B3 with different RAM chips

[John Watlington]

On Nov 23, 2009, at 1:55 PM, Tiago Marques wrote:

> On Sun, Nov 22, 2009 at 8:01 PM, Mitch Bradley <wmb at laptop.org> wrote:
>> Tiago Marques wrote:
>>>
>>> Hi,
>>>
>>> The hardware page for the XO 1.5 doesn't show the clockspeed of the
>>> RAM chips used. CAS 3 looks like very aggressive timings for cheap
>>> DDR2 chips.
>>
>> CL3 is in spec for the chips that we were using but not for a new  
>> chip that
>> we might start using.
>>
>>> Are you running at 400 or 533?
>>
>> 400
>>>
>>> Doesn't the datasheet specify the appropriate timings?
>>>
>>
>> Of course it does.  The jumper is to tell which timing set to use,  
>> depending
>> on which chips are populated.
>
> Ok, I was rather confused when I read:
>
> "I switched the timing to CAS latency 4 (from 3), which ** should meet
> ** specs for this chip."
>
> And was finding rather odd that you didn't had access to specified  
> timings.

We have access to the chip timings, and we have information about how
to program the controller.   But it is still a trial and error process.
Nobody likes to manually verify that the programmable DRAM controller is
actually correctly generating the literally hundreds of timing specs  
for a
given chip.  We instead make our best guess at correct values and then
test for proper operation across time and temperature.

In this case, the chips usually worked with the CAS latency set too low.

wad