D E M O 9 S 0 8 S F 4
J U N E 9 , 2 0 0 9
U S E R G U I D E
9
VX_EN
The VX_EN option header is a 2-pin jumper that connects or disconnects input J1-1 directly to
the target board voltage rail. J1-3 connects directly to the target board ground plane. Use of
this feature requires a regulated input power source. This power input is decoupled to
minimize noise but is not regulated or protected. Care should be exercised when using this
feature; no protection is applied on this input and damage to the target board may result if
excessive voltage is applied. Also, do not attempt to power the target board through this
connector while also applying power through the USB BDM or the PWR connector; damage to
the board may result.
Power may also be sourced to off-board circuitry through the J1 connector. The current
supplied from the USB bus limits current available to external circuitry. Excessive current drain
may damage the target board, the host PC USB hub. The figure below details the VX_EN
header connections.
Figure 4: VX_EN Option Header
ON
OFF
1
2
Enabled
Disabled
VX_EN
CAUTION:
Do not exceed available current from USB-BDM when sourcing power
through connector J1 to external circuitry.
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 smoothes the reset pulse and ensures adequate pulse width.
The MC9S08SF4 MCU applies a multiplexed RESET* input. To use the RESET switch, the
RESET pin must be enabled (SOPT1_RSTPE). Refer to the MC9S08SF4 Reference Manual
for details on configuring using the RESET* input.
LOW VOLTAGE RESET
The MC9S08SF4 applies an internal Low Voltage Detect (LVD) circuitry. The LVD holds the
MCU in reset until applied voltage reaches the appropriate level. The LVD also protects
against under-voltage conditions. Consult the MC9S08SF4 Reference Manual for details on
LVD operation.
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