MCB2140: Use as a Self-Powered USB Device

QUESTION

Is it possible to use MCB2140 evaluation board as the self-powered USB device?

ANSWER

Yes.

The MCB2140 power supply may be changed from a bus-powered USB device to self-powered mode enabling the SoftConnect feature of the LPC2148. 

SoftConnect is especially useful for devices that change USB functionality during runtime. USB debugging with µVision becomes easier because the firmware may be reset without unplugging the board. TO do this, the below changes are required:

Hardware Changes

Remove the following components:

• R37: This disconnects the board power from the bus.
• USB LED: Can be reused to indicate USB SoftConnect status in the changed layout.
• R36: No longer used

Add the following components:

• Connect the base of a PNP transistor (PMBTA92) to P0.31 through a 1.8K Ohm resistor.
• Connect the transistor's emitter to +3.3V on regulator IC3.
• Connect the transistor's base through a 10K Ohm resistor to +3.3V on regulator IC3.
• Wire the Collector pin with 1.5 KOhm to D+. On the Collector you may also add an LED with 330 Ohm to GND, to indicate switching state.

Additional information about the transistor: Nexperia PMBTA92

Software Changes

P0.31 must be configured for SoftConnect as follows:

PINSEL1 |= 0x80004000;

Schematics

MORE INFORMATION

• Refer to Schematics in the MCB2140 User's Guide.

Last Reviewed: Thursday, January 14, 2021