[U-Boot-Users] 83xx DDR2 667Mhz memory

[Jerry Van Baren]

Russell McGuire wrote:
> All,
> 
> Just curious if anyone has done any work with this near past.
> I have had an 8360E board up for several months now and been happily working
> away with different types of Kingston S0-DIMM DDR2 memory.
> 
> 512MB, 1204MB, DDR2 533Mhz memory. All work well, with U-boot.
> Using the SPD detectetion, things have just been automatically detected and
> worked wonderful.
> 
> Yesterday I went down and purchased two additional sticks of 1024MB DDR2
> Crucial 667Mhz memory, as 533Mhz is in short supply. SPD detect still works
> in U-boot and everything is detected correctly. However, the u-boot will now
> hang when it tries to transfer execution into RAM. At first I figured this
> was a memory speed issue, since 667Mhz is not technically supported on the
> 8360E yet <Rev 2... I might be wrong on this?>, So I patched in a quick line
> of code to force the spd detection to limit to 533Mhz.  Still doesn't work.
> 
> Any ideas? Again, same exact u-boot just different memory. SPD sees it all
> just fine, just can't use the RAM after its setup. Currently using a
> slightly older version of U-boot 1.3.0-dirty.
> 
> -Russ

Hi Russ,

Any luck?  I would be suspicious that the number of clocks changed (e.g. 
CL latencies) and the current code did not pick the changes out of the 
SPD fields properly.  I would dump the two SPDs and compare them.  If 
you can get datasheets on the RAM chips that are on the boards, that may 
also be helpful to compare.

Quite often, faster parts have longer latencies.  My SPD decoding is 
knowledge getting a little rusty, but my recollection is that the memory 
controller configuration values and the SPD definitions did not clearly 
map 1:1 (had to massage SPD values to figure out/guess controller 
register values).

HTH,
gvb

P.S., as an added, free, bonus, here is a little diatribe on memory 
speed...  It is slightly dated, obviously, but still valid.

Today we have PowerPCs running at 400-800MHz (and above) with the system 
bus running at 100MHz (and above). However, the systems which are 
running 100MHz on the system/memory bus have typically 18 clock cycles 
latency...

     100MHz / 18 = 5.56MHz

In the IBM PC-XT, created in 1981, the system bus was 4.77MHz!
In the IBM PC-At, created in 1984, the system bus was 8MHz (originally 6MHz)

25 years after the debut of the PC-XT, we are clocking like mad but have 
basically the same data rates on our system bus!

Disclaimers:
     * Bus width has increased from 2 bytes to 16 bytes, with resulting 
linear scaling of data throughput.
     * Pipelining on the memory bus can use many of the latency clock 
cycles to queue up reads and writes, with resulting increases in 
throughput. However...
           o The average pipelining success rate is much less than 100%.
           o I/O typically doesn't support pipelining.
     * IIRC, the IBM-XT could access memory mapped ISA boards with zero 
wait states. Accessing I/O ports (in/out instructions) inserted wait 
states.