memory (CL2) problems in T21.

[bastaz]

Hello everyone

I have my thinkpad T21 with 2 infineon memory modules (2x128MB PC 100). There is written on them that they have CL2 but in windows xp the system shows me (cpuz, aida, sandra) that they work on CL3. Can I change somewhere (registry etc.) to CL2? Do U know why it works on CL3, not on CL2?

thanks in advance

[rkawakami]

Hello and welcome to thinkpads.com!

The CAS latency (CL) rating on the module normally indicates the fastest setting that it can operate at. A PC100 module has been designed and tested to run at 100Mhz (or 10ns cycle times). Access to the first piece of data can be performed in two cycles at the CL2 rating (total or 20ns). If the module is run at CL3, that just means an extra timing cycle is being used and the memory is running slower than it can (a total of 30ns). Running the memory module at the CL3 setting will do no harm, other than slowing your access to the first piece of data.

My (limited) understanding is that the CL is determined by the memory controller chipset inside the laptop and/or the modules you are using. I don't know if there is any Windows registry setting that you can change. Each module has information programmed into it that can tell the motherboard what speeds it supports. This is what is called the SPD (serial presence detect) bytes. Whether or not the motherboard/memory controller inside the Thinkpads uses this information to set the timing system specifications is unknown to me.

[bastaz]

Thanks for the answer. I know what is CL, and thats why i bought CL2 memorys But I dont understand why its working in CL3... I think that in wxp something has changed becouse since month my system is a bit slower than it was earlier... Ehh... I think i will reinstall wxp... someday

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[rkawakami]

I know what is CL, and thats why i bought CL2 memorys But I dont understand why its working in CL3...

I'll answer this in two parts as I do not totally understand the meaning of your statement:

1) If you do not understand why the memory module WORKS at the CL3 timing it is because that setting allows more time to access the first piece of data from the memory. In other words, the module is not running at the fastest speed it has been guaranteed to operate at. A CL2-rated module will operate perfectly at CL3, CL4, even CL5 timings as long as the bus speed is kept at 100Mhz (for the T21 system).

2) If you do not understand why CL3 is being USED then that has to do with the memory controller inside the laptop. It can either look at the memory modules to determine what speed to use (the SPD information) OR it can be fixed to only run at CL3. Since both of the modules you are using are marked CL2, I can only assume that the laptop has been told to always run at CL3 timings. There may be a way of changing that, but I do not think re-installing Windows is the answer.

Also, the SPD information on the modules has bits that define the CL values it will support. By definition if the module has been tested (and passed) at the CL2 timing, then the manufacturer should have set the CL2 bit, as well as the CL3 bit. It is remotely possible that the module does not have the correct SPD information, but I doubt that is causing the usage of CL3 in your system.

Hope that explains it better.