Typewriter User Manual

4- 28 MC68340 USER’S MANUAL MOTOROLA
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.
SWSR $027
76543210
SWSR7 SWSR6 SWSR5 SWSR4 SWSR3 SWSR2 SWSR1 SWSR0
RESET:
00000000
Supervisor Only
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:
F
SYSTEM
= F
CRYSTAL
[2
(2+2W+X)
] × (Y+1)
SYNCR $004
1514131211109876543210
W X Y5 Y4 Y3 Y2 Y1 Y0 RSVD 0 0 SLIMP SLOCK RSTEN STSIM STEXT
RESET:
00111111000UU000
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 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.
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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