deglitching a clock

One of my customers screwed up the pinout on a largish microprocessor chip, *every pin*, and kluged it to work. It looked like a chip sitting on a ball of red yarn.

John

Hello John,

That increases phase noise. It may not matter in the digital world but I wouldn't do that for ADC clocks.

One technique I use if a home-run star system isn't practical is a nicely impedance controlled clock line (or a few) and then tapping off with low-C RF transistors wherever the clock is needed. It raises the hackles of the digital folks during design reviews but after a thorough explanation they like it.

That's how TV distribution used to be done in Europe, one line down from the roof and tapped off with -20dB couplers. At the very end there is a terminator. I use AC termination, mostly.

Regards, Joerg

I've had that happen, too! I laughed out loud reading your post. It is good to know that others in the world have gone through the same pain :) !

Take care, Rob

CATV systems still use backmatched taps, but the tap value drops the further you get from the amp. the systems I worked with were designed for a +10 dBm signal at the port on the tap to allow for four TVs per drop, and up to 4 dB loss in the drop and house wiring. It was an average sized system with 10,000 active drops, and 4,000 homes passed that didn't have service.

The MC68340 is the same family as the MC68332. I saw a lot of MC68340 on the embedded controllers we built for the Microdyne 700 &

Emitter followers?

John

The 32 KHz oscillator/PLL thing is really a pain on the Moto parts. It's a lot easier and much more reliable to just buy a packaged XX MHz oscillator and use it directly.

John

I agree, but the design team insisted that they needed the ability to reprogram the CPU's clock rate, "Just in case." Then to add insult to injury, they mounted the watch type tuning fork crystal vertically, and soldered the bottom edge of the crystal's case to a large pad rather than lay it down and use a drop of RTV to shock mount it. I saw a lot of dead boards with dead crystals. There were so many that I kept 10 spare crystals on my bench. They mounted the less sensitive baud rate crystal the right way, and soldered the top of the can to a pad. All of this was done a year before I was hired so I was stuck with getting the things out of production, and into customer's hands.

Hello John,

Yes, generally. But you need a negative supply and ideally a +8V or higher as well (really, really good bypasses or it'll oscillate). This is usually the case for the systems I work on but the analog folks are often suspicious about me tapping into "their" supplies for lowly digital purposes. Once I tell them that I am an analog guy myself that relieves them from their worries. Now, about that big honking stepper motor...

Regards, Joerg

We once soldered tiny wires to the vias on the *top* of a BGA package to get at some signals we'd thought we wouldn't need to get to on the prototype... that was fun :-)

Try XAPP609 and Answer Record 17697 for a description of local clock placement and routing restrictions (although I think XAPP609 only describes V2, the structure is similar but not identical in S3).

Brian

Thanks! I'll give it a thourough reading.