| 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 |
| CFG_8xx_CPUCLK_MIN - minimum allowed CPU clock |
| CFG_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. |