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Altera_Forum
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19 years ago

DM9000A Ethernet and the 25 Mhz Clock

I realise that the DM9000 chip needs a 25 Mhz clock from a PLL, but I'm confused about how to actually deliver it and handle the other signals.

I would think you could use a PLL like so: 

    pll: sdram_pll PORT MAP (CLOCK_50, DRAM_CLK, ENET_CLK);

Which would connect the -3 ns output of the PLL to the SDRAM and the divided-by-2 output to the Ethernet chip.

The DE2_NET example, however, has the following port maps: 

               // the_DM9000A
   .ENET_CLK_from_the_DM9000A(ENET_CLK),
                 ....
   .iOSC_50_to_the_DM9000A(CLOCK_50),

There is also a PLL which has a 25 Mhz output, but I can't find a place where it connects to the Ethernet chip. If somebody can clear this up it would be much appreciated.

1 Reply

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    19 years ago

    --- Quote Start ---

    originally posted by benway@Mar 17 2007, 05:49 PM

    i realise that the dm9000 chip needs a 25 mhz clock from a pll, but i'm confused about how to actually deliver it and handle the other signals.

    i would think you could use a pll like so: 

        pll: sdram_pll port map (clock_50, dram_clk, enet_clk);

    which would connect the -3 ns output of the pll to the sdram and the divided-by-2 output to the ethernet chip.

    the de2_net example, however, has the following port maps: 

                   // the_dm9000a
   .enet_clk_from_the_dm9000a(enet_clk),
                 ....
   .iosc_50_to_the_dm9000a(clock_50),

    there is also a pll which has a 25 mhz output, but i can't find a place where it connects to the ethernet chip. if somebody can clear this up it would be much appreciated.

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    --- Quote End ---

    In the DE2_NET project, the ENET_CLK signal is generated in the DMA9000A_IF.v file, by using a simple divider from a 50Mhz signal to get a 25MHz signal.

    I agree that it is better to use a PLL to generate the ENET_CLK signal. So you can probably go with PLL code you have.