Reproducing old machines with newer technology (Re: PDP-12 at the RICM)

tony duell ard at
Tue Jul 14 14:27:55 CDT 2015

> That sounds like a bug in the original.  If you have a set of flops clocked by some signal, and it matters that the 
> outputs don’t all change at the same time, then the original wasn’t reliable either.

It is very poor design, and not something that I would do, but it certainly was done in production machines. 
With care you can determine the width of the glitch, and if it's small enough, ignore it. 

But there is a related problem with FPGAs. You learn in introductory digital electronic courses that there are
2 types of counter. The Asynchronous, or ripple, counter where each flip-flop toggles the next when it goes
from 1 to 0. Obviously those do not all change at once, so if you combine them with gates there can be 
quite large glitches. Then there is the synchronous counter where all flip-flops are clocked together. Now to a
good approximation (all the flip-flops have the same delay from clock to output), they do all change together.
So if you now combine the outputs (say you AND some of the Q and Q/ outputs to decode a particular state)
the glitches will be small. That's what is taught. That is what works with TTL, ECL, etc.

Now try it in an FPGA (at least the Xilinx ones I've used). You will find glitches all over the place. The reason
is that the 'wires' linking the outputs of the flip-flops to the gates are not wires at all. They are paths on the
chip through logic multiplexers, buffers, etc that are set when the chip is configured. And they introduce
delays. Delays that are enough to cause glitches that are wide enough to trigger other flip-flops.

My experience of FPGAs is that if you design a circuit for an FPGA it will work. If you take an existing design
feed it into a schematic capture program and compile it for an FPGA then it won't.



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