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MC68341 USER’S MANUAL
MOTOROLA
kHz reference signal. The system frequency is controlled by the frequency control bits in
the upper byte of the SYNCR as follows:
FSYSTEM = FCRYSTAL [2
(2W+X+3Z–1) ]
× (Y+1)
SYNCR
$004
15
14
13
12
11
10
9876543210
W
X
Y5
Y4
Y3
Y2
Y1
Y0
Z
V
0
SLIMP
SLOCK
RSTEN
STSIM
STEXT
RESET:
00111111100U
U
000
U = Unaffected by reset
Supervisor Only
W—Frequency Control Bit
This bit controls the prescaler tap in the synthesizer feedback loop. Setting the bit
increases the VCO frequency by a factor of 4, requiring a time delay for the VCO to
relock (see equation for determining system frequency).
X—Frequency Control Bit
This bit controls a divide-by-two prescaler, which is not in the synthesizer feedback
loop. Setting the bit doubles the system clock speed without changing the VCO speed,
as specified in the equation for determining system frequency; therefore, no delay is
incurred to relock the VCO.
Y5–Y0—Frequency Control Bits
The Y-bits, with a value from 0–63, control the modulus downcounter in the synthesizer
feedback loop, causing it to divide by the value of Y+1 (see the equation for determining
system frequency). Changing these bits requires a time delay for the VCO to relock.
Z—Frequency Control Bit
This bit controls a divide-by-eight. Setting the bit increases the system clock frequency
by a factor of eight, without changing the VCO speed.
V—Frequency Control Bit
This bit controls a divide-by-two on the external clock input (EXTCLK). Clearing the bit
allows the VCO to run at half the speed of the external clock. Changing the bit value
requires a time delay for the VCO to relock.
Bits 5—Reserved
Bit 5 is reserved.
SLIMP—Limp Mode
1 = A loss of input signal reference has been detected, and the VCO is running at
approximately one-half the maximum speed (affected by the X-bit ), determined
from an internal voltage reference.
0 = External input signal frequency is at VCO reference.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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