CAS latency

Column address strobe latency, also called CAS latency or CL, is the delay in clock cycles between the READ command and the moment data is available.[1][2] In asynchronous DRAM, the interval is specified in nanoseconds (absolute time).[3] In synchronous DRAM, the interval is specified in clock cycles. Because the latency is dependent upon a number of clock ticks instead of absolute time, the actual time for an SDRAM module to respond to a CAS event might vary between uses of the same module if the clock rate differs.

RAM operation background
Further information: DRAM § Principles of operation

Dynamic RAM is arranged in a rectangular array. Each row is selected by a horizontal word line. Sending a logical high signal along a given row enables the MOSFETs present in that row, connecting each storage capacitor to its corresponding vertical bit line. Each bit line is connected to a sense amplifier that amplifies the small voltage change produced by the storage capacitor. This amplified signal is then output from the DRAM chip as well as driven back up the bit line to refresh the row.

When no word line is active, the array is idle and the bit lines are held in a precharged[4] state, with a voltage halfway between high and low. This indeterminate signal is deflected towards high or low by the storage capacitor when a row is made active.

To access memory, a row must first be selected and loaded into the sense amplifiers. This row is then active, and columns may be accessed for read or write.

The CAS latency is the delay between the time at which the column address and the column address strobe signal are presented to the memory module and the time at which the corresponding data is made available by the memory module. The desired row must already be active; if it is not, additional time is required.

As an example, a typical 1 GiB SDRAM memory module might contain eight separate one-gibibit DRAM chips, each offering 128 MiB of storage space. Each chip is divided internally into eight banks of 227=128 Mibits, each of which composes a separate DRAM array. Each bank contains 214=16384 rows of 213=8192 bits each. One byte of memory (from each chip; 64 bits total from the whole DIMM) is accessed by supplying a 3-bit bank number, a 14-bit row address, and a 13-bit column address.

Effect on memory access speed

With asynchronous DRAM, memory was accessed by a memory controller on the memory bus based on a set timing rather than a clock, and was separate from the system bus.[3] Synchronous DRAM, however, has a CAS latency that is dependent upon the clock rate. Accordingly, the CAS latency of an SDRAM memory module is specified in clock ticks instead of absolute time.

Because memory modules have multiple internal banks, and data can be output from one during access latency for another, the output pins can be kept 100% busy regardless of the CAS latency through pipelining; the maximum attainable bandwidth is determined solely by the clock speed. Unfortunately, this maximum bandwidth can only be attained if the address of the data to be read is known long enough in advance; if the address of the data being accessed is not predictable, pipeline stalls can occur, resulting in a loss of bandwidth. For a completely unknown memory access (AKA Random access), the relevant latency is the time to close any open row, plus the time to open the desired row, followed by the CAS latency to read data from it. Due to spatial locality, however, it is common to access several words in the same row. In this case, the CAS latency alone determines the elapsed time.

Because modern DRAM modules' CAS latencies are specified in clock ticks instead of time, when comparing latencies at different clock speeds, latencies must be translated into absolute times to make a fair comparison; a higher numerical CAS latency may still be less time if the clock is faster. Likewise, a memory module which is underclocked could have its CAS latency cycle count reduced to preserve the same CAS latency time.

Double data rate (DDR) RAM performs two transfers per clock cycle, and it is usually described by this transfer rate. Because the CAS latency is specified in clock cycles, and not transfers (which occur on both the rising and falling edges of the clock), it is important to ensure it is the clock rate (half of the transfer rate) which is being used to compute CAS latency times.

Another complicating factor is the use of burst transfers. A modern microprocessor might have a cache line size of 64 bytes, requiring eight transfers from a 64-bit-wide (eight bytes) memory to fill. The CAS latency can only accurately measure the time to transfer the first word of memory; the time to transfer all eight words depends on the data transfer rate as well. Fortunately, the processor typically does not need to wait for all eight words; the burst is usually sent in critical word first order, and the first critical word can be used by the microprocessor immediately.

In the table below, data rates are given in million transfers—also known as megatransfers—per second (MT/s), while clock rates are given in MHz, million cycles per second.

