Re: [SI-LIST] : How to identify SSO

Lfresearch (
Sun, 11 Jan 1998 11:38:47 EST

I've been following this ( see bottom of message ) discussion with a lot of
interest. I have faced an ASIC problem recently ( hope to publish what I've
found very soon ) which is directly related to the issue of noise on NON
switching signals.

The subject is low cost controllers, an ASIC was a great way to combine
discrete devices to recoup board space etc etc from earlier versions. However,
the ASIC is used not only to control many lines of I/O leaving the controller,
but signals between the uP, its memory, and an onboard A to D convertor.
Because all signals use the ASIC, the I/O lines have remnents of the really
fast switching on them.

I can't afford to filter each line, the whole product DMC is less than $30! I
have looked at several vendors parts and discovered there is a big difference
between them from this aspect. Being an EMC guy rather than a Silicon guy I
don't fully appreciate why. I'm nervous though that if my current good
supplier goes, my whole controller range will fail EMC tests.

I guess this posting was to make sure that folks designing chips know that the
EMC chaps are very interested in things like "crosstalk" between lines being
switched. Even if it's not considdered of primary importance to the SI

Best regards,

Derek N. Walton
Owner L F Research EMC Design and Test Facility

In a message dated 98-01-08 10:26:35 EST, writes:

<< Andrew Ingraham wrote:
> > One thing that has always concerned me with SSO is the affect of the VCC
> > drop on non-switching signals. Theses signal would have noise on them
> > that would mimic the VCC noise on chip (since their outputs are
> > effectively shorted to VCC).
> Indeed. Even worse, since noise margin is usually smaller in the
> low state than the high state, is the GND bounce killing your noise
> margin on those outputs that are supposed to be LOW.
> You may need to assume that ALL outputs go indeterminate on every
> clock cycle, whether they switch or not. Clock or clock-like
> outputs need special treatment to keep them usable as clocks.

Absolutely. Synchronous systems specifically tolerate noise on
non-switching signals except during sample windows, which moots
the noise issue. Clocks, IMNSHO, should be differential signals
anyway for a whole slew of reasons, this not least.