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

Detlev Zundel dzu at denx.de
Mon Dec 3 16:39:31 CET 2007

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

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?

> 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 ;)

> 3. I assume, the function above is called while the MPC executes this 
> code from flash memory (array init_sequence[] in lib_ppc/board.c). Is 
> there a potential that the U-Boot would hang when it changed the CLKDIV?

If you get the timings wrong, of course things could get hairy, or am
I missing something?


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                       -- Mark Twain
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