■ SCHEMATIC 3. This
circuit is primarily used in
host mode applications.
For instance, a mass
storage application
would have this USB
circuitry interface to a
USB thumb drive.
Ferrite beads are popular components in this Vinculum-II
design. The good news is that a single type of ferrite bead
is used throughout the design. The whole +1.8 volt thing
begins with the 3. 3 volts entering at the Vinculum-II’s
pin 2 which happens to be the Vinculum-II’s internal
voltage regulator input. A basis for the Vinculum-II’s logic
voltage levels must be set to allow its I/O pins to interface
with external logic devices. That’s
where the Vinculum-II’s VCCIO pins
come in. The voltage applied to pins
21, 38, and 54 establish the logic
level of the Vinculum-II’s I/O
subsystem. As you can see in
Schematic 4, the Vinculum-II is a 3. 3
volt device utilizing 3. 3 volt logic
levels. These days, 3. 3 volt and 5.0
volt logic levels coexist more often
than not. So, the Vinculum-II
architects added 5.0 volt tolerance to
all of the inputs.
If the debug module is attached
to the Vinculum-II, there’s no need
for pull-up resistors on the active-low
RESET and PROG lines as the pull-up
resistors are mounted on the debug
module PCB. If you’re considering
running the base system we’ve
designed without the debug module
attached, you’ll want to pull up the
Vinculum-II’s RESET and PROG pins
with a pair of 10K resistors just as
we’ve done with R6 and R7.
Good power supply bypassing
techniques are essential to properly
operating microcontroller circuitry.
So, power supply bypass capacitors C16, C17, C18, and
C19 need to be mounted as close as possible to the
Vinculum-II’s power pins that are directly connected to the
source power supply’s 3. 3 volt output. The Vinculum-II’s
internal clocks and timers are driven from a clock signal
derived from an external 12 MHz crystal. For this design,
I’ve chosen a FOX resin-sealed crystal. The 12 MHz
encapsulated crystal can be had from Mouser by
specifying part number 559-FQ5032B- 12.
COLLECTING THE PARTS
Now that our design is committed to a set of
schematics, we can begin the process of converting the
graphics to silicon and fiberglass. Whenever possible, we’ll
use 0603 SMT components as they are more easily placed
into tight trace-rich spaces. All of the electronic
components that make up our Vinculum-II design can be
had from Mouser.com. Rather than assemble a Vinculum-II unit that is locked into a specific application set, our
hardware will take the shape of a portable module that
can be plugged into an application-specific circuit. The pin
assignments in Schematic 4 represent the default I/O
function set. If you had the opportunity to eyeball last
month’s Design Cycle, you know that we can configure
these default I/O pin assignments to appear on alternate
■ PHOTO 2. It looks like a microcontroller. It acts like a microcontroller. It programs like a microcontroller. It is totally
USB. This is the 64-pin version of the Vinculum-II. You can
also get this IC in 32- and 48-pin variants.
64
September 2010