RE: [ethmac] CRC appending

["Igor Mohor" <igorm@opencores.org>]

Hi.

Thanks for the report. This was a known issue already but I didn't find time
to go into it. Thank you very much for your help. I'll check your solution
tomorrow and update the code if ok.

Regards,
	Igor

> -----Original Message-----
> From: owner-ethmac@opencores.org [mailto:owner-ethmac@opencores.org]On
> Behalf Of Lake, Matt A @ PWC
> Sent: 28. oktober 2002 19:32
> To: 'ethmac@opencores.org'
> Subject: [ethmac] CRC appending
>
>
>
> Recently when simulating the MAC i noticed a problem. I had the MAC set up
> to NOT append FCS but it was appending it.  I started going
> through the code
> and i think i found the problem to correct this.
>
> In eth_txstatem.v you are assigning your StartFCS signal.  the line says
>
> assign StartFCS = ~Collision & StateData[1] & TxEndFrm & (~Pad &
> CrcEn | Pad
> & NibbleMinFl)
>                 | ~Collision & StatePAD & NibbleMinFl;
>
> This, from what i can tell, is incorrect and here is why.  First of all to
> anyalize this we can assume that we are not going to have a collision and
> that both StateData[1] and TxEndFrm will be valid ('1') at some point and
> therefore we dont need to look at them so we just concentrate on:
>
> (~Pad & CrcEn | Pad & NibbleMinFl)| StatePAD & NibbleMinFl
>
> So the behavior that we would want is that if i do not want to
> append a CRC
> that StartFCS should never be a 1.  In my simulation i want to Pad so the
> Pad signal will be a '1' and in the Mode register i tell it to not append
> CRC and when i write the control register of the particular BD
> for Transmit
> i also tell it to not append CRC.  This makes CrcEn a '0'  Now as you can
> see from the above statement that if i am padding and i recive a packet
> smaller than minimum packet length it will Make StartFCS a '1' when it
> finishing padding (denoted by the NibbleMinFl varible that is
> only a '1' if
> the packet is the minumum packet length).  This means that no
> matter what my
> CrcEn is if i am padding and i recive a packet that needs to be padded i
> will append an FCS.  To make this correct i belive it should be.
>
> assign StartFCS = (~Collision & StateData[1] & TxEndFrm & (~Pad | Pad &
> NibbleMinFl)
>                 | ~Collision & StatePAD & NibbleMinFl) & CrcEn;
>
> This will only append the FCS if we have set the appropriate
> things to make
> CrcEn a '1'.  I also wonder if you need the (| ~Collision & StatePAD &
> NibbleMinFl) portion of that because it would seam that you said the same
> thing with your Pad & NibbleMinFl statement, but i would have to
> investigate
> that further to see if it is the truth.
>
> Matt Lake
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