M 5 2 2 3 3 D E M O
S E P T E M B E R 2 5 , 2 0 0 6
12
POWER SELECT
Configuration of applied input power is controlled using 2 option headers. The PWR_SEL
header selects between the on-board voltage regulator and the USB voltage input. The
VX_EN header connects J1-1 directly to the upper voltage rail. The illustrations below show
the different configuration for each option header.
PWR_SEL
Power from the integrated BDM is drawn from the USB bus and is limited to
500mA
. Exces-
sive current drain will violate the USB specification causing the USB bus to disconnect.
CAUTION:
Violating the USB specification will cause the USB bus to disconnect. This will
cause the board to reset.
The on-board voltage regulator (VR1) accepts power input through a 2.1mm barrel connector
(PWR). Input voltage may range from +5V to +15V. VR1 provides a +3.3V fixed voltage out-
put with output current limited to 800mA. Over-temperature and over-current limit built into the
voltage regulator provides limited protection from damage due to excessive stresses.
The user should consider the maximum output current of the selected power source when at-
tempting to power off-board circuitry through connector J1.
Figure 5: PWR_SEL Option Header
•
•
•
Select power input from USB-BDM
VB
VDD
PWR_SEL
•
•
•
Select power input from VR1
VB
VDD
PWR_SEL
VX_EN
The VX_EN option header is a 2-pin jumper that connects the target-board voltage rail to J1-1.
J1-3 is connected directly to the ground plane. This input requires a reg3.3V voltage
source. This power input is decoupled to minimize noise input but is not regulated. Also, no
protection is applied on this input and damage to the target board may result if over-driven. Do
not attempt to power the target board through this connector while also applying power through
the USB-BDM or the PWR connector as damage to the board may result.
Power may be sourced to off-board circuitry through the J1 connector. The current limitation of
the USB bus or the on-board regulator must be considered when attempting to source power
to external circuitry. Excessive current drain may damage the target board, the host PC USB
hub, or the on-board regulator. The figure below details the VX_EN option header connec-
tions.
