Filtering supply lines

[Lenny Baledge]

>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?), 

could be the braklights drawing excess current from your system.

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?


Isolate your supply. IE. Run your cpu off of another battery (9v) for a test
and see if the problem still exists. If the problem is still there, start
checking your serial cable.


>a) better filtering of the power supply: how (please comment on circuit)
>b) better filtering at the LCD supply line (use an L in series with Vcc)
>c) use of a shielded cable for the display (need to tear apart instrument
panel again)

Most definately. Unshielded cable + any appreciable baud rate + 5' or
greater run + car environment = noise on lines.

>d) use a switching power supply instead (circuit board redesign, more
expensive)

 I think a good linear with adequate caps should work, especially if you are
only needing 100ma. But I do not have a lot of experience in automotive
applications.

>   If so, which one should I use? I have seen the recommendations for
>   the National Semiconductor units, but they are for currents in excess of
>   1A, right? For say 200mA, which one would you suggest?
>   Price and availability are issues here...
>
>Any comments would be greatly appreciated!
>
>Thanks,
>
>-Oliver
>
>