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4.3.2.8 SOFTWARE SERVICE REGISTER (SWSR). The SWSR is the location to which
the software watchdog servicing sequence is written. The software watchdog can be
enabled or disabled by the SWE bit in the SYPCR. SWSR can be written at any time, but
returns all zeros when read.
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:
SYNCR
15
14
13
12
W
X
Y5
Y4
RESET:
0
0
1
1
U = Unaffected by reset
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 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.
Bits 7–5—Reserved
Bit 7 is reserved for factory testing.
4-28
Freescale Semiconductor, Inc.
SWSR
7
6
5
4
SWSR7 SWSR6 SWSR5 SWSR4 SWSR3 SWSR2 SWSR1 SWSR0
RESET:
0
0
0
0
F
= F
SYSTEM
CRYSTAL
11
10
9
8
Y3
Y2
Y1
Y0
1
1
1
1
MC68340 USER'S MANUAL
For More Information On This Product,
Go to: www.freescale.com
$027
3
2
1
0
0
0
0
0
Supervisor Only
(2+2W+X)
[2
]
(Y+1)
7
6
5
4
RSVD
0
0
SLIMP SLOCK RSTEN
0
0
0
U
$004
3
2
1
0
STSIM
STEXT
U
0
0
0
Supervisor Only
MOTOROLA
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