
MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
5-7
When the clock synthesizer is used, SYNCR determines the system clock frequency
and certain operating parameters. The W and Y[5:0] bits are located in the PLL feed-
back path, enabling frequency multiplication by a factor of up to 256. When the W or
Y values change, VCO frequency changes, and there is a VCO relock delay. The SYN-
CR X bit controls a divide-by circuit that is not in the synthesizer feedback loop. When
X = 0 (reset state), a divide-by-four circuit is enabled, and the system clock frequency
is one-fourth the VCO frequency (fVCO). When X = 1, a divide-by-two circuit is enabled
and system clock frequency is one-half the VCO frequency (fVCO). There is no relock
delay 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) results in a power-on fsys of 8.388 MHz when fref
is 4.194 MHz.
For the device to operate correctly, the clock frequency selected by the W, X, and Y
bits must be within the limits specified for the MCU.
Internal VCO frequency is determined by the following equations:
or
Table 5-2 shows clock control multipliers for all possible combinations of SYNCR bits.
To obtain clock frequency, find counter modulus in the leftmost column, then multiply
the reference frequency by the value in the appropriate prescaler cell. Shaded cells
exceed the maximum system clock frequency at the time of manual publication; how-
ever, they may be usable in the future. Refer to APPENDIX A ELECTRICAL CHAR-
ACTERISTICS for maximum allowable clock rate.
Table 5-3 shows clock frequencies available with a 4.194 MHz reference and a max-
imum specified clock frequency of 20.97 MHz. To obtain clock frequency, find counter
modulus in the leftmost column, then refer to appropriate prescaler cell. Shaded cells
exceed the maximum system clock frequency at the time of manual publication; how-
ever, they may be usable in the future. Refer to APPENDIX A ELECTRICAL CHAR-
ACTERISTICS for maximum system frequency (fsys).
f
sys
f
ref
128
----------
4Y
1
+
() 2
2W
X
+
()
[]
=
f
VCO
4f
sys if X = 0
=
f
VCO
2f
sys if X = 1
=