Memory timing examples
Memory timing examples (CAS latency only)
Generation Type Data rate Transfer time[lower-alpha 1] Command rate[lower-alpha 2] Cycle time[lower-alpha 3] CAS latency First word[lower-alpha 4] Fourth word[lower-alpha 4] Eighth word[lower-alpha 4]
SDRAM PC100 100 MT/s 10.000 ns 100 MHz 10.000 ns 2 20.00 ns 50.00 ns 90.00 ns
PC133 133 MT/s 7.500 ns 133 MHz 7.500 ns 3 22.50 ns 45.00 ns 75.00 ns
DDR SDRAM DDR-333 333 MT/s 3.000 ns 166 MHz 6.000 ns 2.5 15.00 ns 24.00 ns 36.00 ns
DDR-400 400 MT/s 2.500 ns 200 MHz 5.000 ns 3 15.00 ns 22.50 ns 32.50 ns
2.5 12.50 ns 20.00 ns 30.00 ns
2 10.00 ns 17.50 ns 27.50 ns
DDR2 SDRAM DDR2-400 400 MT/s 2.500 ns 200 MHz 5.000 ns 4 20.00 ns 27.50 ns 37.50 ns
3 15.00 ns 22.50 ns 32.50 ns
DDR2-533 533 MT/s 1.875 ns 266 MHz 3.750 ns 4 15.00 ns 20.63 ns 28.13 ns
3 11.25 ns 16.88 ns 24.38 ns
DDR2-667 667 MT/s 1.500 ns 333 MHz 3.000 ns 5 15.00 ns 19.50 ns 25.50 ns
4 12.00 ns 16.50 ns 22.50 ns
DDR2-800 800 MT/s 1.250 ns 400 MHz 2.500 ns 6 15.00 ns 18.75 ns 23.75 ns
5 12.50 ns 16.25 ns 21.25 ns
4.5 11.25 ns 15.00 ns 20.00 ns
4 10.00 ns 13.75 ns 18.75 ns
DDR2-1066 1066 MT/s 0.938 ns 533 MHz 1.875 ns 7 13.13 ns 15.94 ns 19.69 ns
6 11.25 ns 14.06 ns 17.81 ns
5 9.38 ns 12.19 ns 15.94 ns
4.5 8.44 ns 11.25 ns 15.00 ns
4 7.50 ns 10.31 ns 14.06 ns
DDR3 SDRAM DDR3-1066 1066 MT/s 0.938 ns 533 MHz 1.875 ns 7 13.13 ns 15.94 ns 19.69 ns
DDR3-1333 1333 MT/s 0.750 ns 666 MHz 1.500 ns 9 13.50 ns 15.75 ns 18.75 ns
7 10.50 ns 12.75 ns 15.75 ns
6 9.00 ns 11.25 ns 14.25 ns
DDR3-1375 1375 MT/s 0.727 ns 687 MHz 1.455 ns 5 7.27 ns 9.45 ns 12.36 ns
DDR3-1600 1600 MT/s 0.625 ns 800 MHz 1.250 ns 11 13.75 ns 15.63 ns 18.13 ns
10 12.50 ns 14.38 ns 16.88 ns
9 11.25 ns 13.13 ns 15.63 ns
8 10.00 ns 11.88 ns 14.38 ns
7 8.75 ns 10.63 ns 13.13 ns
6 7.50 ns 9.38 ns 11.88 ns
DDR3-1866 1866 MT/s 0.536 ns 933 MHz 1.071 ns 10 10.71 ns 12.32 ns 14.46 ns
9 9.64 ns 11.25 ns 13.39 ns
8 8.57 ns 10.18 ns 12.32 ns
DDR3-2000 2000 MT/s 0.500 ns 1000 MHz 1.000 ns 9 9.00 ns 10.50 ns 12.50 ns
DDR3-2133 2133 MT/s 0.469 ns 1066 MHz 0.938 ns 12 11.25 ns 12.66 ns 14.53 ns
11 10.31 ns 11.72 ns 13.59 ns
10 9.38 ns 10.78 ns 12.66 ns
9 8.44 ns 9.84 ns 11.72 ns
8 7.50 ns 8.91 ns 10.78 ns
7 6.56 ns 7.97 ns 9.84 ns
DDR3-2200 2200 MT/s 0.455 ns 1100 MHz 0.909 ns 7 6.36 ns 7.73 ns 9.55 ns
DDR3-2400 2400 MT/s 0.417 ns 1200 MHz 0.833 ns 13 10.83 ns 12.08 ns 13.75 ns
12 10.00 ns 11.25 ns 12.92 ns
11 9.17 ns 10.42 ns 12.08 ns
10 8.33 ns 9.58 ns 11.25 ns
9 7.50 ns 8.75 ns 10.42 ns
DDR3-2600 2600 MT/s 0.385 ns 1300 MHz 0.769 ns 11 8.46 ns 9.62 ns 11.15 ns
DDR3-2666 2666 MT/s 0.375 ns 1333 MHz 0.750 ns 15 11.25 ns 12.38 ns 13.88 ns
13 9.75 ns 10.88 ns 12.38 ns
12 9.00 ns 10.13 ns 11.63 ns
