MC68HC908KX8 Datasheet, PDF (122/310 Pages) Motorola, Inc – Microcontrollers

Internal Clock Generator Module (ICG)
Before switching frequencies by changing the N value, the clock monitor
must be disabled. This is because when N is changed, the frequency of
the low-frequency base clock (IBASE) will change proportionally until the
digital loop filter has corrected the error. Since the clock monitor uses
IBASE, it could erroneously detect an inactive clock. The clock monitor
cannot be re-enabled until the internal clock is stable again (ICGS is set).
The following flow is an example of how to change the clock frequency:
• Verify there is no clock monitor interrupt by reading the CMF bit.
• Turn off the clock monitor.
• If desired, switch to the external clock (see 7.5.1 Switching Clock
Sources).
• Change the value of N.
• Switch back to internal (see 7.5.1 Switching Clock Sources),
if desired.
• Turn on the clock monitor (see 7.5.2 Enabling the Clock
Monitor), if desired.
7.5.6 Nominal Frequency Settling Time
Because the clock period of the internal clock (ICLK) is dependent on the
digital loop filter outputs (DDIV and DSTG) which cannot change
instantaneously, ICLK temporarily will operate at an incorrect clock
period when any operating condition changes. This happens whenever
the part is reset, the ICG multiply factor (N) is changed, the ICG trim
factor (TRIM) is changed, or the internal clock is enabled after inactivity
(stop mode or disabled operation). The time that the ICLK takes to adjust
to the correct period is known as the settling time.
Settling time depends primarily on how many corrections it takes to
change the clock period and the period of each correction. Since the
corrections require four periods of the low-frequency base clock
(4*τ_IBASE), and since ICLK is N (the ICG multiply factor for the desired
frequency) times faster than IBASE, each correction takes 4*N*τ_ICLK.
The period of ICLK, however, will vary as the corrections occur.
Technical Data
122
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 — Rev. 1.0
Internal Clock Generator Module (ICG)
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