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4- 12
MC68341 USER’S MANUAL
MOTOROLA
signal reference has been detected. The RSTEN bit in the SYNCR controls whether an
input signal loss causes a system reset or causes the device to operate in limp mode. The
SLOCK bit in the SYNCR indicates when the VCO has locked onto the desired frequency
or if an external clock is being used.
4.2.3.1 PHASE COMPARATOR AND FILTER. The phase comparator takes the output of
the frequency divider and compares it to an external input signal reference. The result of
this compare is low-pass filtered and used to control the VCO. The comparator also
detects when the external crystal or oscillator stops running to initiate the limp mode for
the system clock.
The PLL requires an external low-leakage filter capacitor, typically in the range from 0.01
to 0.1
F, connected between the XFC and VCCSYN pins. The XFC capacitor should
provide 50-M
insulation but should not be electrolytic. Smaller values of the external filter
capacitor provide a faster response time for the PLL, and larger values provide greater
frequency stability. For external clock mode without PLL, the XFC pin can be left open.
4.2.3.2 FREQUENCY DIVIDER. The frequency divider circuits divide the VCO frequency
down to the reference frequency for the phase comparator. The frequency divider consists
of 1) a 2-bit prescaler controlled by the W-bit in the SYNCR and 2) a 6-bit modulo
downcounter controlled by the Y-bits in the SYNCR.
The frequency of CLKOUT can additionally be divided by 2 with the X-bit in SYNCR and
divided by 8 with the Z-bit in SYNCR.
Several factors are important to the design of the system clock. The resulting system clock
frequency must be within the limits specified for the device. The frequency of the system
clock is given by the following two equations:
FSYSTEM = FCRYSTAL [2(
2W+X+3Z–1)] X (Y+1)
The maximum VCO frequency limit must also be observed. The VCO frequency is given
by the following equation:
FVCO = FCRYSTAL[(2)
(4+2W)] X (Y+1) = F
SYSTEM[2
(5–X–3Z) ]
The VCO upper frequency limit must be considered when programming the SYNCR. Both
the system clock and VCO frequency limits are given in Section 12 Electrical
Characteristics. Table 4-2 lists some frequencies available from various combinations of
SYNCR bits with a reference frequency of 32.768-KHz.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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