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I think megafunctions work propably, except that the dual port ram generated needs 2 clocks to read the contents of 2 addresses in memory, unlike writing, which takes only 1 clock to write in 2 addresses simultenously.
So, in case of reading, it's the same as single port ram.
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Even with altsyncram instantiation, it requires two clocks to read two different addresses simult., which is a draw back that makes dual-port same like single-port ram in that point :( .
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In the "RAM: 2-PORT" MegaWizard, you can select "With two read/write ports" and "Single clock". If that doesn't function correctly, you should file a mySupport service request. As far as I know, this MegaWizard setting combination lets you read both ports simultaneously at independent addresses in the same clock cycle.
If you tried instantiating altsyncram directly, then try the MegaWizard. You would then instantiate the MegaWizard variation (the HDL file generated by the MegaWizard), which in turn instantiates altsyncram. You might have gotten something like a parameter wrong in the altsyncram instantiation that the MegaWizard will do correctly.
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