MCU Internal Clock Generator (ICG)
MC1321x Reference Manual, Rev. 1.6
14-12
Freescale Semiconductor
14.4
Initialization/Application Information
14.4.1
Introduction
This section is intended to give some basic direction on which configuration a user would want to select
when initializing the ICG. For some applications, the serial communication link may dictate the accuracy
of the clock reference. For other applications, lowest power consumption may be the chief clock
consideration. Still others may have lowest cost as the primary goal. The ICG allows great flexibility in
choosing which is best for any application.
The following sections contain initialization examples for various configurations.
NOTE
Hexadecimal values designated by a preceding $, binary values designated
by a preceding percent, and decimal values have no preceding character.
Important configuration information is repeated here for reference.
Table 14-3. ICG Configuration Consideration
Clock Reference Source = Internal
Clock Reference Source = External
FLL Engaged
FEI
4 MHz < f
Bus
< 20 MHz.
Medium power (will be less than FEE if oscillator
range = high)
Medium clock accuracy (After IRG is trimmed)
Lowest system cost
(no external components
required)
IRG is on. DCO is on.
1
1
The IRG typically consumes 100
μ
A. The FLL and DCO typically consumes 0.5 to 2.5 mA, depending upon output frequency.
For minimum power consumption and minimum jitter, choose N and R to be as small as possible.
FEE
4 MHz < f
Bus
< 20 MHz
Medium power (will be less than FEI if oscillator
range = low)
Good clock accuracy
Medium/High system cost (crystal, resonator or
external clock source required)
IRG is off. DCO is on.
FLL
Bypassed
SCM
This mode is mainly provided for quick and reliable
system startup.
3 MHz < f
Bus
< 5 MHz (default).
3 MHz < f
Bus
< 20 MHz (via filter bits).
Medium power
Poor accuracy.
IRG is off. DCO is on and open loop.
FBE
f
Bus
range <= 8 MHz when crystal or resonator is
used.
Lowest power
Highest clock accuracy
Medium/High system cost (Crystal, resonator or
external clock source required)
IRG is off. DCO is off.
Table 14-4. ICGOUT Frequency Calculation Options
Clock Scheme
f
ICGOUT
1
P
Note
SCM — Self-clocked Mode (FLL bypassed internal)
f
ICGDCLK
/ R
NA
Typical f
ICGOUT
=
8 MHz out of reset
FBE — FLL bypassed external
f
ext
/ R
NA
FEI — FLL engaged internal
(f
IRG
/ 7)* 64*N / R
64
Typical f
IRG
= 243 kHz
FEE — FLL engaged external
f
ext
* P * N / R
Range = 0 ; P = 64
Range = 1; P = 1
Содержание freescale semiconductor MC13211
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