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Cas Latency and Timing??
- Thread starter MULLDOG
- Start date
Nothing like having an online encyclopedia available at http://en.wikipedia.org/wiki/Cas_latency
"CAS Latency
From Wikipedia, the free encyclopedia
(Redirected from Cas latency)
Jump to: navigation, search
CAS is an abbreviation for column address strobe, or sometimes column address select, both referring to the column of the physical memory location in an array (comprised of columns and rows) of capacitors used in dynamic random access memory modules. Thus CAS Latency (CL) is the time (in number of clock cycles) that elapses after the memory controller sends a request to read a memory location and before the data is sent to the module's output pins.
Data is stored in individual memory cells, each uniquely identified by a memory bank, row, and column. To access DRAM, controllers first select a memory bank, then a row (using the row address strobe, RAS), then a column (using the CAS), and finally request to read the data from the physical location of the memory cell. The CAS Latency is the number of clock cycles that elapse from the time the request for data is sent to the actual memory location until the data is transmitted from the module.
When selecting a RAM card, the lower the CAS latency (given the same clock speed), the better the system performs. Current DDR RAM should have a CAS latency of about 3, or optimally 2. DDR2 RAM can have latencies ranging from 3 to 5.
Comparing between clock speeds gets trickier. CAS Latency only specifies the delay between the request and the first bit. The clock speed specifies the latency between bits. Thus, when reading bursts of data, a higher clock speed can be faster in practice, even with a worse CAS Latency.
For example, consider a 133 MHz CL3 device (7.5 ns per cycle, 3 cycles request latency) versus a 100 MHz CL2 device (10.0 ns per cycle, 2 cycles request latency). The first bit would be available after 22.5 ns (7.5 ns * 3) on the CL3 device and after 20.0 ns (10.0 ns * 2) on the CL2 device, demonstrating the benefit of a lower CAS latency. However when reading a burst of even 4 bits, the higher clock speed wins: 45.0 ns (7.5 * 3 latency + 7.5 * 3 bits after the first) versus 50.0 ns (10.0 * 2 latency + 10.0 * 3 bits after the first).
When a timing is specified for a particular CAS Latency (e.g. CL3 = 5.0 ns, CL2.5 = 6.0 ns), that indicates the clock speed at which that CL is supported. In this example, the RAM could support CL3 at 200MHz or CL2.5 at 166MHz. Most RAM supports multiple clock speeds, with varying performance, hence this notation.
RAM performs no better than the system in which it is installed. RAM with a rating worse than the system's rating will reduce performance, but RAM with a rating higher than the system's rating will not improve performance beyond the system's capabilities."
To go into a good article explaining the different types of memory timings to see how they apply, http://www.techpowerup.com/articles/overclocking/AMD/memory/131
For gaming people love having lower CAS Latency values along with faster memory timings to get that performance. For other applications and hardware setups a higher CAS Latency may actually be the better.
"CAS Latency
From Wikipedia, the free encyclopedia
(Redirected from Cas latency)
Jump to: navigation, search
CAS is an abbreviation for column address strobe, or sometimes column address select, both referring to the column of the physical memory location in an array (comprised of columns and rows) of capacitors used in dynamic random access memory modules. Thus CAS Latency (CL) is the time (in number of clock cycles) that elapses after the memory controller sends a request to read a memory location and before the data is sent to the module's output pins.
Data is stored in individual memory cells, each uniquely identified by a memory bank, row, and column. To access DRAM, controllers first select a memory bank, then a row (using the row address strobe, RAS), then a column (using the CAS), and finally request to read the data from the physical location of the memory cell. The CAS Latency is the number of clock cycles that elapse from the time the request for data is sent to the actual memory location until the data is transmitted from the module.
When selecting a RAM card, the lower the CAS latency (given the same clock speed), the better the system performs. Current DDR RAM should have a CAS latency of about 3, or optimally 2. DDR2 RAM can have latencies ranging from 3 to 5.
Comparing between clock speeds gets trickier. CAS Latency only specifies the delay between the request and the first bit. The clock speed specifies the latency between bits. Thus, when reading bursts of data, a higher clock speed can be faster in practice, even with a worse CAS Latency.
For example, consider a 133 MHz CL3 device (7.5 ns per cycle, 3 cycles request latency) versus a 100 MHz CL2 device (10.0 ns per cycle, 2 cycles request latency). The first bit would be available after 22.5 ns (7.5 ns * 3) on the CL3 device and after 20.0 ns (10.0 ns * 2) on the CL2 device, demonstrating the benefit of a lower CAS latency. However when reading a burst of even 4 bits, the higher clock speed wins: 45.0 ns (7.5 * 3 latency + 7.5 * 3 bits after the first) versus 50.0 ns (10.0 * 2 latency + 10.0 * 3 bits after the first).
When a timing is specified for a particular CAS Latency (e.g. CL3 = 5.0 ns, CL2.5 = 6.0 ns), that indicates the clock speed at which that CL is supported. In this example, the RAM could support CL3 at 200MHz or CL2.5 at 166MHz. Most RAM supports multiple clock speeds, with varying performance, hence this notation.
RAM performs no better than the system in which it is installed. RAM with a rating worse than the system's rating will reduce performance, but RAM with a rating higher than the system's rating will not improve performance beyond the system's capabilities."
To go into a good article explaining the different types of memory timings to see how they apply, http://www.techpowerup.com/articles/overclocking/AMD/memory/131
For gaming people love having lower CAS Latency values along with faster memory timings to get that performance. For other applications and hardware setups a higher CAS Latency may actually be the better.
It can be a little overwhelming at first. But those heavy into OCing and system specifications and other technical data will be regularly looking over this type of information to be informed. That first site is a great one to save for reference when you get stuck on something. While some of this stuff gets lengthy the descriptions are easier to follow then many.