68HC05PL4 datasheet(50/98 Pages) FREESCALE

50 / 98 page

GENERAL RELEASE SPECIFICATION

April 30, 1998

SYSTEM CLOCKS

MC68HC05PL4

8-2

REV 2.0

FMODE — Fast Mode RC select

FMODE selects the oscillating frequency of the internal RC. After power-on-

reset, the default setting is 500kHz.

1 = Internal RC oscillates at 500kHz

0 = Internal RC oscillates at 20kHz

OSCF — OSC running Flag

This bit is set when the external clock (External/crystal) from OSC1 is on.

See also CKOSC bit below.

RCF — RC Running Flag

This bit is set when the internal RC clock is on.

CKOSC — ChecK OSC

The CKOSC bit enables the internal logic for external clock selection. The pro-

cedure below should be followed when switching from RC to external clock.

1. Set the CKSEL1 and CKSEL2 bits for external clock source.

2. If crystal option is used — set the 8-bit timer for counting crystal

stabilization delay (typically 4064 clock cycles).

3. Write a “1” to the CKOSC bit and check for OSCF bit set.

4. If the OSCF bit is not set, no external clock is available, the internal RC

clock will be used as the system clock, irrespective of the setting for

CKSEL1 and CKSEL2.

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