MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-5
Figure 5-3 System Clock Oscillator Circuit
If a fast reference frequency is provided to the PLL from a source other than a crystal,
or an external system clock signal is applied through the EXTAL pin, the XTAL pin
must be left floating.
When an external system clock signal is applied (MODCLK = 0 during reset), the PLL
is disabled. The duty cycle of this signal is critical, especially at operating frequencies
close to maximum. The relationship between clock signal duty cycle and clock signal
period is expressed as follows:
5.3.2 Clock Synthesizer Operation
VDDSYN is used to power the clock circuits when the system clock is synthesized from
either a crystal or an externally supplied reference frequency. 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 a stable operating frequency. When an external system clock
signal is applied and the PLL is disabled, VDDSYN should be connected to the VDD sup-
ply. Refer to the SIM Reference Manual (SIMRM/AD) for more information regarding
system clock power supply conditioning.
A voltage controlled oscillator (VCO) in the PLL 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. 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 a reference signal, either
from the crystal oscillator or from an external source. The comparator generates a con-
trol signal proportional to the difference in phase between the two inputs. This signal
is low-pass filtered and used to correct the VCO output frequency.
32 OSCILLATOR 4M
EXTAL
XTAL
1 M
1.5 k
27 pF*
VSS
RESISTANCE AND CAPACITANCE BASED ON A TEST CIRCUIT CONSTRUCTED WITH A KDS041-18 4.194 MHz 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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