[SI-LIST] Re: AC coupling of SATA and SAS I/O

Richard,
It seems that my statement caused some confusion. Let me clarify the statement.

"If your transmitter has 0V DC bias..."  ==> "If your transmitter operates with 
0V DC bias..."

TK

________________________________
From: Ward, Richard [mailto:richard.ward@xxxxxx]
Sent: Friday, May 08, 2009 12:24 AM
To: Alief; si-list@xxxxxxxxxxxxx; T.K. Jeon
Subject: RE: [SI-LIST] Re: AC coupling of SATA and SAS I/O

Hi Alief,

My initial statement was directed at one of the comments below:
"If your transmitter has 0V DC bias, then I guess you don't need to implement 
AC coupling".
I read this as a statement of 0V common-mode from the Tx.
A large portion of the Serdes out there and used for these standards are VTT 
referenced or GND referenced swings and given the supply ranges for the type of 
systems these go into (often somewhere between 0-0.8V to 0-5V), I don't believe 
the statement above (or I misunderstand it and someone will correct me).
We're agreed on the function of the cap - but the prevous thread seemed to 
contradict that (to me anyway).

The point about the differently referenced systems was for interoperability 
yes, but that fact that it allows a wider range of Si to work with those 
standards if ac caps are there is a benefit to all - and therefore usually a 
good thing for a standard to have.

For hot-swap, one of the protections the circuit designer needs to put in is 
for "things to go wrong" and short. This is a failure mode. It may be a 
transient, but it may be someone, somehow, managed to stuff the wrong connector 
in the slot - my children are experts at that. We could argue this isn't 
strictly hot-swap, but it is a precaution many circuit designers take and label 
under the hot-swap banner. The diff pairs, being the most sensitive signals are 
usually the most difficult to protect. For EM breakdown limits, time is 
obviously one of the major parameters. So if it's a short on a high-speed 
signal that's ac coupled...we've blocked the major dc component and therefore 
reduced the time and the EM risk. That was the only point I was making there - 
caps can be your friends in a failure mode.

Regards,
Richard

________________________________
From: Alief [saifj_m@xxxxxxxxx]
Sent: Thursday, May 07, 2009 11:36 PM
To: si-list@xxxxxxxxxxxxx; T.K. Jeon; Ward, Richard
Subject: Re: [SI-LIST] Re: AC coupling of SATA and SAS I/O
Hi Richard,

Thanks a bunch for your response.

I'm really not sure why you found it necessary to state that "AC caps aren't 
used to set the Rx bias".  Of course, the AC cap does not set the Rx bias.  In 
fact, how can one expect an AC cap to set a bias ?  What the AC cap does is to 
remove the common mode level not set it; and then, of course, that permits the 
Rx to set the common mode wherever it chooses to.

You have made good points about how ac coupling helps differently referenced 
systems work together - I would classify that under interoperability..

Those are all good points worth noting.  However, I am afraid I am not aware of 
any long term effects of hot swap.  Doesn't hot swap only cause transients ?  
Assuming there are no application mismatches or design marginalities/errors how 
could hot swap result in long term effects ?  Would greatly appreciate your 
expounding on that.  Regards,

Alief

--- On Thu, 5/7/09, Ward, Richard <richard.ward@xxxxxx> wrote:

From: Ward, Richard <richard.ward@xxxxxx>
Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O
To: "Alief" <saifj_m@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" 
<si-list@xxxxxxxxxxxxx>, "T.K. Jeon" <tkjeon@xxxxxxxxxxx>
Date: Thursday, May 7, 2009, 9:48 PM

Hi Alief,

My view is a little different from that below.

AC caps aren't used to set the Rx bias, but to allow the Rx to set it's own 
bias point. This is especially useful for rack-to-rack systems where the common 
mode cannot be well controlled. It also has the advantages of:

- allowing VTT referenced and GND referenced systems to work together
- allowing a single Serdes to cover multiple standards
- allowing newer (restricted supply) devices to work with legacy devices
- ... (I'm sure someone on this list has written a book or two on this...)

The hot-swap ability doesn't really demand dc blocking caps, but they give 
advantages. "hot-swap" is a bit of a nebulous term too, often.
One of the risks of hot-swap is shorting the connections (signal-signal or 
signal-supply). Some devices will be designed to withstand the short-term 
current only (not long-term lifetime degradation). Having the caps means a 
long-term effect, actually becomes a short-term one, therefore EM 
considerations are less restrictive on the circuit design side.
I believe PCIE defined them at the Tx for this (shorting) reason (whereas a 
majority of non-pluggable applications put them at the Rx side, why? well it 
depends).

Regards,
Richard

-----Original Message-----
From: 
si-list-bounce@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx>
 
[mailto:si-list-bounce@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx>]
 On Behalf Of Alief
Sent: 07 May 2009 12:41
To: 
si-list@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx>;
 T.K. Jeon
Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O

Thank you for your response T.K.

Yes, a 0V DC bias would establish an automatic reference (in theory); thus it 
would be akin to DC coupling.

However, I am hesitant to make my common mode 0V; because of issues with ground 
loops, ground path parasitics, etc and also because the receiver would see 
negative voltages.

I realize that this is to first order a biasing issue and allows for 
interoperability.

However, are there other considerations ?  I have heard/read that it might 
permit hot swappability.  Is this true.  If I had a known common mode level 
doesn't that make it easier for hot swapping ?

Also, are there other issues which AC coupling helps with ?

Thanks a bunch in advance,

Alief
--- On Thu, 5/7/09, T.K. Jeon 
<tkjeon@xxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=tkjeon@xxxxxxxxxxx>>
 wrote:


From: T.K. Jeon 
<tkjeon@xxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=tkjeon@xxxxxxxxxxx>>
Subject: RE: [SI-LIST] AC coupling of SATA and SAS I/O
To: "Alief" 
<saifj_m@xxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=saifj_m@xxxxxxxxx>>,
 
"si-list@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx>"
 
<si-list@xxxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxxx>>
Date: Thursday, May 7, 2009, 6:55 PM


Alief,

AC coupling is needed to maintain the correct DC bias for receivers. If your 
transmitter has 0V DC bias, then I guess you don't have to implement AC 
coupling.

TK

-----Original Message-----
From: 
si-list-bounce@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx>
 
[mailto:si-list-bounce@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx>]
 On Behalf Of Alief
Sent: Thursday, May 07, 2009 10:46 AM
To: 
si-list@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] AC coupling of SATA and SAS I/O

Greetings everyone,

I am currently looking at SATA and SAS SerDes designs.  It looks like the data 
transmission is AC coupled.  Is this a standards requirement ?

Could anyone enlighten me on why SATA/SAS specifies only AC coupled links [if 
that is in fact the case]; i.e. what was the thinking/justification for 
restricting transmission to ac coupled only.

Also, do the other standards :  Gbit Ethernet, PCI Express, RapidIO, etc. have 
similar restrictions/requirements ?

Thanks v much in advance,

Alief


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