# DDR3 memory transfer rate

I am trying to understand memory transfer rate while working with DDR3. I am getting different numbers.

For example, in Wikipedia https://en.wikipedia.org/wiki/DDR3_SDRAM DRAM has peak transfer rate from 6400 MB/s to 17066MB/s depending on the clock.

While looking at different FPGA development boards I see data in Mbps and it is completely different from the ones in Wikipedia. For example this product offers 1050Mbps at https://store.digilentinc.com/zybo-zynq-7000-arm-fpga-soc-trainer-board/.

For me, 1050Mbps is 131MB/s. Where is the error?

Data rate is dependent on both data width and frequency. Data rate is:

$$(\mathrm{freq}\times2\times\mathrm{width})$$

The x2 is due the the fact that it is DDR - double data rate - memory, so the output data rate is twice the clock frequency.

Typical PC DIMMs are 64bit wide and run between 400MHz (6.4GB/s) and 1066MHz (17.06GB/s) which is where the figures Wiki quotes are from. As a worked example a standard 64bit DIMM running at 800MHz would give a data rate of:

$$800\mathrm{MHz}\times2\times64=102.4\mathrm{Gbps}\space(12.8\mathrm{GB/s})$$

According to the documentation for that board, it has a 32bit memory chip running at most 533MHz, giving a maximum data rate of 34.1Gbps (4.26GB/s).

As to where the 1050Mbps comes from, as far as I can see they are talking about the per-bit data rate of the memory. In other words the $(\mathrm{freq}\times2)$ bit of the calculation.

For a 533MHz DDR module, that equates to 1066Mbps - close to the figure they quote.