RE: Optional PMA interface (OIF)
Vipul, Henning, Stu,
I would recommend that we do the following:
- eliminate the reference clock as a REQUIRED
item in the XBI interface, leaving it as
an implementation-specific issue
- suggest a 156.25 MHz reference clock in an
informative appendix (one is needed anyway
to deal with system timing and clocking
issues that are implementation dependent)
Regards,
- Tom Alexander
PMC-Sierra, Inc.
Vipul Bhatt wrote:
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Henning,
This is a very sensible proposal and I support it. Instead of leaving it
open, I would prefer that we specify or
suggest the frequency of the slower reference clock (156.25 MHz or 78.125
MHz).
Vipul
vipul.bhatt@xxxxxxxxxxx
(408)542-4113
==================
-----Original Message-----
From: owner-stds-802-3-hssg@xxxxxxxx
[mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Henning Lysdal
Sent: Tuesday, June 06, 2000 12:37 AM
To: stds-802-3-hssg@xxxxxxxx
Cc: paw@xxxxxxx
Subject: Optional PMA interface (OIF)
All,
In Ottawa Stuart Robinson presented a proposal to paste the "OIF
interface" (SFI-4 interface,
OIF1999.102), see
http://grouper.ieee.org/groups/802/3/ae/public/may00/robinson_1_0500.pdf
into
.3ae
The idea of reusing the work done by the OIF and the devices developed
to meet their specification
agrees perfectly with the cost and time-to-market objectives of .3ae.
However, in the LAN PHY case,
a minor change has to be made in the 10GE version of the OIF
interface:
The OIF specifies both data and clocks at the 16-bit interface. The
reference clock is specified to be
622.08MHz (for OC-192 rate). In the LAN case (64b66b) this translates
to 644.53MHz. Thus, the
clock-multiplier ratio is x16. The specification allows other optional
reference clocks e.g. 311MHz
(x32).
In my mind these reference clocks are too fast for 10GE. Fast refck's
means bulky and expensive
oscillators. In addition using 644.53MHz refck adds an entirely new
clock-domain in the PHY,
requiring additional clock tolerance compensation (see below). For
Ethernet we obviously need
cheap and small.
If the OIF interface is included in 802.3ae as an optional PMA
interface we should do one of the
following:
1) not specify the reference clock allowing this to be implementation
specific
2) specify a slower reference clock
Actually I like both options. Those who like option 2, please consider
the following:
In a serial LAN PHY you need the following clocks:
312.5MHz or 156.25MHz for XGMII (or 3.125GHz for XAUI)
156.25MHz for the 64 and 66 bit wide interfaces in the 64b66b CODEC
(PCS)
644.53MHz for the 16-bit (OIF) PMA interface
10.3125GHz (line rate)
some of these clocks are needed in both a receive and a transmit
version.
The OIF specification implies that the 644.53MHz interface clock
should be sourced from the
SerDes. Thus the SerDes generates both transmit and receive version of
the 644.53MHz and the
10.3125GHz clocks.
Looking at the list above, 156.25MHz becomes an obvious choice as
reference clock. This implies
that the SerDes clock-multiplier should be x66, requiring a 10GE
specific version of the OIF-style
SerDes.
If you want to implement a serial LAN PHY using a "pure" OIF SerDes
(644.53MHz refck), the
156.25MHz PCS clock should be generated by the PCS chip or sourced
from an additional crystal.
The former requires an extra PLL on-board the PCS chip and the later
increases device count and
requires clock tolerance compensation.
Thus, either way you're in trouble. You can choose to specify a
644.53MHz reference and reuse OIF
SerDes. This complicates PCS design and in some implementations
require an additional crystal
reference. You can also choose to let the SerDes do the job, but then
it is no longer a standard OIF
SerDes.
Being a SerDes designer, I think that the handling of this odd-ratio
clock rate conversion is best
done in the SerDes. From a total PHY cost and complexity perspective
adding an extra crystal
reference (in addtion to an already expensive one) or generating
156.25MHz from 644.53MHz inside
a CMOS PCS chip makes little sense. The only thing gained would be the
ability to reuse OIF
SerDes. Modifying OIF SerDes to include Ethernet specific clock
generation is a minor task that
will give us a lower complexity (cost, power) LAN PHY.
THE BOTTOM LINE:
Specifying an OIF reference clock of 644.53MHz increases serial LAN
PHY complexity significantly.
The reference clock should be left unspecified of specified at
156.25MHz (or half: 78.125MHz).
Stuart:
If you consider this a "friendly amendment", please update your
proposal and I'll be happy to endorse
it for July.
Regards,
Henning
-----------------------------------------
Henning Lysdal
Design Engineer
GiGA A/S - an Intel Company
Mileparken 22
DK-2740 Skovlunde
Denmark
Tel.: +45 70 10 10 62, Fax: +45 70 10 10 63
Direct: +45 44 54 61 54
E-mail: hl@xxxxxxx
Web: www.giga.dk, www.intel.com