11 8.25 ns 9.38 ns 10.88 ns
DDR3-2800 2800 MT/s 0.357 ns 1400 MHz 0.714 ns 16 11.43 ns 12.50 ns 13.93 ns
12 8.57 ns 9.64 ns 11.07 ns
11 7.86 ns 8.93 ns 10.36 ns
DDR3-2933 2933 MT/s 0.341 ns 1466 MHz 0.682 ns 12 8.18 ns 9.20 ns 10.57 ns
DDR3-3000 3000 MT/s 0.333 ns 1500 MHz 0.667 ns 12 8.00 ns 9.00 ns 10.33 ns
DDR3-3100 3100 MT/s 0.323 ns 1550 MHz 0.645 ns 12 7.74 ns 8.71 ns 10.00 ns
DDR3-3200 3200 MT/s 0.313 ns 1600 MHz 0.625 ns 16 10.00 ns 10.94 ns 12.19 ns
DDR3-3300 3300 MT/s 0.303 ns 1650 MHz 0.606 ns 16 9.70 ns 10.61 ns 11.82 ns
DDR4 SDRAM
DDR4-1600 1600 MT/s 0.625 ns 800 MHz 1.250 ns 12 15.00 ns 16.88 ns 19.38 ns
11 13.75 ns 15.63 ns 18.13 ns
10 12.50 ns 14.38 ns 16.88 ns
DDR4-1866 1866 MT/s 0.536 ns 933 MHz 1.071 ns 14 15.00 ns 16.61 ns 18.75 ns
13 13.93 ns 15.54 ns 17.68 ns
12 12.86 ns 14.46 ns 16.61 ns
DDR4-2133 2133 MT/s 0.469 ns 1066 MHz 0.938 ns 16 15.00 ns 16.41 ns 18.28 ns
15 14.06 ns 15.47 ns 17.34 ns
14 13.13 ns 14.53 ns 16.41 ns
DDR4-2400 2400 MT/s 0.417 ns 1200 MHz 0.833 ns 17 14.17 ns 15.42 ns 17.08 ns
16 13.33 ns 14.58 ns 16.25 ns
15 12.50 ns 13.75 ns 15.42 ns
DDR4-2666 2666 MT/s 0.375 ns 1333 MHz 0.750 ns 17 12.75 ns 13.88 ns 15.38 ns
16 12.00 ns 13.13 ns 14.63 ns
15 11.25 ns 12.38 ns 13.88 ns
13 9.75 ns 10.88 ns 12.38 ns
12 9.00 ns 10.13 ns 11.63 ns
DDR4-2800 2800 MT/s 0.357 ns 1400 MHz 0.714 ns 17 12.14 ns 13.21 ns 14.64 ns
16 11.43 ns 12.50 ns 13.93 ns
15 10.71 ns 11.79 ns 13.21 ns
14 10.00 ns 11.07 ns 12.50 ns
DDR4-3000 3000 MT/s 0.333 ns 1500 MHz 0.667 ns 17 11.33 ns 12.33 ns 13.67 ns
16 10.67 ns 11.67 ns 13.00 ns
15 10.00 ns 11.00 ns 12.33 ns
14 9.33 ns 10.33 ns 11.67 ns
DDR4-3200 3200 MT/s 0.313 ns 1600 MHz 0.625 ns 16 10.00 ns 10.94 ns 12.19 ns
15 9.38 ns 10.31 ns 11.56 ns
14 8.75 ns 9.69 ns 10.94 ns
DDR4-3300 3300 MT/s 0.303 ns 1650 MHz 0.606 ns 16 9.70 ns 10.61 ns 11.82 ns
DDR4-3333 3333 MT/s 0.300 ns 1666 MHz 0.600 ns 16 9.60 ns 10.50 ns 11.70 ns
DDR4-3400 3400 MT/s 0.294 ns 1700 MHz 0.588 ns 16 9.41 ns 10.29 ns 11.47 ns
DDR4-3466 3466 MT/s 0.288 ns 1733 MHz 0.577 ns 18 10.38 ns 11.25 ns 12.40 ns
17 9.81 ns 10.67 ns 11.83 ns
16 9.23 ns 10.10 ns 11.25 ns
DDR4-3600 3600 MT/s 0.278 ns 1800 MHz 0.556 ns 19 10.56 ns 11.39 ns 12.50 ns
18 10.00 ns 10.83 ns 11.94 ns
17 9.44 ns 10.28 ns 11.39 ns
16 8.89 ns 9.72 ns 10.83 ns
15 8.33 ns 9.17 ns 10.28 ns
14 7.78 ns 8.61 ns 9.72 ns
DDR4-3733 3733 MT/s 0.268 ns 1866 MHz 0.536 ns 17 9.11 ns 9.91 ns 10.98 ns
DDR4-3866 3866 MT/s 0.259 ns 1933 MHz 0.517 ns 18 9.31 ns 10.09 ns 11.12 ns
DDR4-4000 4000 MT/s 0.250 ns 2000 MHz 0.500 ns 19 9.50 ns 10.25 ns 11.25 ns
18 9.00 ns 9.75 ns 10.75 ns
17 8.50 ns 9.25 ns 10.25 ns
16 8.00 ns 8.75 ns 9.75 ns
DDR4-4133 4133 MT/s 0.242 ns 2066 MHz 0.484 ns 19 9.19 ns 9.92 ns 10.89 ns
