i have a hard time separating board design from signal-integrity.  in a
perfect
world an engineer sets up a set of design rules for the board layout person
to
work from.  but no layout person should be working without knowledge of the
principles involved.

i had a medium-size board with about 150 digital I/O on it, and i had
simulation/
pattern generation software (teradyne's LASAR), and when i started on the
board
i had a test for a small section of the board, and left all the other inputs
undriven.

the board was so noisy i couldn't test anything.  noise on the best signals
was
about 2 volts.  unclamped inputs showed plus and minus 10 volts.  

later on i found that it only took one nasty oscillator to mess things up. 

miker
 

> -----Original Message-----
> From: matt [SMTP:[EMAIL PROTECTED]]
> Sent: Wednesday, March 28, 2001 11:31 AM
> To:   Protel EDA Forum
> Subject:      Re: [PEDA] Reference
> 
> Leaving CMOS logic inputs floating is bad design practice . The inputs are
> high impedance and wideband and pick up noise that translates in outputs
> toggling state randomly at high speed . This has nothing to do with pcb
> layout and is an electrical design issue. You shouldn't have to deal with
> this type of problem unless you're also the person who _designed_ the
> schematic and are not just doing the capture and layout. Protel schematic
> is
> not intended as a digital design tool .
> 
> Best Regards,
> Matt Tudor , MSEE
> Elmar Technologies
> http://personal.lig.bellsouth.net/~mariusrf          consulting
> http://www.rfdatacorp.com           spread spectrum radio telemetry
> 
> -----Original Message-----
> From: Nicholas Cobb <[EMAIL PROTECTED]>
> To: [EMAIL PROTECTED] <[EMAIL PROTECTED]>
> Date: Wednesday, March 28, 2001 9:21 AM
> Subject: [PEDA] Reference
> 
> 
> >In a post about a month ago Mr. Lomax mentioned that leaving a CMOS input
> >open could cause problems on a board.  Are there any references that will
> >help me learn the details that might cause problems like this in
> commercial
> >devices?  Up to this point I have been making circuit boards that will be
> >used only by me.  I am just starting to work on some that will be mass
> >produced.  Any sort of help would be appreciated.
> >Thank you,
> >Nick Cobb
> >

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