Re: gEDA-user: challenge board ALMOST works

[Werner Hoch <werner.ho@xxxxxx>]

On Monday 11 September 2006 05:29, DJ Delorie wrote:
> I've got the prototype all soldered up.  Took about six hours,
> including microscope inspection, mostly for the 01005's.

[...]

> 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.

[...]

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

Whenever the inverter switches, there's a high current peak trough the 
capacitor (C3) and the builtin ESD protective diodes of the inverter 
input. The high current can cause voltage drop or inductive 	
disturbance, both may cause the two oscillators to lock.
See the schematic for the current pathes. The Diodes are inside the 
schmitt-trigger IC.

You should place a resistor somewhere to limit that current. (Second 
schematic.)

When I'm using 7414 as oszillator I usually use the third circuit.
This only works with IC's that have schmitt-trigger inputs.

regards
Werner


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