Have a great day,
* It is common having single and double channel DDR controller in the PC world. Corresponding performance difference has been considered and tested many times. Synthetic benchmarks can show 30% and even 80% differences between the different configurations, but some real-world testing show 3-5% gain. I.e. it is task depended. It is true for the MPC8640 with its 2 DDR controllers. For example moving data in memory by DMA may show 80% increasing of DDR bandwidth for 2 DDR controllers. So the system designer should estimate and decide if there is noticeable gain from doubled memory bus bandwidth for designed system.
There was preliminary version of Freescale app. Note AN3431 “MPC8641D Memory Subsystem Performance Benchmarking and Platform Optimized Solutions” which required for NDA. You may ask Freescale support for this document if it is still available.
** If the DDR memory consists of 2 physical parts it can be connected to the different chip selects of the same DDR controller and share the memory bus or can be connected to the different DDR controllers. In the last case we get fewer loads per signal line and may have better signal quality.
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