Re: gEDA-user: Prototyping with SMTs

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Fri, Mar 10, 2006 at 11:33:55AM -0800, Samuel A. Falvo II wrote:
> I'm old-school.  Whenever and whereever possible, I prefer to
> prototype what I'm working on incrementally.  For example, my Kestrel
> 1 computer is a clear demonstration of this.  It's built on one of
> those cheesy Radio Shack breadboards, runs at 4MHz (although the
> oscillator used in the pictures on the website is 1MHz), and took me
> about two weeks to build.

But building on breadboard is slow and error prone.

PCB guards you against mistakes plus soldering is real fast, at least
with my 125W transformer gun. While the fab house is spinning it's
cogwheels, you can do something else.

As time is money, this may even be cheaper.

CL<
> 
> I would place the board's power first, then test.  Once that test
> works, then I'd place the CPU, along with a NOP generator so that it
> has some (bogus) instructions to fetch.  Test.  Once that passed, I'd
> place the address decoder -- test.  Then the RAM itself -- test.  Etc.
>  I simply would not advance to the next stage without fully testing
> the first stage.
> 
> The problem with SMTs is that they're very poor at allowing someone to
> incrementally build with them.  Wire-wrap sockets for SMT chips are
> expensive if you can find them, and they are usually quite limited in
> the number of pins they support.  One could perhaps find commercial
> SMT-to-DIP or to PLCC conversion modules, thus allowing for easier
> wire-wrapping, but you're still pretty much limited to 68 pins.  Good
> luck trying to do this with a 144-pin FPGA.
> 
> You could always make your own PCBs that break out all the pins of a
> chip onto a bunch of 40-pin headers, I suppose.  However, even this
> has the problem of being very expensive when you think about it, as
> PCB runs of these kinds of boards is typically done in very small
> quantity.
> 
> What are your approaches to dealing with this problem?  Thanks.
> 
> --
> Samuel A. Falvo II