Filtering supply lines

[Brian]

>I still have one problem with this circuitry: All works fine,
>but when I press the brake pedal, a few "trash" pixels appear on the
>LCD display I have connected to the CPU. The CPU does not crash,
>no problem, and if I clear the display, the trash pixels go away.

>I dont know if the LCD is particularly sensitive to the spikes
>introduced by the brake pedal switch (or the cruise control vacuum
>solenoid?), or if the serial line that feeds the display has
>noise on it (I do not have a scope to connect to my car... :-( )
>It would also be possible that the noise is inducted in the (unshielded)
>line to the LCD.

>What should I do?
Try grounding the data line to the LCD so it'd just display a blank screen.  
Then press the brake pedal and see if you get any interference.  If not, the 
noise isn't coming in thru your serial line.  Then try connecting your LCD to a 
battery rather than the car's power supply.  Then try the same thing.  If you 
don't get the interference anymore than your problem is in your supply.  Your 
problem with that probably isn't the positive side of the supply.  I'd check 
your ground.  Being a veteran with car audio I can tell you the horror stories 
about automotive grounds.  They don't always stay at 0v, EPECIALLY when when 
brake, turn, head lights are on and things like windshield wipers really mess 
up grounds too.  You might need to ground your LCD right back to the battery.