Idea: Indestructible pin

Luis Digital

Hello all,

I have an idea or more well a question: Why use not a resistance in the entrance before the diodes of protection in the chips?

Something thus as 1 ohm.

Would function like damper and fuse. Thus it would not produce the unpleasant effect of diodes in short circuit, causing high consume and heat.

Or has more negative effects that positive?

Beau Schwabe

Luis Digital,
·
There already is a bit of resistance (several Ohms)·added to the input of an I/O pin (See attached image)
·
The "A" and "C" transistors would be turned on HIGH "A" or LOW "C" in the case of the pin being defined as an Output and then specifying a HIGH or LOW command.· Transistors "B" and "D" are configured as a reverse biased diode from the PAD to the Positive or Negative voltage rail and under normal circumstances these diodes will·not conduct.· In fact, the reverse biased diodes will act like·very small capacitors ... Notice that there is a secondary stage of the transistors also configured as a reverse biased diode after the resistor.· The reverse biased diode structures are·there as a first line of defense against an input voltage if it·exceeds one of the voltage rails in either direction... The resistor also helps to aid in this defense mechanism.·
·
"Thus it would not produce the unpleasant effect of diodes in short circuit, causing high consume and heat." - In this case, the reverse biased diodes are not the culprit unless you reverse the polarity to the Chip itself... I know very few chips that will survive that torture.· What happens during a short is that you damage transistor "A" or "C" because when they are turned "on" ,· they function as a forward biased diode allowing the rail voltage to be present at the PAD.



·

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 9/6/2008 4:54:56 AM GMT

Luis Digital

By mistake there was placed a pin in short circuit and had survived, but in strange conditions (perhaps the wire collided with something without noting it) to stopped working and now I have a hot chip. Apparently the resistance is stronger than the transistor and did not work as a fuse.

Thanks, has been very illustrative and detailed.