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Why does a GDDR5 memory which is designed to work at 1250MHz gets slow at 1450MHz? Because of ECC check start? Because it is not supposed to work at that frequency(hard-coded?)? Because of some quantum physics related resonance condition? Maybe capacitors get short-circuiting at higher frequencies or coils get more inductant?

Note : it gets faster after 1575 MHz and has a peak at 1650MHz. The slower peak is at 1375 MHz.

Side note : applied many voltages through 1.5V...1.7V but failed to change mem-behaviour(1.5V is standard)

Deep-note : i also have coil-whining when not overclocked. Overclocking gets me rid of coid whining sound.

This is for HD7870 graphics card.

Thanks.

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closed as off topic by Kortuk Feb 17 '13 at 17:15

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  • \$\begingroup\$ This is the type of question you should ask on Super User. Edit and improve your questions there to remove your Q ban. \$\endgroup\$ – Kortuk Feb 17 '13 at 17:15
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One way this sort of peak/valley behaviour can happen with DRAM performance: there are several inter-related speed parameters that have to be satisfied.

One is the simple bus speed that you are adjusting : as you increase it, data transfer to/from DRAM gets faster.

Another is the internal access time between the 2-D memory array and a buffer that holds a single row of values (Row Access Time). Usually after fetching a row, we do a lot of accesses in that row (adjacent pixels share the row) so this has relatively little effect.

Another is the delay between asking for a particular value from that row (i.e. at a particular column address) and that value being available on the output pins (Column Access Time).

Now the problem comes because the Column Access time we need is a fixed value (say 20ns) but the bus speed is variable. But the internals can't usually work in fractions of a clock cycle, so Column Access time is actually expressed as a whole number of bus cycles; the datasheet may call this CAS Latency or CL.

Now if CL = 7 cycles at 1375 MHz but you then increase the bus speed a little, CL has to increase to 8 cycles to meet the Column Access time, so memory accesses slow down, and actual performance drops. Increase bus speed and performance increases again, until eventually 8 bus cycles is too short for Column Access time, and CL has to be increased to 9 cycles.

These are not exact numbers; you would have to find those from the datasheets on the memory fitted to your graphics card.

EDIT : playing with a spreadsheet :

You observed highest performance at 1375 MHz and 1650 MHz.

These frequencies happened to be 275 MHz apart.

Coincidentally (not!) they happen to be 5 * 275 MHz and 6 * 275 MHz. Therefore your observations are consistent with a column access time of 1/275MHz, and CL=5 for frequencies up to 1375 MHz, and CL=6 between 1376 and 1650 MHz.

You can probably find a similar performance peak (and cooler running chips!) at 1100 MHz and CL=4.

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  • \$\begingroup\$ Nice answer. This is hynix i think, will look at datasheet of t2c version. Thanks. \$\endgroup\$ – huseyin tugrul buyukisik Feb 17 '13 at 19:27
  • \$\begingroup\$ If you don't have any luck there, micron.com datasheets are excellent and they probably make an equivalent part. It's probably useful to add a comment if you confirm (or otherwise) the details. \$\endgroup\$ – Brian Drummond Feb 17 '13 at 20:02

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