DDR4-4200 4200 MT/s 0.238 ns 2100 MHz 0.476 ns 19 9.05 ns 9.76 ns 10.71 ns
DDR4-4266 4266 MT/s 0.234 ns 2133 MHz 0.469 ns 19 8.91 ns 9.61 ns 10.55 ns
18 8.44 ns 9.14 ns 10.08 ns
17 7.97 ns 8.67 ns 9.61 ns
16 7.50 ns 8.20 ns 9.14 ns
DDR4-4400 4400 MT/s 0.227 ns 2200 MHz 0.454 ns 19 8.64 ns 9.32 ns 10.23 ns
18 8.18 ns 8.86 ns 9.77 ns
17 7.73 ns 8.41 ns 9.32 ns
DDR4-4600 4600 MT/s 0.217 ns 2300 MHz 0.435 ns 19 8.26 ns 8.91 ns 9.78 ns
18 7.82 ns 8.48 ns 9.35 ns
DDR4-4800 4800 MT/s 0.208 ns 2400 MHz 0.417 ns 20 8.33 ns 8.96 ns 9.79 ns
19 7.92 ns 8.54 ns 9.38 ns
DDR5 SDRAM
DDR5-4800 4800 MT/s 0.208 ns 2400 MHz 0.417 ns 40 16.67 ns 17.29 ns 18.13 ns
38 15.83 ns 16.46 ns 17.29 ns
36 15.00 ns 15.63 ns 16.46 ns
34 14.17 ns 14.79 ns 15.63 ns
DDR5-5200 5200 MT/s 0.192 ns 2600 MHz 0.385 ns 40 15.38 ns 15.96 ns 16.73 ns
38 14.62 ns 15.19 ns 15.96 ns
36 13.85 ns 14.42 ns 15.19 ns
34 13.08 ns 13.65 ns 14.42 ns
DDR5-5600 5600 MT/s 0.179 ns 2800 MHz 0.357 ns 40 14.29 ns 14.82 ns 15.54 ns
38 13.57 ns 14.11 ns 14.82 ns
36 12.86 ns 13.39 ns 14.11 ns
34 12.14 ns 12.68 ns 13.39 ns
30 10.71 ns 11.25 ns 11.96 ns
DDR5-6000 6000 MT/s 0.167 ns 3000 MHz 0.333 ns 40 13.33 ns 13.83 ns 14.50 ns
38 12.67 ns 13.17 ns 13.83 ns
36 12.00 ns 12.50 ns 13.17 ns
32 10.67 ns 11.17 ns 11.83 ns
30 10.00 ns 10.50 ns 11.17 ns
DDR5-6200 6200 MT/s 0.161 ns 3100 MHz 0.323 ns 40 12.90 ns 13.39 ns 14.03 ns
38 12.26 ns 12.74 ns 13.39 ns
36 11.61 ns 12.10 ns 12.74 ns
DDR5-6400 6400 MT/s 0.156 ns 3200 MHz 0.313 ns 40 12.50 ns 12.97 ns 13.59 ns
38 11.88 ns 12.34 ns 12.97 ns
36 11.25 ns 11.72 ns 12.34 ns
34 10.63 ns 11.09 ns 11.72 ns
32 10.00 ns 10.47 ns 11.09 ns
DDR5-6600 6600 MT/s 0.152 ns 3300 MHz 0.303 ns 34 10.30 ns 10.76 ns 11.36 ns
Generation Type Data rate Transfer time Command rate Cycle time CAS latency First word Fourth word Eighth word

Notes
  1. Transfer time = 1 / Data rate.
  2. Command rate = Data rate / 2 for double data rate (DDR), Command rate = Data rate for single data rate (SDR).
  3. Cycle time = 1 / Command rate = 2 × Transfer time.
  4. Nth word = [(2 × CAS latency) + (N − 1)] × Transfer time.

See also

References
  1. Stokes, Jon "Hannibal" (1998–2004). "Ars Technica RAM Guide Part II: Asynchronous and Synchronous DRAM". Ars Technica.
  2. Jacob, Bruce L. (December 10, 2002), Synchronous DRAM Architectures, Organizations, and Alternative Technologies (PDF), University of Maryland
  3. Memory technology evolution: an overview of system memory technologies, HP, July 2008
  4. Keeth, Brent; Baker, R. Jacob; Johnson, Brian; Lin, Feng (December 4, 2007). DRAM Circuit Design: Fundamental and High-Speed Topics. John Wiley & Sons. ISBN 978-0470184752.
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