Hi,To gain space I am thinking about replacing tqfp SRAM (cy7c1380) with bga DDR2. Is there any difference between using a nios on sram or on ddr ? what are the problems with ddr ? thanks.
If the problem is only pcb space, I think cy7c1380, too, is available in a bga package.Anyway, ddr2 like any dram memory have a much lower cost per bit compared to sram, so it could be convenient for saving $$ besides saving space, especially if you need a big memory. The major problem with DDR2 is designing pcb. Unlike sram, ddr2 devices don't use LVTTL signaling but SSTL 18. This signaling standard requires impedance matching and a careful design of traces in order to avoid transmission problems. Apart this point, DDR2 then works much like an usual sdram, so all pro-con comparisions between sram and dram apply.
The read/write cycle times will also be somewhat different.This may not make much difference for cache line read/write, but for random cycles the DDR2 is likely to be slower.