[SI-LIST] Re: Need advice on basic 6-layer stackup

Graham,

To understand where 50 ohms comes from, I suggest that you look at the 
IV curves of the troublesome drivers, with a load line of 40-50 ohms to 
the opposite voltage level.  In other words, plot the high state IV 
curve with a load line of 40-50 ohms to Vol, and vice versa.  The 
intersection of the IV curve and load line represents the launch voltage 
into the net of that impedance.  It's not surprising that the launch 
voltages are really close to the input threshold with low-Z loads.  A 
few ohms difference in load Z shifts the launch voltage considerably.   
Heebie-jeebies is an understandable response.

Basically, 50 ohms is driveable with a large class of circuits. The 
output impedance of  many circuits can approach 50 ohms (depending on 
drive strength selection), giving a built-in series termination effect.  
Higher impedance traces are more easily driven but are more prone to 
crosstalk and are harder to fabricate (narrower traces, thicker 
dielectrics).

The absense of  termination (or a design strategy that comprehends the 
consequences of not terminating)  really leaves you hanging out.  That 
long net of yours could ring 3 or 4 times, at ~1.5 ns per transit of the 
trace.  All depending on where the driver is on the vendor's process 
spread.   Meanwhile, some inputs are probably being overdriven with 
respect to Vin max or overshoot duration. The series terminated line is 
settled in 2 transitions, with minimum or no stress on the receiver.  If 
the ringing net is a clock or reset, you have lost control of the timing 
completely.  An extra threshold crossing on that class of net is fatal.  
If it is a clocked signal, you will be functionally OK if the ringing 
settles out before the next clock (minus setup time).

One thing I can tell you,  if the board was designed some time back, the 
risetime of the parts on the board is probably faster than when it was 
initially designed and tested.  Nets that used to work OK can become 
unreliable when the risetime decreases.  Lots of boards "work" in spite 
of ignoring SI considerations.  For a while.

Regards

Mike

- - - - -

Graham Davies wrote:

>--- In si-list@xxxxxxxxxxxxxxx, steve weir <weirsi@xxxx> wrote:
>
>  
>
>>... the fact that your company's
>>"slow"  "1999 era" product doesn't 
>>work, is testament that something
>>in your shop is sorely lacking.
>>    
>>
>
>This is accepted fact. Specifically what is lacking is a digital 
>circuit designer. More generally, what was lacking is any kind of 
>management of the engineering process. I am trying to rectify the 
>latter but the former will take more time.
>
>  
>
>>Your board house has handed you a
>>massively improved ... 50 ohm
>>stack-up.
>>    
>>
>
>Agreed. It almost works and can be made to work with a minor tweak. 
>The only thing that's troubling me, that I can't seem to get across, 
>is that I don't understand where this 50 ohms came from and why it is 
>the right choice. A number of the chips on the good-stackup board 
>seem to be having trouble driving their nets, leaving signal levels 
>dangerously close to the threshold while reflections rattle around. 
>This just gives me the heebie-jeebies.
>
>  
>
>>That you still have questions ...
>>should tell you to seek education ...
>>    
>>
>
>That's exactly what I'm doing here.
>
>  
>
>>Lee Ritchey's "Right the First Time".
>>Dr. Bogatin's "Signal Integrity Simplified"
>>Dr. Johnson's  "High Speed Digital Design".
>>Tom Granberg's book "Handbook of Digital Techniques 
>>for High-Speed Digital Design..." 
>>    
>>
>
>Thank you for these recommendations.
>
>Graham.
>
>
>
>
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