JP2 - This is a three-pin header with the labels, "I", "R" and "U". The "R" is an abbreviation referring to microcontroller pin, RC2. "I" is an abbreviation referring to the "ICE" female header pin for the RC2 signal. "U" is an abbreviation for the USB VBUS line. When the jumper is in the "R" to "I" position, the RC2 pin connects only to the ICE female header pin, just like most of the other general purpose I/O pins. When the jumper is in the "R" to "U" position, RC2 (which is 5.5V tolerant) can be used to sense when the USB cable has been attached to the host, and when the host is actively providing power to the +5V VBUS line. According to the USB 2.0 specifications, no device should ever pull the D+ or D- lines high (such as with the D+ or D- pull-up resistor) until the host actively powers the +5V VBUS line. This is intended to prevent self-powered peripherals from ever sourcing even small amounts of power to the host when the host is not powered. Small amounts of current could potentially prevent the host (and possibly other USB peripherals connected to that host) from fully becoming depowered, which may cause problems during power-up and initialization. Self-powered peripherals should periodically monitor the +5V VBUS line and detect when it is driven high. Only when it is powered should user firmware enable the USB module and turn on the D+ (for full speed) or D- (for low speed) pull-up resistor, signaling device attach to the host. The recommended method of monitoring the +5V VBUS line is to connect it to one of the microcontroller.s 5.5V tolerant I/O pins through a large value resistor (such as 100 kOhms). The resistor serves to improve the ESD ruggedness of the circuit as well as to prevent microcontroller damage if user firmware should ever unintentionally configure the I/O pin as an output. Peripherals which are purely bus powered obtain all of their power directly from the +5V VBUS line itself. For these types of devices, it is unnecessary to monitor when the VBUS is powered, as the peripheral will not be able to source current on the D+, D- or VBUS lines when the host is not powered.
JP3 - This jumper is located in series with the +5V VBUS power supply line from the USB connector. When the jumper is removed, a current meter may be placed between the header pins to measure the board current which is being drawn from the USB port. Additionally, by removing the jumper cap altogether, JP3 provides a means of preventing the board from consuming USB power.