MC68HC12 Datasheet, PDF (214/478 Pages) Motorola, Inc – The MC68HC912DT128A microcontroller unit (MCU) is a 16-bit device composed of standard on-chip peripherals including a 16-bit central processing unit

Oscillator
• The input ESD resistor from EXTAL to the gate of the oscillator
amplifier was changed to provide a parallel path, reducing
parasitic phase shift in the oscillator.
13.5.1.1 Oscillator Architecture Change from Colpitts to Pierce
The primary difference from the ‘A’ to the ‘P’ versions of the
MC68HC912Dx128 is the architecture, or configuration, of the oscillator.
The previous version (‘A’) is connected in Colpitts configuration, where
the resonator is connected between the EXTAL pin and VSS. This
configuration causes the relatively large parasitics from EXTAL to VSS
react in parallel with the resonator, decreasing gain margin in some
corners. The Pierce configuration places the much-lower EXTAL to
XTAL parasitic capacitances in parallel with the resonator, providing a
much larger gain margin across process, temperature and voltage
variance.
Implementation of the Pierce architecture required the replacement of
the previous P-type, non-inverting source-follower amplifier with an N-
type, inverting, traditional amplifier. Additionally, the EXTAL biasing
circuit on the Colpitts configurations was replaced with a feedback
resistor from XTAL to EXTAL to achieve self-bias. Parametric
differences from the ‘A’ to the ‘P’ versions of the oscillator include:
• Phase shift from EXTAL to XTAL — The phase shift on the ‘P’
version will be approximately 180 degrees (vs. approximately 0
degrees on the ‘A’ version) due to the requirement of an inverting
amplifier in the Pierce configuration).
• DC offset of oscillation on EXTAL and XTAL — The DC offset of
the EXTAL and XTAL nodes on the ‘P’ version will be
approximately 0.7–1.0V (vs. approximately VDD–2V and
VDD–1V, respectively, on the ‘A’ version) due to the different bias
requirements of the N-type inverting amplifier.
• Amplitude of oscillation — The amplitude of oscillation may be
slightly lower on the ‘P’ version than the ‘A’ version due to using
the same Amplitude Level Control (ALC) circuit for both
architectures. The circuit responds slightly differently to the
different DC offsets in the two architectures, resulting in slightly
Technical Data
214
Oscillator
MC68HC912DT128A — Rev 4.0
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