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User’s Manual
M68HC12A4EVB Evaluation Board — Rev. 1
90
Hardware Reference
MOTOROLA
Hardware Reference
4.8 Clock Circuitry
The EVB comes with a 16-MHz crystal oscillator installed in a 14-pin DIP
socket (XY2). The socket wiring allows the use of various types of oscillator
packages. Additionally, there is ancillary circuitry that includes a footprint for
a discrete crystal (Y1). This flexible arrangement facilitates the construction of
custom oscillators. When designing a custom oscillator, refer to the EVB
schematic diagram to locate the applicable components and the headers that
must be changed.
An external clock input can be supplied to the MCU’s EXTAL by installing a
right-angle BNC connector in footprint J7. Refer to the EVB schematic diagram
to locate the headers that must be changed.
4.9 Phase-Locked Loop (PLL)
The PLL can be used to run the MCU on a timebase that differs from the clock
frequency. To alter the timebase, capacitors must be installed between the
MCU’s XFC pin and the PLL’s ground reference, V
SSPLL
. Connection points
E4, E5, E6, E7, E8, and E9 provide space for these capacitors. Header footprint
W37 connects the XFC pin to the capacitors.
For more information, refer to the EVB schematic diagram. More detailed
information on the operation of the PLL is found in the MC68HC812A4
Technical Summary, Motorola document order number MC68HC812A4TS/D.
4.10 Reset
The reset circuit includes a pullup resistor, debounce capacitor, and optional
connection to an installed undervoltage sensing device (U1, as described in
4.11
Low-Voltage Inhibit (LVI)
). The reset circuit drives the MCU’s RESET pin
directly.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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