22 Apr
2011
22 Apr
'11
11:35 a.m.
On 21/04/11 22:02, Tony Duell wrote:
Go look up "ChipScope" and "SignalTap II".
Then tell me you can't spy on the internal logic of an FPGA.
I never said you couldn't monitor internal signals, I said you can't
connectr a 'scope or logic analyser to interesting points in the
circuitry.
And from waht you say below, these tools are none-too-reliable. The last
thing I need when I am debuggign something it so be unable to trust my
instruments. Or as I found when using one version of the Xylinx tools
many years ago 'Is that glitch a real one, or is it something the
simulator has come up with...'
Drop the
design into an FPGA and you go from several huge boards full of
TTL to one Eurocard, an FPGA, and a couple of RAM chips.
With nowhere to connect the 'scope and logic analyser to debug it, no
interesting signals to look at... Might as well jsut buy a modern PC.
If you really want to use a standalone logic analyser (I often do
because it tends to be more reliable) then there's always the option of
pulling a couple of pins out onto solder pads. The 1.27mm TESTPADs in
Ah yes.... But be very careful doing this.
The 'wires' in an FPGA are not direcrt connecitons, they go through
various routing swithces, which introduce delays. If you add connections
to monitor internal signals, it's very likely other signals will be
rerouted and will havw their delays changed. If your design is dodgy to
start with, you may find glitches come and go when you do this.
And of course you can only modify the design to bring out signals if
you have the origianl design files (VHDL, schematics, whatever). As I
said, i know you provide such things, but they can get lost. Whereas I
have, on many coccasions, debugged large boards of SSI/MSI parts with no
docuemtnatiuon at all.
the EAGLE base libraries work quite well IME. Big
enough to be easy to
solder to, small enough that they don't take up much board space.
The DiscFerret ICSP header is as big as it is for one reason only:
I still don;t get this desire to make things as small as possible. What
is the great advantage?
-tony