MC908QT4MPE datasheet(96/184 Pages) FREESCALE

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Oscillator Module (OSC)

MC68HC908QY/QT Family Data Sheet, Rev. 5

Freescale Semiconductor

11.8.1 Oscillator Status Register

The oscillator status register (OSCSTAT) contains the bits for switching from internal to external clock

sources.

ECGON — External Clock Generator On Bit

This read/write bit enables external clock generator, so that the switching process can be initiated. This

bit is forced low during reset. This bit is ignored in monitor mode with the internal oscillator bypassed,

PTM or CTM mode.

1 = External clock generator enabled

0 = External clock generator disabled

ECGST — External Clock Status Bit

This read-only bit indicates whether or not an external clock source is engaged to drive the system

clock.

1 = An external clock source engaged

0 = An external clock source disengaged

11.8.2 Oscillator Trim Register (OSCTRIM)

TRIM7–TRIM0 — Internal Oscillator Trim Factor Bits

These read/write bits change the size of the internal capacitor used by the internal oscillator. By

measuring the period of the internal clock and adjusting this factor accordingly, the frequency of the

internal clock can be fine tuned. Increasing (decreasing) this factor by one increases (decreases) the

period by approximately 0.2% of the untrimmed period (the period for TRIM = $80). The trimmed

frequency is guaranteed not to vary by more than ±5% over the full specified range of temperature and

voltage. The reset value is $80, which sets the frequency to 12.8 MHz (3.2 MHz bus speed) ±25%.

Address: $0036

Bit 7

654321

Bit 0

Read:

RRRRRR

ECGON

ECGST

Write:

Reset:

00000000

=Reserved

= Unimplemented

Figure 11-4. Oscillator Status Register (OSCSTAT)

Address: $0038

Bit 7

654321

Bit 0

Read:

TRIM7

TRIM6

TRIM5

TRIM4

TRIM3

TRIM2

TRIM1

TRIM0

Write:

Reset:

10000000

Figure 11-5. Oscillator Trim Register (OSCTRIM)

MC908QT4MPE Datasheet(PDF) 96 Page - Freescale Semiconductor, Inc

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