> -----Original Message-----
> From: Grebenkemper, John [SMTP:[email protected]]
> Sent: Thursday, April 01, 1999 1:31 PM
> To: [email protected]
> Subject: [SI-LIST] : RE: [SI-LIST]: Long bus or star?
>
> There have been a number of messages recently on using a star
> configuration
> for a multi-drop net. We have successfully used the star terminated
> nets in
> several designs. A paper I presented at DesignCon98 discusses star
> terminations (a PDF copy is available on request).
>
> The star termination can be implemented by using a resistor located
> between
> the junction of the star and each transmission line. This requires N
> equal
> valued resistors for a N net star configuration. To obtain a perfect
> match
> at the star, the value of each resistor needs to be set to (N-2)Zo/N.
> This
> will avoid all reflections at the star, but will not necessarily yield
> the
> best waveform at the loads. Some tuning of these values can improve the
> waveform.
>
> For instance, at N=3 each resistor needs to be set to Zo/3 for a perfect
> match. For an ideal voltage source driver, this will give a maximum
> overshoot of 50%. Increasing the resistor value to 0.6Zo decreases the
> maximum overshoot to 18%. For real drivers, this value would need to be
> somewhat less due to the effective series resistance of the driver.
> SPICE
> simulations can allow one to tune to an optimal resistor value in one's
> system.
>
> This resistor value can be optimized for a larger number of loads in the
> star. Note that it is difficult to get incident wave switching for N>5.
> Equal length nets from the star can improve the waveforms due to some
> cancellations that occur in the reflections. However, quite acceptable
> peformance can be obtained with arbitary length transmission lines on
> the
> arms of the star.
>
> We have found that the propagation time on the net can be considerably
> less
> for a star configuration than for a daisy chain, especially when the
> loads
> off the daisy chain require longer transmission lines due to routing
> constraints. The star may also work better in configurations in which
> any
> receiver can become the driver since each driver now sees a more
> identical
> load.
>
> -John Grebenkemper
> [email protected]
>
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