README.MPC866 1.1 KB

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  1. The current implementation allows the user to specify the desired CPU
  2. clock value, in MHz, via an environment variable "cpuclk".
  3. Four compile-time constants are used:
  4. CONFIG_8xx_OSCLK - input quartz clock
  5. CONFIG_SYS_8xx_CPUCLK_MIN - minimum allowed CPU clock
  6. CONFIG_SYS_8xx_CPUCLK_MAX - maximum allowed CPU clock
  7. CONFIG_8xx_CPUCLK_DEFAULT - default CPU clock value
  8. If the "cpuclk" environment variable value is within the CPUCLK_MIN /
  9. CPUCLK_MAX limits, the specified value is used. Otherwise, the
  10. default CPU clock value is set.
  11. Please make sure you understand what you are doing, and understand
  12. the restrictions of your hardware (board, processor). For example,
  13. ethernet will stop working for CPU clock frequencies below 25 MHz.
  14. Please note that the new clock-handling code is enabled if
  15. CONFIG_8xx_CPUCLK_DEFAULT is defined. Since this mechanism supports
  16. only MPC866 and newer CPUs, this constant MUST NOT be defined for
  17. MPC823/850/860/862 series. The clock generation algorithm for older
  18. chips is different and has not been implemented yet. If you need it,
  19. your patch is welcome.