Re: [SI-LIST] : Daisy-Chain

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From: S. Weir (weirsp@a.crl.com)
Date: Mon Mar 20 2000 - 12:44:52 PST


Jeff,

The primary value of those resistors is to decouple the parasitic
capacitance of each load from the T-line. For the parallel case, if R
matches Zo, then it limits the impedance bump at each drop to -Zo/2. To
the extent that skew can be tolerated, the bigger the R, the better as in
the limit Zo is undisturbed, but also in the limit no AC signal gets to any
pin.

Most commonly, matching series R's are used at opposing ends as series
terms on bidirectional data lines, where there are only loads/drivers at
the two ends.

Regards,

Steve.
At 09:28 AM 3/20/2000 -0800, you wrote:
>Normally just lurk...but would like to add my .02 here. I too have
>experienced the same phenomona in the past. The solution I wuold like to
>suggest is to end terminate using your favorite method (AC, thev. equiv, R
>to ground, etc.). But additionally put a resisistor of value Zo just
>before the net connects to each receiver. Your net should daisy chain to
>each of the resistors, the other side of the resistor connects to the
>receiver input pin. Provided you keep this "stub" as short as possible,
>I've found this to work well. Your trace will look like the following:
>
>Driver
>---------------------------------------------------------------------------
>--End Term
> R R R R R R
> Pin Pin Pin Pin Pin Pin
>
>I would be very curious to hear what the list has to say. I would also
>like to hear if this suggestion cleans up you simulation.
>
>Regards
>Jeff
>>----- Original Message -----
>>From: <mailto:kt_tham@uti2000.co.jp>Tham Kok Tong
>>To: <mailto:si-list@silab.eng.sun.com>si-list@silab.eng.sun.com
>>Cc: <mailto:kt_tham@hotmail.com>kt_tham@hotmail.com ;
>><mailto:smtai@uti2000.co.jp>smtai@uti2000.co.jp
>>Sent: Monday, March 20, 2000 12:26 AM
>>Subject: [SI-LIST] : Daisy-Chain
>>
>>Hi,
>>
>>I have a trace which connects in daisy-chain from 1 driver(transceiver)
>>to 6 receivers ( 2 ASICs (BGA type), 2 ROMs, 1 flash memory and 1
>>transceiver).
>>The connecting sequence is 1) ASIC1, 2) ASIC2, 3), transceiver, 4) ROM1,
>>5) ROM2, 6) Flash memory.
>>My problem is the simulation result shows reflection occurs at the
>>receiver(non-monotonic waveform) nearest to the driver. Is it a good
>>idea to put a pulldown resistor at that receiver(resistor value = Zo)?
>>
>>I welcome any suggestions or advices on this matter. Thank you.
>>
>>Regards,
>>KT.THAM
>>
>>
>>
>>
>>
>>_________________________________________________________
>>
>>$B!!!!(BUltimate Technologies Inc.
>> SI Group
>> KT.THAM
>>
>> 1462-6F, Shindencho, Nagano-Shi 380-0835, Japan
>>$B!!(B TEL$B!'(B $B!!(B026-267-7256
>>$B!!!!(BFAX$B!'(B 026-267-7259
>>________________________________________________________

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