FPGAs and PDP-11's - test-drb@ccmp.vtda.org

30 Aug 1999

It's certainly possible to buy adapters for some of the larger qfp packages,
and making one is a REAL pain in the *ss.  It's bad enough that these
adapters, by and large, don't have a solder mask, so you have to wear a lupe
and check all your solder joints twice .  .  .

Making a device which allows you to "play" with an FPGA is a really useful
trick.  You can bring out whicever signals you think you need, but you need
to be aware of the timing differences between the internal signal and the
one you're allowed to "see" from outside.  I/O blocks generate delay and
are
probably best registered.  Likewise, if you use an external signal to
stimulate some internal mechanism, it is wise to bring it in through a pair
of synchronous registers in order to avoid metastability due to setup or
hold time violations.

In times pretty much gone by, one ran the risk of having the FPGA pinout
change due to rerouting.  Todays tools generally support pin-locking, i.e.
firmly binding the signal to a predefined pin, shuffling other resources
instead of reassigning pins when rerouting a circuit.  This can, however,
impact timing.  READ THE FINE PRINT!

Dick
-----Original Message-----
From: Tony Duell &lt;ard(a)p850ug1.demon.co.uk&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Monday, August 30, 1999 5:56 PM
Subject: Re: FPGAs and PDP-11's

...
 >
> There are packaging options which might be utilized to help you in
"scoping"
...
 > out your circuit.  One of many complaints I have
about FPGA's is that they
...
 > have far too many pins to suit me.  Well, since
there are lots of pins, you
...
 > can obtain a socketable adapter for the package
and then work your way
> through the design, changing the "bond-out" by routing the signals you
want
...
 > to compare to I/O blocks associated with otherwise
unused pins.  That way
> you might be able to help yourself figure out strange timing effects between
...
   internal and
external signals, perhaps even allowing you to see the inner
 workings at least to some extent. 
I've done this many times (in fact, IMHO it's the _only_ way to see what
an FPGA circuit is really doing), but a word of warning :

When you add the extra outputs, you recompile the circuit and probably
change the routing of some signals. Now, these signals will therefore
suffer different routing delays which means that glitches may move about,
appear 'from nowhere' or vanish.

Yes, I know that a well-designed FPGA circuit won't have problems with
routing delays. But if you're new to FPGA design, especially if you've
done a lot with TTL, you won't expect your wires to give sigificant
(longer than switching time of a gate) delays.

>
> The handiest package I've run into for these is the PLCC84, for which you
> can obtain a socket compatible with a wire-wrap adapter.  That would allow
...
 > you to do what I described above without
interfering with the 40-odd signals
...
   you might want
to inject into your existing application.  This same thing
 might be achievable with a larger package, but I have some doubts about
 making the transition from the FPGA package to a wire-wrap socket.  You
 might have to make an adapter PCB or buy one from Samtech or Emulation
 Technology.  Those get expensive. 

You can get SIL strips of wire-wrap socket pins that you can cut to
length. If you solder/glue these to a suitable piece of matrix board you
can make up a socket to hold just about any PGA chip, or indeed a
solder-type PLCC socket.

Most FPGAs (certainly from Xilinx) are available in PGA packages -- at a
price.

BGA and PQFP are a little harder to handle, and you may well have to make
up an adapter board.

-tony