Datasheet

Oscillator Module (OSC)
Functional Description
MC68HC908QY4•MC68HC908QT4•MC68HC908QY2•MC68HC908QT2•MC68HC908QY1•MC68HC908QT1
MOTOROLA Oscillator Module (OSC) 103
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8.4.1 Internal Oscillator
The internal oscillator circuit is designed for use with no external
components to provide a clock source with tolerance less than ±25%
untrimmed. An 8-bit trimming register allows the adjust to a tolerance of
less than ±5%.
The internal oscillator will generate a clock of 12.8 MHz typical (INTCLK)
resulting in a bus speed (internal clock ÷ 4) of 3.2 MHz. 3.2 MHz came
from the maximum bus speed guaranteed at 3 V which is 4 MHz. Since
the internal oscillator will have a ±25% tolerance (pre-trim), then the
+25% case should not allow a frequency higher than 4 MHz:
3.2 MHz + 25% = 4 MHz
Figure 8-2 shows how BUSCLKX4 is derived from INTCLK and, like the
RC oscillator, OSC2 can output BUSCLKX4 by setting OSC2EN in
PTAPUE register. See Section 12. Input/Output (I/O) Ports.
8.4.1.1 Internal Oscillator Trimming
The 8-bit trimming register, OSCTRIM, allows a clock period adjust of
+127 and –128 steps. Increasing OSCTRIM value increases the clock
period. Trimming will allow the internal clock frequency value fit in a ±5%
range around12.8 MHz.
There’s an option to order a trimmed version of MC68HC908QY4. The
trimming value will be provided in a known FLASH location, $FFC0. So
that the user would be able to copy this byte from the FLASH to the
OSCTRIM register right at the beginning of assembly code.
Reset loads OSCTRIM with a default value of $80.