[USRP-users] Ethernet over SFP+ in custom FPGA design (X3x0)

Sugandha Gupta sugandha.gupta at ettus.com
Tue Nov 28 19:34:32 EST 2017

I can answer the question related to the padding. I am not sure about the

Ettus Padding: An ethernet frame has 6 bytes of destination MAC address and
6 bytes of Source MAC address. Since we use 64 bits/8 bytes of data in one
clock cycle, we add a 6 byte padding in front of the ethernet packet. The
new packet becomes:
< 6 bytes padding> 80 00
20 7A 3F 3E 80 00 20 20
and so on...

[image: Inline image 1]

We just ignore the 6 bytes of padding in the eth_dispatch block. It is used
to align the incoming packet to make classification easier.

- Sugandha

On Tue, Nov 28, 2017 at 12:09 PM, Christian Lenz <
christian.lenz at freedelity.com> wrote:

> Ian and Sugandha,
> thank you very much for your comments and also for the attached file.
> Sadly, another three questions remain for the moment:
> 1. In the attached file, there is a series of 48? bits named "Ettus
> Padding". Is this an Ettus specific bit sequence and where can I find
> information on this?
> 2. If powering up and programming the FPGA, the clock from the PLL is not
> present because it must be programmed over SPI first. According to the HDL
> source files and pin constraints, the corresponding SPI master module in
> the FPGA is clocked  from a 125MHz clock. I assume this clock to be present
> without any configuration, is this true?
> 3. Do you know where to find the missing pages of the X3xx schematics
> document (13-17)? I would assume to find the 125MHz clock source on page 17.
> Thanks a lot!
> Zitat von Ian Buckley <ianb at ianbuckley.net>:
> Christian,
>> CHDR packets are encapsulated in UDP/IP between Host and USRP. See the
>> attachment.
>> PHY+ MAC functionality live under the x300_sfpp_io_core. However these
>> blocks do not encapsulate/decapsulate the network packets.
>> All the ethernet/IP/UDP framing fields are added by chdr_eth_framer on
>> egress, and will be removed on ingress by etc_dispatch (if needed).
>> -Ian

Sugandha Gupta
Staff Software Engineer
Ettus Research
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