[En-Nut-Discussion] Some questions about the Expansion Port

[Harald Kipp]

Hi Patrick,

At 19:12 06.04.2004 +0200, you wrote:
>Hi,
>
>First of all, congratulations for this very interesting project !

Many thanks, also in the name of all our contributors.




>I'm evaluating the Ethernut 2 board as a mean to add Ethernet connectivity to
>a FPGA-based demonstrator. Currently, we communicate with the demonstrator
>via a PC and a USB connexion. The goal would be to use Ethernet instead.
>
>According to the Hardware Manual, the Expansion Port can be used to connect
>Ethernut to an external device equipped with it's own processor.

Sure, there are several options to do so, like
RS232, SPI, I2C etc. But they may be all too slow.




>I have several questions about the Ethernut and the Expansion Port :
>
>   - Where can I find the specs (timing...) for the Expansion Port digital 
> IO ports ?

As the ports are directly connected to the ATmega128, you
will best check the ATmega128 datasheet. The Ethernut Board
runs with a 14.7456 MHz clock.



>   - Can I access the Ethernut SRAM through the Expansion Port (ie can I have
>     a DMA on the external device to read / write the SRAM) or do I need the
>     ATmega to read / write data on the Expansion Port.

The ATmega doesn't allow bus sharing or any kind of DMA,
so you would have to halt the CPU. This might be at least
complicated.

However, the Ethernut Board hasn't used up all address
space. So you may map part of _your_ RAM to the ATmega
address bus available at the expansion port and may be
use some simple port handshake to lock/release access
to this RAM.


>   - In both cases, what would be the throughput ? I need to transfer quite a
>     lot of data from the LAN to my demonstrator (image processing).

Feeding Ethernet is the bottleneck for this tiny CPU. Do
not expect more than about 250 kByte per second. OK,
"a bit more" may be squeezed out of it, but probably
not more than 400 kByte, which is still far below
10 MBit Ethernet. However, the LAN controller on Ethernut 2
is more reliable. Missing transmission are no problem
with TCP/IP, but degrade performance.

(For other members of the list: Even with the latest
contribs, the Realtek is acting weird and missing
incoming or simply not transmitting outgoing packets.)




>If some of these answers are available on your web site, please just let 
>me know
>where. I think I've looked everywhere, but I might have missed some 
>documentation.


I wish they would be, because this question comes up
quite often. At least we have a reference in the
mailing list now.

Regards,
Harald