8 Dec
2007
8 Dec
'07
10:41 a.m.
On Sat, Dec 08, 2007 at 10:33:23AM -0800, Bob Armstrong wrote:
Right.
Ethan Dicks
[ethan.dicks at usap.gov] wrote:
Thanks for any nudges towards a solution.
Implementing the combinatorial logic for a seven segment decoder is
trivial even for the simplest GAL. But, as somebody else pointed out, the
latches in a simple GAL are permanently tied to output pins, so a 4 bit
counter plus a seven segment decoder would need 4+7=11 output pins. A 16V8
has only 8 outputs, and a 22V10 has only 10, which rules both of them out. Now if you only wanted it to latch the data rather
than count, that'd be
possible with a 16V8.
I could get away with an external counter easily enough, but I was
musing aloud about a way to cut the part-count in half.
Hmm... Having said all that, it might be possible
to put a state machine
into a 16V8 that had a 7 bit current state and a next state function that
corresponded to "counting" is seven segments. In theory that'd work, but
I
don't know off hand if there are enough terms inside a 16V8 to implement
that next state function. I'll leave that as an exercise for the reader :-)
Indeed. Someone else already made that suggestion, and I'm thinking
about it. The good news is that it's easy to prototype one up and
see how it works on a breadboard.
Another thing to keep in mind is that parts like
these are really power
hungry, even compared with traditional 74xx parts.
True. Power is not really an issue here. The LEDs are going to pull
a fair amount of power on their own, not including counting and driving.
Are you back down there again?? Did you see the NBC
news team that
visited a few weeks ago?
Yep... I arrived back here about 5 weeks ago. I did see the NBC crew
on their brief stop-over, including Ann Curry making a bit of fun of
our soft-serve ice-cream machine (they arrived around 01:00 and departed
around 03:00, and I happened to be up when they arrived).
-ethan
--
Ethan Dicks, A-333-S Current South Pole Weather at 8-Dec-2007 at 18:30 Z
South Pole Station
PSC 468 Box 400 Temp -25.2 F (-31.8 C) Windchill -53.8 F (-47.6 C)
APO AP 96598 Wind 15.5 kts Grid 12 Barometer 678.3 mb (10692 ft)
Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html