Nuts and Volts - September 2010

Vinculum-II Embedded Dual USB Host Controller IC; Vinculum-II IDE; FT232RL: www.ftdichip.com

projects. The TC1262-3.3 LDO is capable of supplying loads up to 500 mA with a typical dropout voltage of 350 mV at full load. Designed for use in battery-powered applications, this voltage regulator can operate reliably with a pair of 1.0 µF ceramic capacitors attached to its input and output pins. Recall my First Law of Embedded Computing which states, “Nothing is free.” There is a “price” to pay for the TC1262-3.3 LDO voltage regulator’s feature set. In this case, the price is cheap as it only costs us 80 µA of power supply current to use it in our designs.

The TC1262-3.3 LDO input must be able to accept voltage from the USB portal and from an external source. If we decide to run the Vinculum-II in host mode, we’ll need to supply the VBUS voltage at the host USB connector in the same manner your PC USB portals supply VBUS power to downstream devices. Our voltage regulator design is laid out in Schematic 2.

■ SCREENSHOT 5. Folks from the planet Krypton have an advantage over us Earthlings as they can check the top and bottom of their ExpressPCB layouts simultaneously.

included to keep those nasty little sparks that tend to bridge electronic components and your fingertips from doing some unwanted soldering inside of the Vinculum-II IC. The ferrite beads FB1 and FB2 are in position to thwart EMI that may try to creep into the 5 volt supply.

VINCULUM-II CORE DESIGN HOST INTERFACE DESIGN

If you study Schematics 2 and 3, you will conclude that any host USB interface has the ability to supply VBUS power to the TC1262-3.3 LDO. However, it is not politically correct to power a host USB portal externally. For instance, you wouldn’t want to attempt to force VBUS power from an external downstream USB device into the USB portal of your thousand-dollar laptop. So, when operating as a host, the presence of the HOST JMPR x provides an electrical path to provide 5.0 volts to the downstream USB device. Since the host provides VBUS via its USB interface, 5.0 volts will probably be supplied externally. That means we need to provide an electrical path between the external 5.0 volt power supply and the input of the voltage regulator. This is easily accomplished by installing the 5V0 JMPR. Note that the VBUS JMPR and 5V0 JMPR are mutually exclusive as it pertains to having those jumpers installed. Our embedded dual USB host controller supports a pair of USB host portals which are identified as P0 and P1. The USB portals drawn up in Schematic 3 are standard fare for most FTDI parts. ESD suppressors CR1, CR2, CR3, and CR4 are Like many of today’s low voltage core microcontrollers, the Vinculum-II needs supporting electrical components for its internal voltage regulators. Pins 3 and 7 of the Vinculum-II IC grinning in Photo 2 form the business end of the Vinculum-II’s internal 1.8 volt power system. The 1.8V VCC PLL IN (pin 3) acts as the +1.8 volt supply to the internal clock multiplier. A 100 nF capacitor must be attached between this pin and ground. The output of the internal +1.8 volt regulator emanates from pin 7 of the Vinculum-II. Schematic 3 shows a pair of filter capacitors hanging from the +1.8 volt output pin. Take another look at Schematic 3 and you’ll see a ferrite bead standing guard on the internal clock multiplier +1.8 volt input pin.

■ PHOTO 1. Rising from concept to reality, the EDTP

Vinculum-II debug module.

5V0

■ SCHEMATIC 2.

If you’re a regular Nuts & Volts Design Cycle reader, there’s no rocket science portrayed here either. The TC1262-3.3 LDO voltage regulator is a solid and proven electronic part shown here in a field tested design.