blob: c8256a14cf2432d7670201dc5c0915038e0b5191 [file] [log] [blame]
wdenk68766092004-01-29 09:22:58 +00001The current implementation allows the user to specify the desired CPU
2clock value, in MHz, via an environment variable "cpuclk".
3
wdenk66ca92a2004-09-28 17:59:53 +00004Four compile-time constants are used:
wdenk68766092004-01-29 09:22:58 +00005
wdenk66ca92a2004-09-28 17:59:53 +00006 CONFIG_8xx_OSCLK - input quartz clock
7 CFG_8xx_CPUCLK_MIN - minimum allowed CPU clock
8 CFG_8xx_CPUCLK_MAX - maximum allowed CPU clock
9 CONFIG_8xx_CPUCLK_DEFAULT - default CPU clock value
wdenk68766092004-01-29 09:22:58 +000010
11If the "cpuclk" environment variable value is within the CPUCLK_MIN /
12CPUCLK_MAX limits, the specified value is used. Otherwise, the
13default CPU clock value is set.
14
wdenk75d1ea72004-01-31 20:06:54 +000015Please make sure you understand what you are doing, and understand
16the restrictions of your hardware (board, processor). For example,
17ethernet will stop working for CPU clock frequencies below 25 MHz.
18
wdenk66ca92a2004-09-28 17:59:53 +000019Please note that the new clock-handling code is enabled if
20CONFIG_8xx_CPUCLK_DEFAULT is defined. Since this mechanism supports
21only MPC866 and newer CPUs, this constant MUST NOT be defined for
22MPC823/850/860/862 series. The clock generation algorithm for older
23chips is different and has not been implemented yet. If you need it,
24your patch is welcome.