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Altera_Forum's avatar
Altera_Forum
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17 years ago

memory usage by DCFIFO in cyclone III

I'm trying to get an understanding of the memory usage by the DCFIFO MegaWizard function in a cyclone III application.

For example, in my application I'm looking at a FIFO that is 18 bits wide by 8K long. In the MegaWizard, the usage is listed as 18 M9K blocks.

The internal memory in a Cyclone III is 9 bit blocks, right? So shouldn't this pack into 16 M9K blocks? Is there some reason that the 9th bit isn't being used by the FIFO MegaFunction?

Thanks.

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Thanks gmpstr

    The check box is grayed out unit I set it to a 8K x 18 FIFO. Once I do I'm able to pack it into 16 M9Ks.
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    See Table 4-1 of the following:

    http://www.altera.com/literature/hb/cyc3/cyc3_ciii51004.pdf

    I would assume the M9Ks are set up in the 8192 x 1 configuration. (8192deep x 1wide) Thus, for 18 wide, it would take 18 M9Ks.

    To pack into 16 M9Ks, it would have to stack 16 of the 512 x 18 configuration.

    The 8192x1 configuration is probably faster and uses less logic then the 512x18 configuration. In 9.0, there is a checkbox called Reduce RAM usage, may be in other versions also. Checking this box yields a 18wide x 8k deep fifo that uses 16 M9Ks. Unchecked, it uses 18 M9ks.