Aaron Toney wrote: > > > This is what I'll be trying first... My primary concern is how to go > > > about buffering/protecting the inputs; touch + static = fried CMOS! > > Remember that the ESD problem comes about when a your body ground > is floating at a much higher potential then environmental earth ground, > usually several tens of kilovolts, and then you touch a output pin > connected to a chip which is grounded to something close to earth ground. > The shunting of the several kilovolt potential difference generates > enough current to fry a hole through the gate oxide of the chip. For a > device worn on the body if you make sure that it is grounded to the body > of its wearer then no potential difference will build up. ( Though this > can be a lot more uncomfortable then it sounds ). Uh, Guys... *Please*, say you're using a 1 Meg or so series resistor in that grounding the wearable to yourself, for safety's sake. You want to limit currents should you hook to 117 or 220 VAC, to a mere tingle if that, no more. > > Modern CMOS designs generally include clamping diodes at all I/O pads to protect > > against ESD. I've never zapped a PIC pin, for example. > > Not true. While the diode protection on the inputs of modern > devices have greatly reduced the damage done to a chip by ESD discharge > you are still damaging the chips. Usually this will be expressed > through the hot carrier effect, and electron punch through permanently > scaring the gate oxide. The I/O pins still work just less and less > efficiently. Much like a slow blow fuse they can take a lot of "hits" > before they finally stop working. Good to augment those diodes. Best bets: Use either a pair of small signal diodes (like 1N4148 or 1N914) "backwards" (cathode to Vcc on the top diode, anode to input pin, then the next's cathode to the input pin, anode to ground), or, a 5.1V Zener or 6.8V unidirectional Transient Voltage Supressor, with cathode to the input pin, anode to ground - then a small series resistor (say 1k or so?) to make things even safer, either will minimize the protection diode currents on the PIC pin inputs. > Aaron Toney, Circus Systems (206) 297-9015 Mark -- I re-ship for small US & overseas businesses, world-wide. (For private individuals at cost; ask.) -- Subcription/unsubscription/info requests: send e-mail with subject of "subscribe", "unsubscribe", or "info" toWear-Hard Mailing List Archive (searchable): http://wearables.blu.org
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