D E M O 9 S 1 2 X D T 5 1 2
J U L Y 1 8 , 2 0 0 8
11
VX_EN
The VX_EN option header is a 2-pin jumper that connects/disconnects input J1-1 directly to
the target board +5V voltage rail. J1-3 is directly connected to the ground plane. Use of this
feature requires a reg5V input power source. This power input is decoupled to mini-
mize noise input but is not regulated. Care should be exercised when using this feature; no
protection is applied on this input and damage to the target board may result if over-driven.
Also, do not attempt to power the target board through this connector while also applying
power through the USB-Multilink BDM or the PWR connector; damage to the board may result.
Power may also be sourced to off-board circuitry through the J1 connector. Current output to
external circuitry is limited by the current supplied from the USB bus or the on-board regulator.
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 header connections.
Figure 4. VX_EN Option Header
ON
OFF
1
2
Enabled
Disabled
VX_EN
CAUTION:
Do not exceed available current supply from USB-Multilink BDM or on-board regu-
lator, when sourcing power through connector J1 to external circuitry.
RESET SWITCH
The RESET switch provides a method to apply 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.
LOW VOLTAGE RESET
The MC9S12XDT512 utilizes an internal Low Voltage Reset (LVR) circuit. The LVR holds the
MCU in reset until applied voltage reaches an appropriate level. The LVR also protect against
under-voltage conditions. Consult the MC9S12XDT512 reference manual for details LVR op-
eration.
TIMING
Default timing configuration for the DEMO9S12XDT512 is a low-power, low-noise Pierce os-
cillator. In this mode, oscillator swing is typically 1V
PP
. The Pierce oscillator input is supplied
by a 4 MHz, fundamental frequency, crystal oscillator at location Y1. The crystal oscillator ex-
hibits a frequency tolerance of 30ppm. An optional socket at X1 is also provided to support
alternate MCU input frequencies. This socket supports full- and half-size can clock oscillators.
