If JTAG debugging is necessary with external power is ON. (????) In addition to jumper on PWR1, jumper on [2-3] on JTG_PWR1 must be placed. External power can be provided directly between DVCC2 and DGND2 headers. In addition for USB power option for the entire board, R15 must be populated and jumper be placed on PWR2 at position [2-3].
When powered by the mains supply PWR2 header can also be treated as a current consumption header by placing an ammeter across. Also, when powered via JTAG, the current consumption header will be no longer PWR2, instead the ammeter can be connected across [1-2] of header JTAG_PWR2.