AVR-Chat

At 01:18 PM 6/25/2006, you wrote:
>    Have you tried to put a 100 nF Capacitor to ground or a 10K resistor to
>VCC and made the same test ? With the internal pullup the cap may be enough
>to cope with external noise...

The original PCB had space for the cap and resistor, I'd just left them out 
to see if they could be omitted from future designs.
  Well I had a little time this afternoon, so I opened up the project and 
added a 10k / 2nf cap.

  Now keep in mind the design has the isp connector physically located 
right next to a FET switcher doing a 12v to 200 volt conversion. I'd almost 
expect connecting the reset pin to the tab of the fet via finger 
resistance/ body capacitance to overwhelm the internal 60k pull up resistor.

   Not a problem for me, because after the case is closed I don't expect 
any kerfinger pokin anyway.

But note, when I did add the parts mentioned above, the sensitivity to 
finger touch did in fact go away. I did not try the experiment with only 
the cap, It's a bit of a pain to dismount the pcb more then once in order 
to solder in the parts.

  It's really just a noise issue. In a low noise environment I'd think the 
built is resistor is fine, especially if you don't have a long trace 
connected to the pin acting as an antenna to couple in noise. I do wish the 
reset pin was closer to the other isp pins. Bit of extra effort routing 
that one signal.

  In a really high noise situation, I'd still turn off the reset fine in 
the fuse settings and use a boot loader to get code into the chip.

Without anything definitive from the data sheets, I guess this is just more 
grist for the mill.

Regards,



carl

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Henry Carl Ott   N2RVQ
http://users.rcn.com/carlott/
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