[SI-LIST] Re: effects of pattern on transmission line - DDR2

Well, even with ideal Vtt regulation, you get into data pattern 
dependent threshold crossing times (Inter-symbol Interference) with this 
example. The timing of the threshold crossing of a transition is a 
function of the data bits preceeding it, producing data-dependent jitter.

With the alternating block, the driver output never reaches full Vol or 
Voh; a threshold set at the midpoint of the swing produces minimum bias 
distortion (near-equal 1-0 width).  When the line is allowed to remain 
in one state for sufficient time, the line reaches steady-state Vol or 
Voh.  Now, the first edge transmitted after the steady-state voltage has 
been reached has further to go to reach Vth, which means it happens 
later on the edge than it did in the alternations case.  Couple that 
with a return transition on the next edge, and you have a 
narrower-than-ideal bit with minimum overdrive.

Vtt regulation (as contrasted to Vth) may cause the driver's operating 
point to shift slightly, as the load line moves along the driver's IV 
curve.  I try to derive Vth at the receiver from the driver's Vddq 
supply with an RC filter, and derive the reference for the Vtt supply 
likewise.  This tends to make everything track proportionally, at least 
at low to mid-frequency.

Regards

Mike

steve weir wrote:

> Shane, in the end it is Vtt at the differential amplifier input.  But, most 
> of us only get to see it on the PWB.
> 
> Steve.
> At 01:57 PM 1/15/2004 -0800, San Miguel, Shane wrote:
> 
>>By VTT supply, do you mean on-die or external (supplied by the system or
>>tester)?
>>
>>Shane San Miguel
>>
>>-----Original Message-----
>>From: Steve Horne [mailto:sch@xxxxxxxxxxxxxx]
>>Sent: Thursday, January 15, 2004 1:52 PM
>>To: weirsp@xxxxxxxxxx
>>Cc: San Miguel, Shane; silist
>>Subject: Re: [SI-LIST] Re: effects of pattern on transmission line -
>>DDR2
>>
>>
>>Another issue that can affect this is what the other bits'
>>transitions are doing to the VTT termination supply.  Having all
>>the other bits have an opposite state than the victim (before,
>>during and after the isolated 1) may yank the VTT supply in an
>>unfavorable direction, affecting the victim bit.  At least this
>>was the case in DDR1.  Your VTT decoupling is therefore an
>>important variable in this analysis.
>>
>>Steve H.
>>
>>steve weir writes:
>> > Shane, what you are dealing with is group delay.  You can find it
>>described
>> > in any filter literature.  The low frequency components of the long
>>periods
>> > before and after the single pulse travel at a different rate than the
>>high
>> > frequency components of the pulse.  This alters the pulse baseline
>>and
>> > shape.  The isolated pulse will appear larger, and more unipolar than
>>the
>> > later training pulses which will be almost uniform about the origin,
>>and
>> > very close in shape to each other, as the system has stabilized.
>> >
>> > Steve.
>> >
>> > At 11:31 AM 1/15/2004 -0800, San Miguel, Shane wrote:
>> > >I have two data patterns of interest (I'll try text) both are a 4
>>burst
>> > >of 4 type
>> > >
>> > >
>> > >----<xxxxxxxxxxxxxxx>----
>> > >
>> > >
>> > >
>> > >----_________| |______------
>> > >
>> > >
>> > >
>> > >In the first pattern we have an alternating 010101 type of pattern.
>>The
>> > >second pattern is all zero's with a 1 in the middle.  The first
>>pattern
>> > >(more traditional) is intended to stabilize the transmission line
>>before
>> > >the measurement is made.  We forget the first and last couple of
>>edges
>> > >and make rise/fall time measurements on each of the edges in the
>>middle
>> > >of the train.
>> > >
>> > >
>> > >
>> > >My thinking is that the second pattern will produce a slower edge
>> > >because you have no activity and the BAM!  You have to fill up all
>>this
>> > >trace capacitance and by the time you do that, the data is done.
>> > >
>> > >
>> > >
>> > >I guess my "rule of thumb" is what is stumping me.  I look at the
>>first
>> > >pattern and go "ok, stable DC level, more repeatable results".  I
>>look
>> > >at the second pattern and go "look at that inductive kick - what a
>> > >reflection" or "how much is that edge going to roll over"...
>> > >
>> > >
>> > >
>> > >Anyone care to educate me on the transmission line behavior with the
>>two
>> > >patterns?
>> > >
>> > >
>> > >
>> > >Shane San Miguel
>> > >
>> > >
>> > >
>> > >
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> 
> 
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