problems with edge triggering of ext interrupt0

Have you bypassed the supply properly(and close to the controller's Vcc pin)

??

i did not understand what u mean by bypassing power supply..all i have done is solder a wire from regulater to vcc pin and of course ground

place a cap 104 between VCC pin and GND remember to place the CAP closer to the VCC... :-)

Have you bypassed the supply properly(and close to the controller's Vcc pin)?

oops sorry for being so vague and shyam thanks for elaborating.

the board is hand soldered by me.. does anyone have experience with making their own boards? i have a feeling theres more to the dev-board than just the minimum circuitry of crystal and capacitors+ power on reset cktry..

Look here for a 50 buck 89XXX experiment board.......
http://amateurworld.in/pmd_31/Low%20Cost%2089C51%20Experiment%20Board%20PCB.html

zen bEaR, I am really not sure if i am correct. Bust just a thought came to my mind..

is it possible that the light is getting reflected somehow to sensor? and thats why a false signal is coming.. Please check the o/p of sensor on CRO. hope it help..

nope, sensor is fine.i have a piece of photograph negative covering the detector to ensure that visible light is not triggering the circuit. besides, 1 channel of cro is always connected to the int0 pin and the other to the output so i can see whats happening all the time.

this is what i have seen- when i connect the cro between vcc and gnd--lots of noise. the power supply should be a straight 5v dc but it is a very fuzzy and noisy line around 5v.

all the output pins suffer from the same problem..lots of noise.

when i remove the uC from the socket, the supply is clean ( ive used an 78L05 with 220 uf capacitors at input and output..the unregulated supply is actually a very clean 8v from an old motorolla cell phone travel charger capable of 800mA)

somehow, the crystal oscillator is causing all the noise..
question is, how do i suppress it?

try this.. connect a 100nF cap close to VCC pin of controller and see if that helps. well regarding oscillator i am not sure.. but sometimes the body of crystal is connected to ground and sometimes they have a local ground plane under crystal on PCBs.

ok. i'll try that. thanks

btw, do u know what a ground plane is with respect to a pcb?
also, hasn't anyone here tried to assemble their own dev board? it would be really nice to share their experience and see if they had the same problems i am facing :|

another curious thing. my atmel programer came with a small demo board.. even that thing is behaving in the same way. loads of noise between vcc and gnd

i think you have bought Atmel series programmer from SP road i bought the same programmer.. and even i got a demo board.. but i did not use it.. coz i didnt like it

i want you to try what i said i hope it will help.