MC68HC908QY2 MOTOROLA [Motorola, Inc], MC68HC908QY2 Datasheet - Page 104

• Manufacturer Part Number: MC68HC908QY2
• Description: Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

Oscillator Module (OSC)
8.4.1.2 Internal to External Clock Switching
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MC68HC908QY4•MC68HC908QT4•MC68HC908QY2•MC68HC908QT2•MC68HC908QY1•MC68HC908QT1
NOTE:
When external clock source (external OSC, RC, or XTAL) is desired, the
user must perform the following steps:
Once transition to the external clock is done, the internal oscillator will
only be reactivated with reset. No post-switch clock monitor feature is
implemented (clock does not switch back to internal if external clock
dies).
1. For external crystal circuits only, OSCOPT[1:0] = 1:1: To help
2. Set CONFIG2 bits OSCOPT[1:0] according to
3. Create a software delay to wait the stabilization time needed for
4. After the manufacturer’s recommended delay has elapsed, the
5. After ECGON set is detected, the OSC module checks for
6. The OSC module then switches to the external clock. Logic
7. The OSC module first sets the ECGST bit in the OSCSTAT
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.
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.
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.
ECGON bit in the OSC status register (OSCSTAT) needs to be set
by the user software.
oscillator activity by waiting two external clock rising edges.
provides a glitch free transition.
register and then stops the internal oscillator.
Oscillator Module (OSC)
Table
8-2. The
MOTOROLA