MC68332 Datasheet, PDF (23/88 Pages) Freescale Semiconductor, Inc – 32-Bit Modular Microcontroller

Freescale Semiconductor, Inc.
3.3 System Clock
The system clock in the SIM provides timing signals for the IMB modules and for an external peripheral
bus. Because MCU operation is fully static, register and memory contents are not affected when the
clock rate changes. System hardware and software support changes in the clock rate during operation.
The system clock signal can be generated in three ways. An internal phase-locked loop can synthesize
the clock from an internal or external frequency source, or the clock signal can be input from an external
source.
Following is a block diagram of the clock submodule.
22 pF2
VSSI
EXTAL
330k
10M
22 pF2
VSSI
XTAL
VDDSYN
XFC1
0.1µF
0.1µF
.01µF
XFC PIN
VDDSYN
VSSI
CRYSTAL
OSCILLATOR
PHASE
COMPARATOR
LOW-PASS
FILTER
VCO
W
FEEDBACK DIVIDER
Y
X
SYSTEM CLOCK CONTROL
SYSTEM
CLOCK
CLKOUT
1. MUST BE LOW-LEAKAGE CAPACITOR (INSULATION RESISTANCE 30,000 MΩ OR GREATER).
2. RESISTANCE AND CAPACITANCE BASED ON A TEST CIRCUIT CONSTRUCTED WITH A DAISHINKU DMX-38 32.768-kHz CRYSTAL.
SPECIFIC COMPONENTS MUST BE BASED ON CRYSTAL TYPE. CONTACT CRYSTAL VENDOR FOR EXACT CIRCUIT.
SYS CLOCK
BLOCK 32KHZ
Figure 7 System Clock Block Diagram
3.3.1 Clock Sources
The state of the clock mode (MODCLK) pin during reset determines the clock source. When MODCLK
is held high during reset, the clock synthesizer generates a clock signal from either a crystal oscillator
or an external reference input. Clock synthesizer control register SYNCR determines operating frequen-
cy and various modes of operation. When MODCLK is held low during reset, the clock synthesizer is
disabled, and an external system clock signal must be applied. When the synthesizer is disabled, SYN-
CR control bits have no effect.
A reference crystal must be connected between the EXTAL and XTAL pins to use the internal oscillator.
Use of a 32.768-kHz crystal is recommended. These crystals are inexpensive and readily available. If
an external reference signal or an external system clock signal is applied through the EXTAL pin, the
XTAL pin must be left floating. External reference signal frequency must be less than or equal to max-
imum specified reference frequency. External system clock signal frequency must be less than or equal
to maximum specified system clock frequency.
MC68332
MC68332TS/D
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MOTOROLA
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