This tool calculates the latency associated with Random Access Memory (RAM). It also calculates the clock cycle time.
Enter
- Transfer rate associated with DDR memory
- CAS Latency
Formula
Clock Cycle Time (ns) = 2000/Module Speed
where Module speed is specified in Mega Transfers per second.
Latency = CAS latency * Clock cycle time
Convert from RAM clock rate (MHz) to transfer rate (MT/s)
Example Calculation
Let’s use the calculator to find the RAM latency for DDR4. The speed of DDR4-3200 is 3200 MT/s. The CAS latency for this module is 22 clock cycles. With these numbers, the calculator gives DDR4 latency of 13.75 ns.
Background
What is DDR?
DDR, also known as Double Data Rate, is a type of memory technology used in modern computer systems. It is a type of SDRAM (Synchronous Dynamic Random Access Memory) that allows data transfer on both the rising and falling edges of the clock signal. This feature effectively doubles the data transfer rate compared to SDR (Single Data Rate) memory.
What is CAS Latency?
Column address strobe latency, also called CAS latency or CL, is the delay in clock cycles between the READ command and the availability of the first bit of output. In asynchronous DRAM, the interval is specified in nanoseconds (absolute time). In SDRAM, the interval is specified in clock cycles.
CAS latency is the delay in clock cycles, between the internal READ command and the availability of the first bit of output
RAM Latency Table
The table below shows RAM latency for different generations of DDR
| Technology | Module Speed (MT/s) | Clock Cycle Time (ns) | CAS Latency | Latency (ns) |
|---|---|---|---|---|
| DDR | 400 | 5 | 3 | 15 |
| DDR2 | 800 | 2.5 | 6 | 15 |
| DDR3 | 1600 | 1.25 | 11 | 13.75 |
| DDR4 | 3200 | 0.62 | 22 | 13.75 |
| DDR5 | 4800 | 0.42 | 40 | 16.67 |
In general, the CAS latency increases with every generation of DDR memory. However as the clock speed and transfer rate increases, newer standards have better performance than prior. CAS latency numbers can be found in the data sheet associated with the memory hardware.
How to pick Memory?
In general
- Identify the highest memory speed (DDR5 for instance) supported by both the processor and motherboard
- Select the lowest latency memory at that speed. Lower latency means smaller delays in data retrieval and therefore superior system performance.