[people/ms/u-boot.git] / doc / README.MPC866

The current implementation allows the user to specify the desired CPU
clock value, in MHz, via an environment variable "cpuclk".

Three compile-time constants are used:

	CFG_866_OSCCLK            - input quartz clock
	CFG_866_CPUCLK_MIN        - minimum allowed CPU clock
	CFG_866_CPUCLK_MAX        - maximum allowed CPU clock
	CFG_866_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 for now the new clock-handling code has been enabled
for the TQM866M board only, even though it should be pretty much
common for other MPC859 / MPC866 based boards also. Our intention
here was to move in small steps and not to break the existing code
for other boards.
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
	4	Three compile-time constants are used:
	5
	6	CFG_866_OSCCLK - input quartz clock
	7	CFG_866_CPUCLK_MIN - minimum allowed CPU clock
	8	CFG_866_CPUCLK_MAX - maximum allowed CPU clock
	9	CFG_866_CPUCLK_DEFAULT - default CPU clock value
	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
68766094 WD	19	Please note that for now the new clock-handling code has been enabled
	20	for the TQM866M board only, even though it should be pretty much
	21	common for other MPC859 / MPC866 based boards also. Our intention
	22	here was to move in small steps and not to break the existing code
	23	for other boards.