mc68hc908qf4 Freescale Semiconductor, Inc, mc68hc908qf4 Datasheet - Page 93

• Manufacturer Part Number: mc68hc908qf4
• Description: M68hc08 Microcontrollers Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

11.3.1.1 Internal Oscillator Trimming
11.3.1.2 Internal to External Clock Switching
MC68HC908QF4 — Rev. 1.0
MOTOROLA
WARNING:
NOTE:
Figure 11-3
oscillator, OSC2 can output BUSCLKX4 by setting OSC2EN in PTAPUE register.
See
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 4.0 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.
Bulk FLASH erasure will set location $FFC0 to $FF and the factory
programmed value will be lost.
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 precharge an
2. Set CONFIG2 bits OSCOPT[1:0] according to
3. Create a software delay to wait the stabilization time needed for the selected
4. After the manufacturer’s recommended delay has elapsed, the ECGON bit
5. After ECGON set is detected, the OSC module checks for oscillator activity
6. The OSC module then switches to the external clock. Logic provides a glitch
7. The OSC module first sets the ECGST bit in the OSCSTAT register and then
Section 13. Input/Output (I/O)
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.
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.
in the OSC status register (OSCSTAT) needs to be set by the user software.
by waiting two external clock rising edges.
free transition.
stops the internal oscillator.
Freescale Semiconductor, Inc.
For More Information On This Product,
shows how BUSCLKX4 is derived from INTCLK and, like the RC
Go to: www.freescale.com
Oscillator Module (OSC)
Ports.
11.7 CONFIG2
Oscillator Module (OSC)
Functional Description
Options. The
Data Sheet
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