that make up the
debug module. Note the silkscreen dot that is associated
with LED1. This dot represents the cathode of the LED. As
you can see in Screenshot 1, the cathode of LED1 is tied
to the cathode of LED2 with a copper trace. LED2 is
located directly to the right of LED1 and is not identified
with a silkscreen legend. LED2’s current limiting resistor,
R8, is mounted directly above LED2.
Screenshot 2 adds the bottom layer traces and
legends under the component layer. Notice that some of
the component layer vias don’t have any bottom or top
layer traces connected to them. These are pass-through
vias that electrically connect the top and bottom ground
planes. In some areas of the PCB, pockets are formed that
isolate a grounded component lead from electrical
ground. So, to complete the ground circuit, we simply
place a via inside of the ground-less pocket which
connects the upper and lower ground planes. The bottom
■ SCREENSHOT 1. This screen capture is filtered to show
only the component bearing pads and the supporting
component-side traces.
■ SCREENSHOT 3. The legend associates directly with the
pins of the interface connector that is located at the far right
of the board.
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September 2010
■ SCREENSHOT 2. This is a composite view of the top and
bottom trace routing. No rocket science here.
side plane, traces, and legends are shown in Screenshot 3.
ExpressPCB does not offer bottom-side silkscreen. So, the
legends that associate with the debug interface pins are
etched into the bottom side copper. Over the years doing
hundreds of PCBs, I’ve discovered that using labels of any
kind is always a good practice. The pass-through vias
appear as holes in this screen capture because the vias are
defined as solid pads instead of thermal pads. To see what
a thermal pad looks like, look at pin 3 of the debug
interface connector. The bottom side plane you see in
Screenshot 3 was constructed in the same way as the top
side plane shown in Screenshot 4. Both the top and
bottom planes are grounded, and you can see the thermal
pads on some of the components that provide the
electrical contact to ground. If you are from the planet
Krypton, the debug module looks like Screenshot 5 when
you kick in your x-ray vision. The graphics that make up
Screenshot 5 come to life in Photo 1. Now that we have
some programmer/ debugger hardware, the next logical
step is to design and assemble a basic Vinculum-II
embedded dual USB host controller circuit.
A BASIC HARDWARE DESIGN
Our host controller can act as both a host and a
downstream device. So, we must design the Vinculum-II
power supply circuitry with that in mind. To meet that
requirement, our power supply design needs to be flexible
as far as its inputs are concerned.
VINCULUM-II POWER SUPPLY DESIGN
We’ve successfully used the Microchip TC1262-3.3
LDO voltage regulator in a number of Design Cycle
■ SCREENSHOT 4. The use of ground planes greatly
reduces the complexity of the printed circuit board by
reducing the number of individual ground traces.
