S912XEG128J2MAA Datasheet, PDF (502/1324 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 12 Pierce Oscillator (S12XOSCLCPV2)
12.3 Memory Map and Register Definition
The CRG contains the registers and associated bits for controlling and monitoring the oscillator module.
12.4 Functional Description
The XOSC module has control circuitry to maintain the crystal oscillator circuit voltage level to an optimal
level which is determined by the amount of hysteresis being used and the maximum oscillation range.
The oscillator block has two external pins, EXTAL and XTAL. The oscillator input pin, EXTAL, is
intended to be connected to either a crystal or an external clock source. The XTAL pin is an output signal
that provides crystal circuit feedback.
A buffered EXTAL signal becomes the internal clock. To improve noise immunity, the oscillator is
powered by the VDDPLL and VSSPLL power supply pins.
12.4.1 Gain Control
In LCP mode a closed loop control system will be utilized whereby the amplifier is modulated to keep the
output waveform sinusoidal and to limit the oscillation amplitude. The output peak to peak voltage will be
kept above twice the maximum hysteresis level of the input buffer. Electrical specification details are
provided in the Electrical Characteristics appendix.
12.4.2 Clock Monitor
The clock monitor circuit is based on an internal RC time delay so that it can operate without any MCU
clocks. If no OSCCLK edges are detected within this RC time delay, the clock monitor indicates failure
which asserts self-clock mode or generates a system reset depending on the state of SCME bit. If the clock
monitor is disabled or the presence of clocks is detected no failure is indicated.The clock monitor function
is enabled/disabled by the CME control bit, described in the CRG block description chapter.
12.4.3 Wait Mode Operation
During wait mode, XOSC is not impacted.
12.4.4 Stop Mode Operation
XOSC is placed in a static state when the part is in stop mode except when pseudo-stop mode is enabled.
During pseudo-stop mode, XOSC is not impacted.
MC9S12XE-Family Reference Manual Rev. 1.25
502
Freescale Semiconductor