Post by thread:[PIC] noise and interference problems on a 18F252

Hi guys, i've been working with pics for several years and always heard that one of the weakness of pics is that it is a uC that get many noise and interference. Well, i'm living this problem now. I have a PIC18F252 based circuit that uses CCP to count pulses from a sensor. It all works fine, but: If someonse uses one of those devices made of piezzoeletrical crystal to fire kitchen ovens, the pic starts counting pulses from nowhere! I can even disconnect the sensor, or add as many filters as i want, but the pic still counts pulses from nowhere. Any help, oppinion, hint will be greatly accepted. thanks for your attention Rafael.

> can even disconnect the sensor, or add as many filters as i > want, but the pic still counts pulses from nowhere. Hi Rafael, have you tried putting the PIC in a grounded metal box and using grounded shielded wire to the sensor and PSU ? I have a product that counts pulses from a sensor and sits next to a 5W radio and petrol engine. Without proper shielding it goes crazy. Can you also decrease the impedance of the sensor signal so it's not so prone to interference ? Perhaps drive the line with an opamp or transistor, add a couple of ferrite beads and bypass caps (eg small ceramics)

Hi Jinx, thanks for answering. I'll quote your message: Jinx wrote: >Hi Rafael, have you tried putting the PIC in a grounded metal >box and using grounded shielded wire to the sensor and PSU ? > > Yes :-( Used an old ATX power supply metal box, and grounded the sensor, my circuit and the power source to the box. I got less noise, but still got it. >I have a product that counts pulses from a sensor and sits next >to a 5W radio and petrol engine. Without proper shielding it >goes crazy. Can you also decrease the impedance of the sensor >signal so it's not so prone to interference ? > I can try that... >Perhaps drive the >line with an opamp or transistor, add a couple of ferrite beads >and bypass caps (eg small ceramics) > > I also tried some of those ideas. Then i'll tell how wired my problem is: If i disconnect ALL the wires and sensors from the board, and let only the power supply and serial connection, the problem persists. Yes, if i "click" the piezzoeletrical ligth firer just near the metal box, the pic counts pulses! Without any wire connected! Could the problem be my board design? Or is the pic too sensible? what options more i have? again, thanks for your attention []s Rafael.

I know a couple of people on the list have mentioned having their products tested for EMI interference and what they did to stop it, hopefully they'll be able to give you some advice > Used an old ATX power supply metal box, Does this box have any holes in it ? If it's like every other PC supply I'm guessing it does. The noise will find its way in through those holes. Try blocking all the holes with aluminium foil, see if that makes a difference. Ideally you want an unbroken metal barrier all around the circuit, which includes the wires http://en.wikipedia.org/wiki/Faraday_cage > and grounded the sensor, my circuit and the power source to > the box. I got less noise, but still got it Were the connections with shielded (eg good co-ax) wire ? > If i disconnect ALL the wires and sensors from the board, > and let only the power supply and serial connection, the problem > persists. Yes, if i "click" the piezzoeletrical ligth firer just near the > metal box, the pic counts pulses! Without any wire connected! Sounds like the sensor wire either isn't the problem or isn't the only one. Have you looked at all the PIC pins with a 'scope to see where the noise is ? Could be difficult to measure without having a roll of kitchen cooking foil handy to seal any gaps you make to get a probe in > what options more i have? Do you use a crystal and have its case grounded ? In the product of mine I mentioned, as unlikely as it sounds, the software doesn't work properly without the crystal case grounded Are you sure the PIC is counting pulses or is it resetting ? Or even both ? MCLR can be sensitive and it doesn't take much noise to cause problems. I'd focus on keeping noise off Vcc, Vss and MCLR

Rafael Vidal Aroca wrote: >> Can you also decrease the impedance of the sensor signal so it's not so >> prone to interference ? > If i disconnect ALL the wires and sensors from the board, and let only > the power supply and serial connection, the problem persists. Check your design for any high impedance points, and give all that don't have to be high impedance a low impedance path. Ideally, there shouldn't be any; everything should have a low impedance. Of course, that is not always possible (for example you may have to read a high impedance sensor), but then you need to look closely at the remaining high impedance locations -- and by now you know exactly where they are. For test purposes, you can try to make all remaining high impedance points low impedance (for example, short out inputs, or use a low value resistor to ground or supply voltage). If the problem is gone, you can revert one high impedance point after the other to high impedance and try to find where the problem comes from. To make detecting reset problems easier, you can pulse a pin right after reset, and only then. This allows you to see whether there are unexpected resets (if you can't see them easily from your firmware's other behavior). Gerhard

At 15.34 2005.10.01 +1200, you wrote: >I know a couple of people on the list have mentioned having >their products tested for EMI interference and what they did >to stop it, hopefully they'll be able to give you some advice > >> Used an old ATX power supply metal box, > >Does this box have any holes in it ? If it's like every other >PC supply I'm guessing it does. The noise will find its way >in through those holes. Try blocking all the holes with >aluminium foil, see if that makes a difference. Ideally you >want an unbroken metal barrier all around the circuit, which >includes the wires Only signals with wavelength smaller than the holes will go through, like a low-pass filter. Thus many small holes are better than one big hole (considering equal surface). That's why it's typical to see many small holes for venting than fewer bigger ones, expecially where high frequences can be a problem.

Electron wrote: > Only signals with wavelength smaller than the holes will go > through, like a low-pass filter. No, selecting for small wavelengths is a high pass filter. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com

> Electron wrote: >> Only signals with wavelength smaller than the holes will go >> through, like a low-pass filter. > > No, selecting for small wavelengths is a high pass filter. But unfortunately certain hole patterns seem to function as slot antennas or even a lens (Luneberg) as I have just discovered (on my cicrowave). And that is true even if there is no obvious relationship between the hole size and pattern, and the wavelength. Peter