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My problem is that with only 2 FIFO and 1 FFT on my cyclone IV, I'm approaching full efficiency of memory bits. But I use only 1/10 LEs of the FPGA.
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Do you mean efficiency or utilisation? You use the memory efficiently when you use the memories without wasting any bits. If you created a 1k ram, but this ram is placed on an M9K, you have only used about 10% of the ram, and the other 90% is wasted, which is not very efficient. You can use up your ram very quickly like this.
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However, I would like to know: If I use an external memory of FPGA chip, wouldn't FPGA be really slower than if I used only the internal memory?
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THere are trade offs:
FPGA ram is has high bandwidth (because you access so much of it in parrallel) but has low capacity.
External ram will be much lower bandwidth (as you only have one data bus, but the bandwidth will be determined by bus speed) but can have a huge capacity.
So it really comes down to your application.
Example - if you are running video with a frame buffer, at larger resolutions a single frame would easy use most or all of the FPGA memory - so you need an external ram to store it.
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