MOTOROLA
Chapter 15. Clocks and Power Control
15-15
Part IV. Hardware Interface
the serial interface, serial communication controller, and serial management controllers to
continue operating at a Þxed frequency, even when the rest of the MPC860 is operating at
a reduced frequency.
SYNCCLK defaults to VCOOUT, but can be reduced in frequency by a frequency divider.
This frequency divider is controlled by SCCR[DFSYNC].
Figure 15-11. SYNCCLK Divider
The synchronization clock frequency is:
Limitations on SYNCCLK include:
¥
SYNCCLK must always have a frequency at least as high as GCLKx.
¥
SYNCCLK must and be at least two times the maximum serial clock rate used by
the serial ports in the system.
¥
If the time-slot assigner (TSA) is used, SYNCCLK must be at least 2.5 times the
maximum serial clock rate of the TSA.
15.3.2 The PIT and RTC Clock (PITRTCLK)
The PIT and RTC clock is generated either from EXTCLK or the crystal oscillator circuit
(OSCM). This input source can be divided by either 4 or 512. The PITRTCLK source and
divide factor are selected by SCCR[RTSEL] and SCCR[RTDIV].
When used by the real-time clock (RTC), the PITRTCLK source is Þrst divided as
determined by RTDIV, and then divided in the RTC circuits by either 8192 or 9600.
Therefore, in order for the RTC to count in seconds, the clock source must satisfy:
(EXTCLK or OSCM) / [(4 or 512) x (8192 or 9600)] = 1
The RTC will operate with other frequencies, but it will not count in units of seconds.
If there were only one clock source for the system, this requirement would limit the set of
desirable frequencies at which to operate the MPC860 . However, the MPC860 provides
two independent clock sources, EXTCLK and OSCM. To allow for maximum ßexibility in
system frequency selection independent of real-time clock operation, it is recommended
that a 32.768 KHz or 38.4 KHz crystal with the OSCM be used for the PITRTCLK source
if the RTC is to be used.
dfsync
cpm
synCclk
vccOut
SYNCCLK
freq
VCOOUT
freq
2
2
DFSYNC
´
(
)
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Содержание MPC860 PowerQUICC
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Страница 78: ...I iv MPC860 PowerQUICC UserÕs Manual MOTOROLA Part I Overview ...
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Страница 226: ...8 32 MPC860 PowerQUICC UserÕs Manual MOTOROLA Part II PowerPC Microprocessor Module ...
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Страница 274: ...III iv MPC860 PowerQUICC UserÕs Manual MOTOROLA Part III Configuration ...
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Страница 325: ...MOTOROLA Part IV Hardware Interface IV v Part IV Hardware Interface ...
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