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| 型号： | MC68CK338CPV14B1 |
| 厂商： | FREESCALE SEMICONDUCTOR INC |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 32-BIT, 14.4 MHz, MICROCONTROLLER, PQFP144 |
| 封装： | PLASTIC, TQFP-144 |
| 文件页数： | 43/133页 |
| 文件大小： | 944K |
| 代理商： | MC68CK338CPV14B1 |
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MC68CK338
MOTOROLA
MC68CK338TS/D
17
Figure 6 System Clock Oscillator Circuit
When an external system clock signal is applied (PLL disabled, MODCLK = 0 during reset), the duty
cycle of the input is critical, especially at operating frequencies close to maximum. The relationship be-
tween clock signal duty cycle and clock signal period is expressed:
When the system clock signal is applied directly to the EXTAL pin (PLL is disabled, MODCLK = 0 during
reset), or the clock synthesizer reference frequency is supplied by a source other than a crystal (PLL
enabled, MODCLK = 1 during reset), the XTAL pin must be left floating. In either case, the frequency of
the signal applied to EXTAL may not exceed the maximum system clock frequency (PLL disabled) or
the maximum clock synthesizer reference frequency (PLL enabled).
3.3.2 Clock Synthesizer Operation
VDDSYN is used to power the clock circuits when the phase-locked loop is used. A separate power
source increases MCU noise immunity and can be used to run the clock when the MCU is powered
down. A quiet power supply must be used as the VDDSYN source. Adequate external bypass capacitors
should be placed as close as possible to the VDDSYN pin to assure stable operating frequency. When
an external system clock signal is applied and the PLL is disabled, VDDSYN should be connected to the
VDD supply. Refer to the SIM Reference Manual (SIMRM/AD) for more information regarding system
clock power supply conditioning.
A voltage controlled oscillator (VCO) generates the system clock signal. To maintain a 50% clock duty
cycle, the VCO frequency (fVCO) is either two or four times the system clock frequency, depending on
the state of the X bit in SYNCR. A portion of the clock signal is fed back to a divider/counter. The divider
controls the frequency of one input to a phase comparator. The other phase comparator input is the
reference signal connected to the EXTAL pin. The comparator generates a control signal proportional
to the difference in phase between the two inputs. The signal is low-pass filtered and used to correct
the VCO output frequency.
Filter circuit implementation can vary, depending upon the external environment and required clock sta-
bility. Figure 7 shows a recommended system clock filter network. XFC pin leakage must be kept within
specified limits to maintain optimum stability and PLL performance.
An external filter network connected to the XFC pin is not required when an external system clock signal
is applied and the PLL is disabled. The XFC pin must be left floating in this case.
338 OSCILLATOR
EXTAL
XTAL
10M
*
4.7 k
*
22 pF*
VSSI
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.
*
R1
C1
C2
R2
Minimum External Clock Period
Minimum External Clock High/Low Time
50 %
Percentage Variation of External Clock Input Duty Cycle
–
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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