MC9S12XD256MAL Datasheet, PDF (122/1348 Pages) Freescale Semiconductor, Inc – Freescale’s Scalable Controller Area Network

Chapter 3 Pierce Oscillator (S12XOSCLCPV1)
3.2.3 XCLKS — Input Signal
The XCLKS is an input signal which controls whether a crystal in combination with the internal loop
controlled (low power) Pierce oscillator is used or whether full swing Pierce oscillator/external clock
circuitry is used. Refer to the Device Overview chapter for polarity and sampling conditions of the XCLKS
pin. Table 3-1 lists the state coding of the sampled XCLKS signal.
.
Table 3-1. Clock Selection Based on XCLKS
XCLKS
Description
1
Loop controlled Pierce oscillator selected
0
Full swing Pierce oscillator/external clock selected
3.3 Memory Map and Register Definition
The CRG contains the registers and associated bits for controlling and monitoring the oscillator module.
3.4 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 selection of loop controlled
Pierce oscillator or full swing Pierce oscillator/external clock depends on the XCLKS signal which is
sampled during reset. 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 V_DDPLL and V_SSPLL power supply pins.
3.4.1 Gain Control
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.
3.4.2 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.
MC9S12XDP512 Data Sheet, Rev. 2.21
122
Freescale Semiconductor