projects / J-u-boot.git / blame
| Commit | Line | Data |
|---|---|---|
| 68766094 WD |
1 | The current implementation allows the user to specify the desired CPU |
| 2 | clock value, in MHz, via an environment variable "cpuclk". | |
| 3 | ||
| 66ca92a5 | 4 | Four compile-time constants are used: |
| 68766094 | 5 | |
| 66ca92a5 | 6 | CONFIG_8xx_OSCLK - input quartz clock |
| 6d0f6bcf JCPV |
7 | CONFIG_SYS_8xx_CPUCLK_MIN - minimum allowed CPU clock |
| 8 | CONFIG_SYS_8xx_CPUCLK_MAX - maximum allowed CPU clock | |
| 66ca92a5 | 9 | CONFIG_8xx_CPUCLK_DEFAULT - default CPU clock value |
| 68766094 WD |
10 | |
| 11 | If the "cpuclk" environment variable value is within the CPUCLK_MIN / | |
| 12 | CPUCLK_MAX limits, the specified value is used. Otherwise, the | |
| 13 | default CPU clock value is set. | |
| 14 | ||
| 75d1ea7f WD |
15 | Please make sure you understand what you are doing, and understand |
| 16 | the restrictions of your hardware (board, processor). For example, | |
| 17 | ethernet will stop working for CPU clock frequencies below 25 MHz. | |
| 18 | ||
| 66ca92a5 WD |
19 | Please note that the new clock-handling code is enabled if |
| 20 | CONFIG_8xx_CPUCLK_DEFAULT is defined. Since this mechanism supports | |
| 21 | only MPC866 and newer CPUs, this constant MUST NOT be defined for | |
| 22 | MPC823/850/860/862 series. The clock generation algorithm for older | |
| 23 | chips is different and has not been implemented yet. If you need it, | |
| 24 | your patch is welcome. |