TTL computing

From: Richard Erlacher <edick_at_idcomm.com>
Date: Wed Apr 10 08:20:48 2002

see below, plz.

Dick

----- Original Message -----
From: "Ben Franchuk" <bfranchuk_at_jetnet.ab.ca>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, April 09, 2002 11:23 PM
Subject: Re: TTL computing


>
> > I've seen/heard of very few FPGA's getting much past 30% utilization,
based on
> > the gate counts proffered by the marketing droids. Remember that they
count a
> > 3-input gate as 3 2-input gates and a flipflop as 14 rather than 6 gates.
> > Therefore, if you use a 32-bit LUT, the marketing people take credit for
32 *
> > 14 gates, plus a huge multiplexing tree counted as though it were built
from a
> > sea of 2 input gates. Just the same, regardless of how you count 'em, the
> > utilization under most circumstances remains well under half the available
> > resources.
>
> Well regardless of how you count them, the chip has no free macro cells
> left, and routing is very full with only a few free I/O pins.
>
AFAIK, the devices the vendors characterize as having "macrocells" are not
FPGA's, but, rather, are CPLD's. CPLD's are quite easy to utilize more fully
than FPGA's. If the devices you're using are CPLD's, e.g. 3000, 5000, 7000 or
9000-series Altera devices, you can, in fact use every macrocell.

FPGA's claim to be quite a bit richer in registers, having as many as 16
registers per contact, but, as I said, it's unusual to be able to use more
than 1 in 3 of those registers before clock distribution or other routing
resources become scarce.
>
> > Some of the configurations that were easily produced using the old Altera
> > APLUS+ software in a 24 macrocell Classic (EP910) can't be implemented in
a
> > Classic EP1810 (twice as large, twice as many macrocells) with the
> > current-generation software because some of the constructs (bidirectional
> > macrocells, latches, etc.) of which the old software was capable don't
exist
> > in the new software. It seem strange ...
>
> Lets not forget everything in BGA pin format too. That is the other
> reason why it so full, I can't move to a larger FPGA and still stay in a
> 84 pin PLCC package.
>
Well, the package isn't the issue where utilization of resources is concerned,
though it certainly determines whether it's convenient to use a given device.
At today's costs, the BGA devices are economical, but if even one of your run
of 10 boards fails because of a flawed device attachment, the economy is in
the toilet. So far, I still prefer the QFP's, since one can ensure their
attachment to the PCB. Unfortunately, they don't lend themselves to
prototyping, since, for example, the wirewrap adapters for a QFP-208 costs
quite a bit (10x) more than the largest CPLD or FPGA in that package.
>
> > If you get well above that level of utilization, you must be VERY good!
> Nope just luck and trial and error ... does feature X fit ... nope , how
> about Y ... then feature Z..
>
> > This particular application is an enhanced parallel port monitor, and the
> > two-port register set is perfect for the application. The pair receives
data
> > in bytes and coughs it up in nybbles for display. In fact, I have a
problem
> > with the range of SRAMs that are avaisable today, as they're too deep,
making
> > them slow, and too narrow, meaning one has to use too many. I need 8
words of
> > 256Kbits more often than I need 256K words of 8 bits. Since FPGA's are
just
> > SRAMs with feedback, it's obvious that shallow SRAMs of sufficient speed
(<250
> > ps Taac) can be fabricated, but they just don't find their way into
useable
> > packages.
>
> $%#! packages all 8 bits wide, how the **** do you find 4 bit wide
> memory for all the 12 bit cpu's out there.
>
12-bit CPU's out there ??? Everybody knows that 12-bitters haven't existed
since the '70's! After all, they stop existing on the day the last one is
shipped. The device manufacturers stop considering a market as viable once
the potential for 100K pieces per week per manufacturer is no longer there.
>
> > It has to do with layout. If you have 4 NAND gates in a package, costing
what
> > 4 gates would, you'd be tempted to share the gates among different
circuits.
> > If those circuits are more than 10mm apart, the routing uses up too much
real
> > estate, so a single gate is reasonable.
>
> Other than inverting a chip select, I really can't see why such glue is
> needed.
>
Well, you can't have it both ways. Either you respin the PCB or you respin
the programmable logic. If you respin the PCB, which takes less time and
costs WAY less money than the latter, you may need to add a NAND or an OR, or
whatever.
Received on Wed Apr 10 2002 - 08:20:48 BST