Dose your LCD rolls?

[surajbarkale]

Okey i has one 89C52 based project which measures displacement by using an incremental rotary encoder & displays it on 16X2 Alphanumeric LCD. I purchased a 230V -> 5V SMPS to power this thing. And guess what the LCD started rolling :o I replaced the SMPS by trusted X'mer & 7805 & everything was okey. But as soon as i used SMPS the LCD starts rolling. I put a 100uF filtering capacitor near the power connector & 0.1uf ceramic capacitors on all IC's but no avail. Can anybody explain this ???

[surajbarkale]

Seems that i am left on my own :( well i wil try to get an oscilloscope on scene & post any updates

[MP]

suraj, have you tried 0.01 uf? This is a high frequency ripple problem. The larger the cap, the lower the frequency.
Another thing to check is did you ground the unused pins of the display? Also try adding a small cap across the 5 volt buss on the display as close to the chips on it as possible.
How often does your display update? A fast refresh seems to add to the problem in some cases.

MP

[surajbarkale]

Thankx MP,

Display refresh is 1 sec (i have used an ISR to update display). As i am using all14 pins of display there is no question of grounding them. but i shall check out adding 0.01uF

[trigger]

Try to add a resister (say 10ohm carbon film; 1/4W should be ok except you need to drive high current components)/choke coil (10uH I think is ok) between the supply and the unit, plus a ceramic capacitor to ground.

This may help a little bit if the ripple of the supply is high.