setup, hold, and signal timing skew inconsistencies

Walter Bridgewater wally at
Tue May 11 17:08:53 PDT 1999

* From the T10 Reflector (t10 at, posted by:
* wally at (Walter Bridgewater)

By duty-cycle skew, I mean the difference in time between when
clock (REQ/ACK) is low and when clock is high.

Ideally, for fast-80 DT, for each level of the clock (low/high), you would
have 12.5ns  (25ns/2).  But, because of, almost normal considerations,
you might have a clock (ACK/REQ) at the receiving chip of
11.3/13.7ns,  low/high time or high/low time.

So, another way of describing duty-cycle skew, is to say how much
the clock deviates from the ideal 12.5/12.5ns  high/low timing.

The duty-cycle skew could be as bad as 10/15ns and not violate
clause 9.2.20 in table 32 of SPI-3r5.  

The are many things that cause duty-cycle skew, besides the obvious
ones in the transmitter and receiver chips.  Things like a 
different delay time in the plus and minus signals in the cable
will cause duty-cycle skew.  For a given pair of wires in a cable,
1 of the wires could be longer than the other, but we really don't
even have any cable spec for this at all. 

So, even if the transmitter does a good job of having low duty-cycle
skew, other things can still increase it, beyond the control of
the transmitter.


> * From the T10 Reflector (t10 at, posted by:
> * Richard Moore <r_moore at>
> *
> Wally,
> How big is this duty-cycle skew? I was trying to figure out what
> it means and my best guess is that this is due to a difference in
> slew rates on rise and fall of the clock signal -- am I right? If
> so, perhaps "slew rate skew" would be a better name for it. And if
> it's a slew rate problem, then shouldn't the effect of this be
> allocated to the transmitter, rather than the cable, in the timing
> budget?
>  -- Richard
> >
> >* From the T10 Reflector (t10 at, posted by:
> >* wally at (Walter Bridgewater)
> >*
> >Jim,
> >
> >I don't think it's fine to say that DT technology is better 
> >than ST technology, there aren't any facts to support this.
> >
> >
> >Every time I have tried to study this issue, letting all other things
> >be equal, it appears that ST is better.  The duty-cycle skew is the
> >main thing that makes DT un-desireable.  All things being equal, and
> >using cables of great bandwidth, ST is much better than DT, since ST
> >doesn't have the duty-cycle skew problem.  
> >
> >As you degrade the cable bandwidth, is there a point at which having
> >the slower clock speed, but with more skew pays off?  By the time
> >the cables have degrade this far from the ideal, is the ISI from
> >both the clk and the data stopping and starting killing the performance
> >and any advantage from having slower clock with more skew lost?
> >
> >
> >Wally
> >
> >
> >> 
> >> * From the T10 Reflector (t10 at, posted by:
> >> * Jim McGrath <Jim.McGrath at>
> >> *
> >> 
> >> Richard,
> >> 
> >> I think its fine for us to say that DT technology is better than ST
> >> technology, even slow speed ST technology - I think it would simplify
> >> everyone's life going forward if DT could be used as the 
> >preferred protocol
> >> for LVD signaling.  I could also see where some of the 
> >timings for ST at
> >> lower speeds might have been specified in a way that make ST 
> >look worse than
> >> DT, even if that is not technically true - I think we have 
> >gotten a lot
> >> better over time at doing the timing budgets.  
> >> 
> >> On your specific points, if we've made a mistake somewhere 
> >in going over
> >> from SCSI-2 to SPI-3  in the ST timings, then maybe George 
> >can comment on
> >> whether these issues are in practice editorial in nature?
> >> 
> >> Jim
> >> 
> >> 
> >> >  
> >> >  -- Richard Moore
> >> >     QLogic Corp.
> >> >  
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