MCU Internal Clock Generator (ICG)
MC1321x Reference Manual, Rev. 1.6
14-2
Freescale Semiconductor
•
Frequency-locked loop
— A frequency-locked loop (FLL) stage takes either the internal or
external clock source and multiplies it to a higher frequency. Status bits provide information when
the circuit has achieved lock and when it falls out of lock. Additionally, this block can monitor the
external reference clock and signals whether the clock is valid or not.
•
Clock select block
— The clock select block provides several switch options for connecting
different clock sources to the system clock tree. ICGDCLK is the multiplied clock frequency out
of the FLL, ICGERCLK is the reference clock frequency from the crystal or external clock source,
and FFE (fixed frequency enable) is a control signal used to control the system fixed frequency
clock (XCLK). ICGLCLK is the clock source for the background debug controller (BDC).
The module is intended to be very user friendly with many of the features occurring automatically without
user intervention. To quickly configure the module, go to
Section 14.4, “Initialization/Application
and pick an example that best suits the application needs.
14.1.1
Features
Features of the ICG and clock distribution system:
•
Several options for the primary clock source allow a wide range of cost, frequency, and precision
choices:
— 32 kHz–100 kHz crystal or resonator
— 1 MHz–16 MHz crystal or resonator
— External clock
— Internal reference generator
•
Defaults to Self-clocked Mode
to minimize start-up delays
•
Frequency-locked loop (FLL) generates 8 MHz to 40 MHz (for bus rates up to 20 MHz)
— Uses external or internal clock as reference frequency
•
Automatic lockout of non-running clock sources
•
Reset or interrupt on loss of clock or loss of FLL lock
•
Digitally-controlled oscillator (DCO) preserves previous frequency settings, allowing fast
frequency lock when recovering from Stop3 Mode
•
DCO will maintain operating frequency during a loss or removal of reference clock
•
Post-FLL divider selects 1 of 8 bus rate divisors (/1 through /128)
•
Separate self-clocked source for real-time interrupt
•
Trimmable internal clock source supports SCI communications without additional external
components
•
Automatic FLL engagement after lock is acquired
•
Selectable low-power/high-gain oscillator modes
Содержание freescale semiconductor MC13211
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