[U-Boot-Users] MPC83xx HRCW

[Andre Schwarz]

Jerry Van Baren schrieb:
> Andre Schwarz wrote:
>> Jerry Van Baren schrieb:
>>> Andre Schwarz wrote:
>>>> In "cpu/mpc83xx/start.S" there is a comment :
>>>>
>>>> /*
>>>>  * The Hard Reset Configuration Word (HRCW) table is in the first 64
>>>>  * (0x40) bytes of flash.  It has 8 bytes, but each byte is repeated 8
>>>>  * times so the processor can fetch it out of flash whether the flash
>>>>  * is 8, 16, 32, or 64 bits wide (hardware trickery).
>>>>  */
>>>>
>>>> This does _not_ hold true for all configurations. Flash is simply 
>>>> one of many options ...
>>>> Maybe it's true for the Freescale boards.
>>>>
>>>> Other sources of the HRCW can be hard-coded strapping pins or an 
>>>> I2C EEPROM.
>>>>
>>>> Why is there a need to define the HRCW ?
>>>>
>>>> regards,
>>>> Andre Schwarz
>>>
>>> Hi Andre,
>>>
>>> The HRCW in flash (could be other memory or a FPGA register) is a 
>>> processor feature which a board may or may not use.  I am not 
>>> familiar with the whole 83xx family, but I presume the feature is 
>>> part of the whole family.
>>>
>>> As you point out, there are other ways of configuring the processor 
>>> on power up, and it is board-specific which way is used on the 
>>> particular board.
>>>
>>> For the boards that support the HRCW, obviously the definition in 
>>> the first 64 bytes of flash is necessary.  For other boards, it is 
>>> unnecessary.  FWIIW, the Freescale eval boards that I have 
>>> experience with allow the HRCW to come from flash, i2c, or an FPGA 
>>> (BCSR).
>>>
>>> To date, having a potentially unused HRCW definition in memory has 
>>> not been an issue - people either use it or ignore it.  If it is an 
>>> issue, you could use conditionals to disable it.  I'm sure the 83xx 
>>> custodian (Kim Phillips) would consider patches to do that.  ;-)  
>>> Note that there is a possibility that some of the code assumes the 
>>> presence of a HRCW, so you would have to inspect and/or regression 
>>> test as part of a conditionalization patch.
>>>
>> ok - so should be no problem to #define the HRCW to "0x0" since it 
>> won't be used at all - just occupies some memory.
>> I just wanted to be sure that the #defined HRCW is not used as a 
>> reference at all in any code !
>
> Theoretically, there is no problem.  I don't know if there are any 
> implicit uses of the HRCW - that would be part of the need to inspect 
> and/or regression test.
>
> I suspect that the CPU frequency determination code uses it, since 
> part of the CPU PLL multiplier comes from the HRCW (IIRC - I get 
> confused between the 82xx and 83xx families sometimes).
>
Using the HRCW during SystemInit is no problem - but the _real_ one from 
the CPU register and not a #defined one.
The CPU won't start anyway if the HRCW is invalid.

I've just #defined my HRCW to "0" without any problems.

Sorry for the confusion.

regards,
Andre
> [snip]
>
> Best regards,
> gvb


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