68HC908RF datasheet(71/188 Pages) MOTOROLA

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Internal Clock Generator Module (ICG)

Functional Description

MC68HC908RF2 — Rev. 4.0

Data Sheet

MOTOROLA

Internal Clock Generator Module (ICG)

The amplifier is enabled when the ECGON bit is set and stop mode is not enabled.

When the amplifier is enabled, it will be connected between the OSC1 and OSC2

pins. In its typical configuration, the external oscillator requires five external

components:

Crystal, X1

Fixed capacitor, C1

Tuning capacitor, C2 (can also be a fixed capacitor)

Feedback resistor, RB

Series resistor, RS (included in the diagram to follow strict Pierce oscillator

guidelines and may not be required for all ranges of operation, especially

with high frequency crystals). Refer to the crystal manufacturer’s data for

more information.

6.3.3.2 External Clock Input Path

The external clock input path is the means by which the microcontroller uses an

external clock source. The input to the path is the OSC1 pin and the output is the

external clock (ECLK). The path, which contains input buffering, is enabled when

the ECGON bit is set and stop mode is not enabled.

6.3.4 Clock Monitor Circuit

The ICG contains a clock monitor circuit which, when enabled, will continuously

monitor both the external clock (ECLK) and the internal clock (ICLK) to determine

if either clock source has failed based on these conditions:

•

Either ICLK or ECLK has stopped.

•

The frequency of IBASE < frequency EREF divided by 4

•

The frequency of ECLK < frequency of IREF divided by 4

Using the clock monitor requires both clocks to be active (ECGON and ICGON are

both set). To enable the clock monitor, both clocks must also be stable (ECGS and

ICGS both set). This is to prevent the use of the clock monitor when a clock is first

turned on and potentially unstable

NOTE:

Although the clock monitor can be enabled only when the both clocks are stable

(ICGS or ECGS is set), the clock monitor will remain enabled if one of the clocks

goes unstable.

The clock monitor only works if the external slow (EXTSLOW) bit in the

configuration register is properly defined with respect to the external frequency

source.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

68HC908RF Datasheet(PDF) 71 Page - Motorola, Inc

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