Re: [SI-LIST] : Daisy-Chain

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From: sweir (weirsp@a.crl.com)
Date: Wed Mar 22 2000 - 12:46:00 PST


Yes, this is becoming a very common issue, tougher and tougher to deal with
given falling Tr's and micro packaging.

Regards,

Steve.

At 07:16 PM 3/22/00 +0900, you wrote:
>Thanks guys.
>
>Jeff's suggestion is sound good but couldn't be use in a high density PC
>board where space is premium. Example: at both ASICs (BGA), there is no
>space to put any series resistors.
>
>The transceiver (driver) I'm using has a current drive capability of
>24mA. Is it possible that this driver is too strong and caused
>reflections at the first receiver?
>
>
>Is this is too strong buffer which can cause reflection from device
>nearest to driver?
>
>Regards,
>KT.THAM
>
>
>
>
>At $B8a8e(B 12:44 00/03/20, S. Weir wrote:
>>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$B!'(B 026-267-7259
>>>>________________________________________________________
>$B"#(B $B8fCf(B
>$B"#(B $BMM(B
>
>$B$$$D$bBgJQ$*@$OC$K$J$C$F$*$j$^$9!#(B
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>
>_________________________________________________________
>
>$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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