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MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-6
4.3 System Clock
The system clock in the SCIM2E provides timing signals for IMB modules and the
external bus interface. MC68F375 MCUs are fully static MCU designs; register and
memory contents are not affected by clock rate changes. System hardware and soft-
ware support clock rate changes during operation.
CAUTION
The SCIM2E system clock is different from the SCIM. Do not refer to
SCIM documentation for SCIM2E clock information.
4.3.1 System Clock Sources
The system clock signal can be generated from one of three sources. An internal
phase-locked loop (PLL) can synthesize the clock from either a slow or fast reference.
The clock signal can also be input directly from an external 2X frequency source.
The slow reference is typically a 32.768 KHz crystal; the fast reference is typically a
4 MHz crystal. The slow and fast references may be provided by sources other than a
crystal. Keep these clock sources in mind while reading the rest of this section.
The system clock source is determined upon RESET assertion by the state of the
VDDSYN/MODCLK and FASTREF/PF0 pins. In addition to selecting the system clock
source, these pins govern the functionality of the W, X, and Y bits in the synthesizer
control register (SYNCR) and the equation that determines the MCU operating fre-
quency. Table 4-5 summarizes system clock source information. For more
The parameter “fREF” refers to the frequency of the clock source connected to the
EXTAL pin. The parameter “
fSYS” refers to the operating frequency of the MCU and
Table 4-5 System Clock Sources
VDDSYN/
MODCLK
FASTREF/
PF0
Clock Mode
SYNCR W/X/Y Bit
Assignments and
Reset Values
MCU Operating
Frequency Equation
0
X1
NOTES:
1. In external clock mode, the FASTREF/PF0 pin has no effect on clock operation.
External Clock
W : has no effect
X=0b1 : Divider
Y[2:0] = 0b000 : Divider
1
0
Slow Reference
W = 0b0 : Multiplier
X = 0b0 : Divider
Y[5:0] = 0b11111 : Multiplier
1
Fast Reference
W[2:0] = 0b011 : Multiplier
X = 0b0 : Divider
Y[2:0] = 0b111 : Divider
f
sys
f
ref
2
---------
2X
–
() 2
Y
()
------------------------------ for Y
7
≤
,
=
f
sys
4f
ref
Y1
+
() 2
2W
X
+
()
=
f
sys
f
ref
W1
+
()
2X
–
() 2
Y
()
------------------------------, for Y 7
=
≤
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Freescale Semiconductor, Inc.
For More Information On This Product,
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