mc68hc912dg128 Freescale Semiconductor, Inc, mc68hc912dg128 Datasheet - Page 409

• Manufacturer Part Number: mc68hc912dg128
• Description: M68hc12 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

20.3.5 What Do I Do In Case The Oscillator Does Not Start-up?
MC68HC912DG128 — Rev 3.0
MOTOROLA
quiescent point is reached. Controlling this final peak to peak amplitude
can be performed by three means:
1. Reducing the values of C1 and C2. This decreases the loading so
2. Using VDDPLL=VSS (i.e. shutting off the PLL). Doing so
3. Also, placing a high value resistor (>1M ) across the EXTAL and
1. First, verify that the application schematic respects the principle of
2. Re-consider the choice of the tuning capacitors.
3. The oscillator circuitry is powered internally from a core VDD pad
4. If possible, consider using a resonator with built-in tuning
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that the necessary gm value required to sustain oscillation can be
smaller. The consequently smaller current will be reached with a
larger ‘extal’ swing.
increases the starting current by approximately 50%. All other
parameters staying the same, a larger ‘extal’ swing will be required
to reduce this starting current to its quiescent value.
XTAL pins significantly increased the oscillation amplitude.
Because this complicates the design analysis as it transforms a
pure susceptance j C1 into a complex admittance G+j C1,
Motorola cannot promote this application trick.
operation, i.e. crystal mounted between EXTAL and VSS,
Capacitor C1 between XTAL and EXTAL, Capacitor C2 between
XTAL and VSS, nothing else. This is not the conventional MCU
application schematic of the Pierce oscillator as it can be seen on
other HC12 derivatives!
and the return path is the VSSPLL pad. Verify on the application
PCB the correct connection of these pads (especially VSSPLL),
but also verify the waveform of the VDD voltage as it is imposed
on the pad. Sometimes external components (for instance choke
inductors), can cause oscillations on the power line. This is of
course detrimental to the oscillator circuitry.
capacitors. They may offer better performances with respect to
their discrete elements implementation.
Appendix: CGM Practical Aspects
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A Few Hints For The CGM Crystal Oscillator Application
Appendix: CGM Practical Aspects
Technical Data
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