Oscillator Module (OSC)
Functional Description
MC68HC908QY/QT Family — Rev. 3
Data Sheet
MOTOROLA
Oscillator Module (OSC)
97
11.3.1 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 adjustment 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 11-3 shows how BUSCLKX4 is derived from INTCLK and, like the RC
oscillator, OSC2 can output BUSCLKX4 by setting OSC2EN in PTAPUE
11.3.1.1 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
allows the internal clock frequency to be set to 12.8 MHz ± 5%.
All devices are programmed with a trim value in a reserved FLASH location,
$FFC0. This value can be copied from the FLASH to the OSCTRIM register
($0038) during reset initialization.
Reset loads OSCTRIM with a default value of $80.
WARNING:
Bulk FLASH erasure will set location $FFC0 to $FF and the factory
programmed value will be lost.
11.3.1.2 Internal to External Clock Switching
When external clock source (external OSC, RC, or XTAL) is desired, the user must
perform the following steps:
1.
For external crystal circuits only, OSCOPT[1:0] = 1:1: To help precharge an
external crystal oscillator, set PTA4 (OSC2) as an output and drive high for
several cycles. This may help the crystal circuit start more robustly.
2.
Set CONFIG2 bits OSCOPT[1:0] according to Table 11-2. The oscillator
module control logic will then set OSC1 as an external clock input and, if the
external crystal option is selected, OSC2 will also be set as the clock output.
3.
Create a software delay to wait the stabilization time needed for the selected
clock source (crystal, resonator, RC) as recommended by the component
manufacturer. A good rule of thumb for crystal oscillators is to wait 4096
cycles of the crystal frequency, i.e., for a 4-MHz crystal, wait approximately
1 msec.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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