[SI-LIST] Re: AC coupling of SATA and SAS I/O
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: Alief <saifj_m@xxxxxxxxx>
- Date: Fri, 08 May 2009 08:54:43 -0700
In some systems, either the RX or TX could be unpowered for extended
periods of time. Especially as systems turn "greener". AC coupling
avoids potential failures that can occur because of a DC path between
some combination of RX and TX (across technology, vendors, process points).
Thanks,
Vinu
Ward, Richard wrote:
> 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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