Thursday, July 31, 2014

Ethernet transmission

In the last post I described how receiving ethernet frames was working.

Transmitting ethernet frames looked like it should follow a fairly similar process, so I spent a little time working on this.

I have structured it so that full-duplex operation is possible, with separate state machines controlling transmission and reception.

The first step was to see if I could cause anything to come out of the ethernet port at all.  Here I hit a bit of a problem, because all modern ethernet cards automatically check the ethernet Frame Check Sequence (FCS), and discard frames that are bad.  This meant that I would need to implement the FCS, and form completely valid ethernet frames first-up.

I much prefer to be able to make incremental advances, knowing that I have addressed particular steps as I go along, so I really wanted something that would let me receive invalid ethernet frames.  Then it dawned on me that the solution was to connect two C65GS's together, since the ethernet receive side doesn't (yet) check the FCS or pretty much anything else about the frames when receiving them.


This also means that I can check things without having to worry about one end feeding crazy bonjour packets all the time.

This let me quickly confirm that in fact nothing was coming out of the ethernet port with my first attempt.

I took a guess that the transmit side might not start transmitting if you don't immediately start with the ethernet preamble code.  After fixing that, and with back-to-back Nexys4 boards running as C65GSs, I was able to cause simple frames to be sent from one to the other.  I just stuffed a sequence of bytes in to the transmit buffer at $FFDE800, and then set the frame length in $FFDE043 - $FFDE044, and then wrote $01 to $FFDE045, and voila, the frame was sent to the other side with a reassuring blink on the ethernet led:

.sffde043 ff 00 01

You can see the momentus frame as received at the other end here.  Sadly no "Watson, come here" or "One small step" here.

.Mffde800                                                       
 :FFDE800 FE 00 5C C2 76 86 0A 0B 0D 0D 0E 0F 07 08 09 01
 :FFDE810 02 03 04 05 07 07 08 09 02 02 03 04 05 06 00 00
 :FFDE820 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE830 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE840 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE850 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE860 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE870 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE880 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE890 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE8A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE8B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE8C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE8D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE8E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE8F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 :FFDE900 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

The first two bytes are the received frame length (I can see that there is an out-by-one error here somewhere, my guess is on the transmit side), then the next four are the FCS as I calculate it as the packet is received, then the packet data starts after, from 0A onwards.  I think I am calculating the FCS incorrectly, as it never matches that of any real received frame sent by an actual computer, so I will need to look into that.

But anyway, this is good progress.

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