CAS Latency

vbimport

#1

Would anyone like to shed some light on the meaning of DDR memory latency ratings eg:CAS Latency 2-2-2-6-T1 (because i want to know if the DDR400mhz has a good latency value in light of the read first at the top of the fourm)


#2

Lower is always better, if it works.

SDRAM CAS Latency Time - This controls the time delay (in clock cycles - CLKs) that passes before the SDRAM starts to carry out a read command after receiving it. This also determines the number of CLKs for the completion of the first part of a burst transfer. In other words, the lower the latency, the faster the transaction.
Note that some SDRAM modules may not be able to handle the lower latency and will become unstable and lose data. Therefore, set the SDRAM CAS Latency Time to 2 for optimal performance if possible but increase it to 3 if your system becomes unstable.
Interestingly, increasing the CAS latency time does have an advantage in that it will enable the SDRAM to run at a higher clockspeed, thereby giving you an edge in overclocking your system. So, if you hit a snag while overclocking, try increasing the CAS latency time.

SDRAM Cycle Time Tras/Trc - This feature toggles the minimum number of clock cycles required for the Tras and the Trc of the SDRAM.
Tras refers to the SDRAM’s Row Active Time, which is the length of time in which the row is open for data transfers. It is also known as Minimum RAS Pulse Width.
Trc, on the other hand, refers to the SDRAM’s Row Cycle Time, which determines the length of time for the entire row-open, row-refresh cycle to complete.
The default setting is 6/8 which is more stable and slower than 5/6. The 5/6 setting cycles the SDRAM faster but may not leave the row open long enough for data transactions to complete. This is especially true at SDRAM clockspeeds above 100MHz.
Therefore, you should try 5/6 for better SDRAM performance and only increase it to 6/8 if your system becomes unstable or if you are trying to get the SDRAM to run at a higher clockspeed.

SDRAM RAS-to-CAS Delay - This option allows you to insert a delay between the RAS (Row Address Strobe) and CAS (Column Address Strobe) signals. This delay occurs when the SDRAM is written to, read from or refreshed. Naturally, reducing the delay improves the performance of the SDRAM while increasing it reduces performance.
Therefore, reduce the delay from the default value of 3 to 2 for better SDRAM performance. However, if you are facing system stability issues after reducing the delay, reset the value back to 3.

SDRAM RAS Precharge Time - This option sets the number of cycles required for the RAS to accumulate its charge before the SDRAM refreshes. Reducing the precharge time to 2 improves SDRAM performance but if the precharge time of 2 is insufficient for the installed SDRAM, the SDRAM may not be refreshed properly and it may fail to retain data.
So, for better SDRAM performance, set the SDRAM RAS Precharge Time to 2 but increase it to 3 if you face system stability issues after reducing the precharge time.

SDRAM Cycle Length - This controls the time delay (in clock cycles - CLKs) that passes before the SDRAM starts to carry out a read command after receiving it. This also determines the number of CLKs for the completion of the first part of a burst transfer. In other words, the lower the latency, the faster the transaction.
Note that some SDRAM modules may not be able to handle the lower cycle length and will become unstable and lose data. Therefore, set the SDRAM Cycle Length to 2 for optimal performance if possible but increase it to 3 if your system becomes unstable.
Interestingly, increasing the cycle length does have an advantage in that it will enable the SDRAM to run at a higher clockspeed, thereby giving you an edge in overclocking your system. So, if you hit a snag while overclocking, try increasing the SDRAM Cycle Length.

SDRAM Leadoff Command - This option allows you to adjust the leadoff time needed before the data stored in the SDRAM can be accessed. In most cases, it is the access time for the first data element in a burst. For optimal performance, set the value to 3 for faster SDRAM access times but increase it to 4 if you are facing system stability issues.

http://www.adriansrojakpot.com/Speed_Demonz/BIOS_Guide/BIOS_Guide_02a.htm