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The current implementation allows the user to specify the desired CPU
clock value, in MHz, via an environment variable "cpuclk".

Three compile-time constants are used:

	CFG_866_OSCCLK            - input quartz clock
	CFG_866_CPUCLK_MIN        - minimum allowed CPU clock
	CFG_866_CPUCLK_MAX        - maximum allowed CPU clock
	CFG_866_CPUCLK_DEFAULT    - default CPU clock value

If the "cpuclk" environment variable value is within the CPUCLK_MIN /
CPUCLK_MAX limits, the specified value is used. Otherwise, the
default CPU clock value is set.

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Please make sure you understand what you are doing, and understand
the restrictions of your hardware (board, processor). For example,
ethernet will stop working for CPU clock frequencies below 25 MHz.

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Please note that for now the new clock-handling code has been enabled
for the TQM866M board only, even though it should be pretty much
common for other MPC859 / MPC866 based boards also. Our intention
here was to move in small steps and not to break the existing code
for other boards.