[U-Boot-Users] MPC85xx: Question about Local Bus initialization

Jens Gehrlein sew_s at tqs.de
Tue Dec 4 08:56:02 CET 2007

Hi Detlev,

thanks for the reply.

Detlev Zundel schrieb:
> Hi Jens,
>> I took a look into the files board/mpc8560ads.c and board/tqm85xx.c and 
>> found something strange:
>> 1. In the function local_bus_init() the current CLKDIV is read from the 
>> register LCRR as was set by Hardreset. After that, the decision is made, 
>> wether the DLL has to be enabled/disabled/overridden. Inside the if-else 
>> blocks the new CLKDIV is changed. But IMO the CLKDIV has to be set 
>> before the query.
>> This is the current code:
>> 	clkdiv = lbc->lcrr & 0x0f;
>> 	lbc_hz = sysinfo.freqSystemBus / 1000000 / clkdiv;
>> 	if (lbc_hz < 66) {
>> 		lbc->lcrr = CFG_LBC_LCRR | 0x80000000;	/* DLL Bypass */
>> 	} else if (lbc_hz >= 133) {
>> 		lbc->lcrr = CFG_LBC_LCRR & (~0x80000000); /* DLL Enabled
>> 	...
>> This may be the situation on other 85xx boards, too. I didn't check them 
>> all.
>> What was the intention, DLL modification dependent on the clock set by
>> the MPC at hardreset or dependent on the targeted frequency?
> I am definitely not the specialist on these chips, but checking the
> 8555 and 8560 manuals, it seems that LCRR comes out of reset with
> 0x8000,0008.  I cannot find any reference to LCRR from the POR
> description.
Agreed. 0x80000008 is the hard reset value.

> Keeping this in mind, "clkdiv" in the above code will always be 8 and
> so it looks to me indeed like the tests really were intended to check
> (CFG_LBC_LCRR & 0x0f).  Can anyone more knowledgable in this area
> confirm this suspicion?
In the current implementation CLKDIV is not modified earlier. Therefore,
lbc->lcrr & 0x0f == CFG_LBC_LCRR & 0x0f is in this code only true, if 
CFG_LBC_LCRR is defined equally to the MPC's hard reset value.
But what if I desire another clock frequency and other timings? In the 
current code the decision to bypass or not to bypass the DLL is made, 
_before_ it is changed. Shouldn't it be "clkdiv = (CFG_LBC_LCRR & 
0x0f);" in the first line?

I'm not sure. And that's is the reason for my question.
I hoped that the Freescale developers had something to say about it.

>> 2. The variable is named lbc_hz, but it contains a value in units of 
>> MHz. I suggest to use the name lbc_mhz or to use Hertz values by 
>> removing the division by 1,000,000 and replacing 66 and 133 by 66666667 
>> and 133333333.
>> What's your opinion?
> Personally I would second your reasoning.  But for others to comment,
> it is really best to implement your proposed change and send a diff to
> comment on.  Such a suggestion is more likely by several magnitudes to
> be considered ;)
No problem. It was just a question.


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