gEDA-user: challenge board ALMOST works

[DJ Delorie <dj@xxxxxxxxxxx>]

I've got the prototype all soldered up.  Took about six hours,
including microscope inspection, mostly for the 01005's.

The design "works".  I say "works" because the oscillators oscillate
(about 400Hz), the xor gates xor, the led drives work just fine,
although I could have used larger resistors on the leds - the
schematic says 220 ohms, but for some reason I bought 150 ohms.  I
think I was thinking "green led" which has a higher voltage drop.
Still within spec even so.

See:
http://www.delorie.com/pcb/smd-challenge/smd-challenge.sch
http://www.delorie.com/pcb/smd-challenge/smd-challenge.pcb
http://www.delorie.com/pcb/smd-challenge/smd-challenge-sch.png
http://www.delorie.com/pcb/smd-challenge/smd-challenge.png

However, the two oscillators tend to self-synchronize.  If I let it
run, one LED is on and the other is off.  If I put scope probes on the
buffered outputs (pins 6 and 8), I can see that the oscillators are
running, they're just in lock-step.  Not even out of phase.

If I put a scope probe on either feedback line (pins 1 or 13), they
drop out of sync and the two LEDs pulse back and forth (about 17Hz).

Also, if I put probes on both xor outputs, they drop out of sync, too.
Sometimes they stay unlocked if I remove the "on led" led's probe but
leave the "off led" probe there.

Ideas?  The only thing I can think of is inductive or capacitive
coupling, but at 400 Hz?

Oh, runs just fine without the XOR chip, but I tried two different xor
chips, both do the same thing.

DJ


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user