SYSTEM INTEGRATION MODULE
MC68332
4-12
USER’S MANUAL
When the clock synthesizer is used, control register SYNCR determines operating fre-
quency and various modes of operation. The SYNCR W bit controls a three-bit pres-
caler in the feedback divider. Setting W increases VCO speed by a factor of four. The
SYNCR Y field determines the count modulus for a modulo 64 down counter, causing
it to divide by a value of Y + 1. When W or Y values change, VCO frequency changes,
and there is a VCO relock delay. The SYNCR X bit controls a divide-by-two circuit that
is not in the synthesizer feedback loop. When X = 0 (reset state), the divider is en-
abled, and system clock frequency is one-fourth VCO frequency; setting X disables
the divider, doubling clock speed without changing VCO speed. There is no relock de-
lay when clock speed is changed by the X bit.
Clock frequency is determined by SYNCR bit settings as follows:
The reset state of SYNCR ($3F00) produces a modulus-64 count.
For the device to perform correctly, system clock and VCO frequencies selected by
the W, X, and Y bits must be within the limits specified for the MCU. Do not use a com-
bination of bit values that selects either an operating frequency or a VCO frequency
greater than the maximum specified values in
shows clock control multipliers for all possible combinations of SYNCR bits.
shows clock frequencies available with a 32.768-kHz reference and a max-
imum specified clock frequency of 20.97 MHz.
Table 4-7 Clock Control Multipliers
To obtain clock frequency in kilohertz, find counter modulus in the left column, then multiply reference frequency by value
in appropriate prescaler cell.
Modulus
Prescalers
Y
[W:X] = 00
[W:X] = 01
[W:X] = 10
[W:X] = 11
000000
4
8
16
32
000001
8
16
32
64
000010
12
24
48
96
011111
16
32
64
128
000011
20
40
80
160
000100
24
48
96
192
000101
28
56
112
224
000110
32
64
128
256
000111
36
72
144
288
001000
40
80
160
320
001001
44
88
176
352
001010
48
96
192
384
001011
52
104
208
416
001100
56
112
224
448
001101
60
120
240
480
001110
64
128
256
512
001111
68
136
272
544
010000
72
144
288
576
010001
76
152
304
608
F
SYSTEM
F
REFERENCE
4 Y 1
+
(
)
2
2W X
+
(
)
[
]
=
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
.
..