(V)HDL Toolsets

Paul Koning paulkoning at comcast.net
Thu May 21 15:20:46 CDT 2020

> On May 21, 2020, at 3:56 PM, Jay Jaeger <cube1 at charter.net> wrote:
> On 5/21/2020 11:55 AM, Paul Koning wrote:
>> ...
>> This sort of question is why I found starting with the simulator is helpful.  In a simulation you can specify delays directly.  So for my 6600, I have the gate delay (5 ns) and the wire delays (1.3 ns per foot, in the twisted pair, or 25 ns for coax cables including tx/rx circuit).  Actually, I only include wire delays for "long" wires; the design clearly uses wires longer than needed in various places for delay reasons, but my guess is that short wires are not time sensitive.  That may be wrong; I need to run it again without that assumption to see if it helps.
> I do indeed plan on starting with simulation - just not for that reason.
> Its just easier to debug then the FPGA proper.  ;)
> I suppose I could figure out the actual wire list, and thus wire
> lengths, but it would be have to be limited only to inter-panel wires,
> and even that much would be painful and very time consuming.  But yeah,
> it makes sense to model gate delays in a general way and then perhaps
> lower them to see what happens at higher speeds, as you suggest below.

As I said, for most machines it is not likely to be useful to model wire lengths.  For the 6600, it is mandatory, and obvious from the design files: when you see a 96 inch wire connecting two modules one inch apart, you know there is a reason why that wasn't a 4 inch wire instead.  Not to mention when "adjust to get x ns pulse" shows up in an annotation.

One good example of this is the master clock oscillator.  In most 6600s it's a 10 MHz crystal clock, but in the first 7 units built, it's a 4 stage ring oscillator, with 96 inch wires to produce 25 ns delay between the four main phases.  The later version gets rid of the ring oscillator, but retains the four buffers with wire delays, as n * 25 ns phase shifters.

If I were working on, say, a PDP-11, I wouldn't expect to have to deal with any of this sort of craziness.  But a 6600 is at, if not over, the hairy edge of possible speed for when it was built.  Even the peripheral processors are well optimized, so they can run many of their instructions in 1 microsecond -- which is the memory cycle time.  Fetching and executing a new opcode every cycle is a pretty hard task.  The PPUs are actually pipelined, though the descriptions you'll read about them don't make this clear at all.

Come to think of it, another nice optimization example is the process context switch, which in the 6000 series is a single instruction -- 16 read/modify/write core memory cycles 100 ns apart, so the whole thing including pipeline drain and restart takes 3 or so microseconds.  Watching that in simulation is fun.


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