[SI-LIST] Power Distribution Design For Async Processor
- From: Rick Collins <gnuarm.2006@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 28 May 2011 17:42:01 -0400
Green Arrays is building chips of asynchronous processor arrays.
"Varying capacitor values create impedance peaks as well as nulls in
the frequency domain and should be avoided."
This is in the data sheet for the GA144 device. I am sure it is well
intentioned and I am sure the folks at GA know their chip, but I
don't think this advice is what I would hear from various board
design experts.
In a class I took some time ago with Lee Ritchey, he showed how using
multiple values of caps in the power distribution system (PDS) will
provide a lower impedance across the board... in spite of the
impedance "peaks" caused by the resonances of the cap and parasitic
inductance. These peaks are mitigated by the equivalent series
resistance of the caps. They still exist, but in a well design
system are lower than the impedance at that frequency if only one
value of cap is used. I don't recall Lee showing us an example where
he was trying to lower the PDS impedance at only specific frequencies
corresponding with the nulls.
The authors of the data sheet seem to feel designing a PDS with a low
impedance over a broad bandwidth is important for the GA chips more
so than in In a system where the noise frequencies appear to be
fixed. I doubt that anyone targets specific frequencies when they
design a PDS. After all, most systems use multiple frequencies and a
board designer isn't going to want to respin a board because someone
changed the frequency of the oscillator. Besides, the null
frequencies depend on the values of parasitics that aren't controlled.
Any comments on the likely validity of the GA recommendation to use a
single value of ceramic cap rather than multiple values?
One of the things from Lee Ritchey's course that has stuck with me is
when he talked about FAEs saying if you don't abide by the advice in
the app notes (which Lee was disagreeing with) that they wouldn't
guarantee the board would work. But if you do abide by the app note
advice, they should then guarantee that the board will work. I
suppose not... eh?
Rick
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