T W R - S 0 8 L L / L H U S E R G U I D E
A P R I L 2 7 , 2 0 1 0
U S E R G U I D E
9
Figure 2: V_SEL Option Header, JP8
VR1
Enables voltage input from on-board regulator. This option should be selected when
powered from the integrated OSBDM (default)
BATT
J
P
8
VR1
Enables voltage input from battery holder. This option should be selected when
battery-powered operation is desired
BATT
J
P
8
Power from the integrated BDM is drawn from the USB bus and is limited
to
500 mA
total.
This current limit accounts for the total current supplied over the USB cable to the BDM circuit,
the target board, and any connected circuitry. Current drain in excess of 500 mA violates the
USB specification and causes the USB bus to disconnect the offending device. This may
cause the board to exhibit power cycling where the board appears to turn on then off
continually. In this case, LED400 will flash on and off. If this condition is allowed to persist,
damage to the host PC or the target board result.
The on-board voltage regulator (VR1) creates the +3.3V rails used by the target MCU from the
+5V input provided by the OSBDM.
The battery holder at BATT accepts a 3.3V, 23mm, 190mAH lithium, coin-cell battery, PN CR-
2325.
VDD_EN
The VDD_EN option header enables and disables VDD to the target board peripherals. Power
to the MCU is unaffected by this option jumper. Removing this option jumper, along with
others, allows the user to take accurate MCU current measurements. The figure below shows
the VDD_EN option jumper connections.
Figure 3: VDD_EN Option Header, JP9
ON
OFF
JP9
Enabled (*)
Disabled
(*)
– Default Position
RESET SWITCH
The RESET switch applies an asynchronous RESET to the MCU. The RESET switch is
connected directly to the RESET* input on the MCU. Pressing the RESET switch applies a
low voltage level to the RESET* input. A pull-up bias resistor allows normal MCU operation.
Shunt capacitance ensures an adequate input pulse width.