i.MX6 - question about DDR memory bandwidth utilization

[marcin]

Hi All,

Our board has i.MX6DL with 32@400MHz DDR3 modules. The memory has a 800 Mb/s per pin, which gives 3200 MB/s. Of course this is theoretical value only, but based on http://www.xilinx.com/support/documentation/white_papers/wp383_Achieving_High_Performance_DDR3.pdf, page 6, Table 1, bandwidth efficiency (clock_cycles_transferring_data / total_clock_cycles * 100) can vary (for optimized memory controller) between 57 % - 90 %.

The problem is that based on values from i.MX MMDC profiling mechanism, we can't get efficiency larger than 50 %.  I found that Francisco Dominguez in thread https://community.freescale.com/message/328259#328259, also wrote about 50 % of efficiency. The question is why?

The second question is about changing way of DDR connection from x32 to x64. What is real improvement in bandwidth? I think that double a bandwidth is not possible. Most of AXI masters are 64-bit, so improvement is possible only due to use of upsizers in NIC-301, am I right?

Thanks,

Marcin

[Yuri]

  Theoretical memory bandwidth calculations, assuming that data are provided at
every clock edge, are very "theoretical". Real DDR data access burst needs some
preparation stage : bus arbitration, RAS phase, CAS phase,CAS Latency and only
after that we can get data  at every clock edge. Comparing 64-bit and 32-bit performance,

only the recent stage (data  at every clock edge) provides some improvement.