Nuts and Volts - September 2010

■ 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.