Digital circuits and analog devices
paulkoning at comcast.net
Fri Dec 2 09:57:50 CST 2016
> On Dec 2, 2016, at 12:59 AM, Tony Duell <ard.p850ug1 at gmail.com> wrote:
> On Fri, Dec 2, 2016 at 2:31 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
>> So I have this memory of a set of law promulgated by an engineer at DEC, one
>> of which was something to the effect that 'all digital circuits are made out
>> of analog devices'. However, my memory doesn't recall where I saw this, and my
>> Google-fu is not strong enough to turn it up. Can anyone help?
> It's one of Don Vonada's laws. I am pretty sure I first read them in
> 'Computer Engineering', that book produced by DEC. I think the
> original is 'Digital circuits are made from analog parts'
> Another of the laws is 'There is no such thing as ground' which
> (a) means voltmeters have 2 leads, so you can take whatever
> you want as a reference
> (b) all connections have impedance (the inductance is the important
> bit in general) so even if you have 2 points connected to a wire you
> call 'ground' they won't necessarily be at the same voltage all the time.
There are probably lots of people who have spoken or written along these lines. I heard the "everything is analog" from my manager Tony Lauck, the head of the DECnet architecture and A-D group and a polymath (as were a number of others in that group). He specifically emphasized the point in connection with metastability issues. But since the statement is both true and all too often forgotten, I assume it has been said many times before.
On the bit about grounds, I still have tucked away somewhere a very funny (but also serious) memory by Bob Steward (lead designer of a number of VAXen) entitled "Do not cut your grounds". He described how a poorly chosen PCB layout turned a ground plane into a slot antenna, among other things.
These are the sort of issues that supercomputer designers at CDC or Cray all understood thoroughly -- you can't build machines like that without this knowledge. But a lot of logic designers working at slower speeds weren't taught properly and/or didn't really get it, and they might not get in trouble right away.
A somewhat related area is the many different ways in which a number of chip designers messed up the random number generators needed in half duplex Ethernet. I remember Intel as a particular offender, but probably not the only one. AMD got it right in the Lance partly because of a lot of coaching from the DEC Ethernet designers.
More information about the cctalk