--- /dev/null
+The current implementation allows the user to specify the desired CPU
+clock value, in MHz, via an environment variable "cpuclk".
+
+Four compile-time constants are used:
+
+ CONFIG_8xx_OSCLK - input quartz clock
+ CONFIG_SYS_8xx_CPUCLK_MIN - minimum allowed CPU clock
+ CONFIG_SYS_8xx_CPUCLK_MAX - maximum allowed CPU clock
+ CONFIG_8xx_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.
+
+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.
+
+Please note that the new clock-handling code is enabled if
+CONFIG_8xx_CPUCLK_DEFAULT is defined. Since this mechanism supports
+only MPC866 and newer CPUs, this constant MUST NOT be defined for
+MPC823/850/860/862 series. The clock generation algorithm for older
+chips is different and has not been implemented yet. If you need it,
+your patch is welcome.