RE: [SI-LIST] : re: [SI-LiST]: PLL clock buffer chips and the fee dback loop

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From: John Draut ([email protected])
Date: Wed Apr 05 2000 - 12:15:12 PDT


Andy,

Your solution should be fine if you run a star type series termination tree,
that is the clock oscillator output immediately feeds into 2 series
resistors (between 10 - 33 ohm depending on line impedance) which in turn
travel down equal length lines to their respective loads.

Regards,

John Draut
Manager, Board & System Engineering
Sebring Networks
200 E. Hacienda Avenue
Campbell, CA 95008
Phone: +1 (408) 871-7430
FAX: +1 (408) 374-2652
e-mail: [email protected]
 

-----Original Message-----
From: Andy Peters [mailto:[email protected]]
Sent: Wednesday, April 05, 2000 11:48 AM
To: [email protected]
Subject: [SI-LIST] : re: [SI-LiST]: PLL clock buffer chips and the
feedback loop

Thanks to all who've responded. Here's what I intend to do. I found a TI
part (CDCF2509) with built-in source terminations and nine outputs. The
oscillator will feed the reference input. The five "1Y" outputs will feed
an FPGA and its four SDRAMs (two of the SDRAMs are on top of the board, the
other two are on the bottom). One of the "2Y" outputs will feed a third
FPGA's clock input. A second "2Y" output will feed a second CDCF2509. The
five "1Y" outputs of the second PLL buffer will feed the second FPGA and its
four SDRAMs. The "2Y" outputs of the second PLL buffer will be disabled.

The board has a clocks-only layer, so we don't think we'll have too much
trouble trying to keep the clock line lengths all equal.

The only "gotcha" here is that the VME interface control chip, a Cypress
(@#$%!) part, requires an 80 MHz clock, but that part has a built-in
power-on reset circuit that enables it to configure itself. That
configuration fails if the clock is not running, and the ~1 ms PLL buffer
startup time is too long. It seems as if the only solution is to run two
traces from the oscillator - one to the first buffer and the second to the
VME chip. I don't think it's a problem keeping those traces equally short,
and the oscillator data sheet says it can drive +/- 16mA.

Again, much thanks to the gurus.

-a

-----------------------------------------
Andy Peters
Sr Electrical Engineer
National Optical Astronomy Observatories
950 N Cherry Ave
Tucson, AZ 85719
520 318 8191
[email protected]

"Money is property; it is not speech."
            -- Justice John Paul Stevens

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