Appendix A: CGM Practical Aspects
A Few Hints For The CGM Crystal Oscillator Application
68HC(9)12DG128 Rev 1.0
MOTOROLA
Appendix A: CGM Practical Aspects
381
XTAL pins significantly increased the oscillation amplitude.
Because this complicates the design analysis as it transforms a
pure susceptance j
ω
C1 into a complex admittance G+j
ω
C1,
Motorola cannot promote this application trick.
What Do I Do In
Case The
Oscillator Does
Not Start-up
1.
First, verify that the application schematic respects the principle of
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!
Re-consider the choice of the tuning capacitors.
If the quartz or resonator is of a high frequency type (e.g. above
10MHz), consider using VDDPLL=0V. Obviously, this choice
precludes the use of the PLL. However, in this case the oscillator
circuitry has more quiescent current available and the starting
transconductance is thus significantly higher.
The oscillator circuitry is powered internally from a core VDD pad
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.
If possible, consider using a resonator with built-in tuning
capacitors. They may offer better performances with respect to
their discrete elements implementation.
2.
3.
4.
5.
3-cgmpa