T W R - 9 S 1 2 G 1 2 8
J U N E 2 , 2 0 1 0
U S E R G U I D E
9
connecting external power to the board if desired. An on-board regulator is used to create the
board operating voltage from this input.
Use of the on-board regulator requires input voltage b7.V and +27V. However, input
voltage should be kept as low as possible to reduce device self-heating.
CAUTION:
This boards does not apply reverse polarity protection on inputs E1 &
E2. Polarity for each input is clearly marked. Reverse input polarity
will damage the board.
Power Select
Option headers PWR_SEL selects the input power source for the target board. When
powered from the Tower System, the OSBDM voltage output is disabled.
Figure 3: PWR_SEL Option Header
JP5
1
● ●
2
Select TWR voltage input
PWR_SEL
● ●
Select OSBDM voltage input (default)
● ●
Select on-board regulator input
RESET SWITCH
The RESET switch applies an asynchronous RESET input 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.
LOW VOLTAGE RESET
The MC9S12G128 applies a Power-On Reset (POR) circuit and an internal Low Voltage
Reset (LVR) circuit to ensure proper device operation. The POR circuit holds the MCU in
reset until applied voltage reaches an appropriate level. The LVR forces the device into reset
if input voltage falls too low, protecting against brown-out conditions. A user configurable Low-
Voltage Detect (LVD) with interrupt output is also available. Consult the MC9S12G128
reference manual for details of POR, LVR, and LVD operation.
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