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MC68349 USER'S MANUAL
MOTOROLA
4.3.3 Clock Synthesizer Control Register (SYNCR)
The SYNCR can be read or written only in supervisor mode. The reset state of SYNCR
produces an operating frequency of 8.39 MHz when the PLL is referenced to a 32.768-
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
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 speed 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 outside 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 prescaler, which is outside the synthesizer feedback
loop. Clearing the bit decreases the system clock speed by a factor of eight without
changing the VCO speed, as specified in the equation for determining system
frequency; therefore, no delay is incurred to relock the VCO.
Bits 6–5—Reserved by Motorola
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.