
MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-13
4.3.5 External Clock Mode
In external clock mode, the clock source, which should be 2x the desired system fre-
quency, must be driven onto the EXTAL pin. This clock is used to generate the system
clock directly (the VCO is turned off). At reset, the system clock frequency is one-half
the external clock frequency. If this frequency is the the maximum specified system
clock frequency, it must not violate strict minimum duty cycle requirements. A block
diagram of external clock mode is show in Figure 4-4.
In this mode, the six-stage Y divider and the one-stage X divider are placed in the clock
output path such that the input clock may be divided down by as much as 128 to pro-
duce the system clock. When this is done, it is not necessary to meet the input duty
cycle restrictions. The Y bit divider is a six-stage divider chain whose output tap is con-
trolled by the three Y register bits. The X bit divider is a single-stage divider which is
bypassed when X is set to 1. X is 1 and Y is 0 after reset, so that the system clock is
the same as the external clock.
NOTE
Setting Y to 7 has the same effect as setting it to 6; the maximum divi-
sor is 26. The X and Y bit dividers are in the output clock path.
Therefore, changing the X or Y bits in this mode causes the fre-
quency to change without a delay.
Table 4-7 CLKOUT In Fast Reference Mode with 4.0 MHz Reference
Y
X=0
W=000
W=001
W=010
W=011
W=100
W=101
W=110
W=111
0=000
2,000,000
4,000,000
6,000,000
8,000,000
10,000,000 12,000,000 14,000,000 16,000,000
1=001
1,000,000
2,000,000
3,000,000
4,000,000
5,000,000
6,000,000
7,000,000
8,000,000
2=010
500,000
1,000,000
1,500,000
2,000,000
2,500,000
3,000,000
3,500,000
4,000,000
3=011
250,000
500,000
750,000
1,000,000
1,250,000
1,500,000
1,750,000
2,000,000
4=100
125,000
250,000
375,000
500,000
625,000
750,000
875,000
1,000,000
5=101
62,500
125,000
187,500
250,000
312,500
375,000
437,500
500,000
6=110
31,250
62,500
93,750
125,000
156,250
187,500
218,750
250,000
7=111
31,250
62,500
93,750
125,000
156,250
187,500
218,750
250,000
Y
X=1
W=000
W=001
W=010
W=011
W=100
W=101
W=110
W=111
0=000
4,000,000
8,000,000
12,000,000 16,000,000 20,000,000 24,000,000 28,000,000 32,000,000
1=001
2,000,000
4,000,000
6,000,000
8,000,000
10,000,000 12,000,000 14,000,000 16,000,000
2=010
1,000,000
2,000,000
3,000,000
4,000,000
5,000,000
6,000,000
7,000,000
8,000,000
3=011
500,000
1,000,000
1,500,000
2,000,000
2,500,000
3,000,000
3,500,000
4,000,000
4=100
250,000
500,000
750,000
1,000,000
1,250,000
1,500,000
1,750,000
2,000,000
5=101
125,000
250,000
375,000
500,000
625,000
750,000
375,000
1,000,000
6=110
62,500
125,000
187,500
250,000
312,500
375,000
187,500
500,000
7=111
62,500
125,000
187,500
250,000
312,500
375,000
187,500
500,000
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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