MAC7200 datasheet(524/976 Pages) FREESCALE

524 / 976 page

Memory Map/Register Definition

MAC7200 Microcontroller Family Reference Manual, Rev. 2

446

Freescale Semiconductor

24.3

Memory Map/Register Definition

The CRG contains the registers and associated bits for controlling and monitoring the oscillator module.

24.4

Functional Description

The OSC_ALC_HIP7A 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 ALC oscillator

mode or 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

24.4.1

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 Section A.8, “Oscillator Characteristics”.

24.4.2

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 Section Chapter 23, “Clock and Reset Generator

(CRG).

24.4.3

Doze Mode Operation

When the system is in Doze Mode, the oscillator operates as normal.

Table 24-1. Clock Selection Based on XCLKS

XCLKS

Description

External Clock Mode (External 3.3V clock required)

ALC Mode (Internal oscillator clock)

MAC7200 Datasheet(PDF) 524 Page - Freescale Semiconductor, Inc