Nuts and Volts - February 2009

■ SCREENSHOT 1. Upon power-up, this is the menu that greets WLAN Phoenix users. This message bounced through an SP3232 on the WLAN Phoenix to the SP3243EU on the CP2103 Evaluation Kit board and out of the CP2103 USB interface to the Tera Term Pro terminal emulator application running on my laptop.

CP2103 has its own five volt to three volt voltage regulator. The CP2103's on-chip voltage regulator allows the CP2103 to use the USB bus power or be powered by an external power source. In USBese, powered by an external source is termed self-powered.

There are three configurations in which the CP2103's V pin can

DD participate. In USB Bus-Powered mode, the V pin is internally DD connected to the CP2103's internal voltage regulator output. The CP2103's VBUS and REGIN (CP2103 voltage regulator input) pins are both tied to the incoming VBUS source in USB Bus-Powered mode. The USB Bus-Powered configuration enables the CP2103's voltage regulator output to be used to power external circuitry. With 100 mA of available current capacity, the CP2103's internal three-volt voltage regulator can easily power a typical PIC microcontroller. If you examine Schematic 2, you'll see that the CP2103 Evaluation Kit is configured as a USB Bus-Powered device. The 4. 7 μF tantalum capacitor hanging on the V pin is recommended when the DD

CP2103's internal voltage regulator is servicing devices in addition to the CP2103 itself. The additional 0.1 μF capacitor attached between the

V pin and ground is for noise

DD suppression. USB Self-Powered mode is in effect when an external five volt power source is connected to the CP2103's REGIN pin. The VBUS source is also connected to the VBUS sense input pin and the internal voltage regulator is enabled, providing three volts on the V pin

DD for possible use by supporting circuitry. If 3V is applied to the V and DD REGIN pins, the internal voltage regulator is disabled and the CP2103 is considered a USB self-powered device. The VBUS sense input is fed by the VBUS source in the configuration, which is defined as USB Self-Powered, Regulator Bypassed mode.