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Maximum voltage on input pins — Parallax Forums

Maximum voltage on input pins

Stanley FredericksonStanley Frederickson Posts: 19
edited 2005-03-31 10:37 in General Discussion
On one of my recent SX projects I mistakenly shorted my battery (6V) directly to one of the pins that I had configured to be an input when·running the SX from a regulated 5V supply.· From reading the datasheet this appears to be explained by the fact that the maximum input pin voltage is Vdd + 0.6V.

It took my by surprise since I have been using the SX chips in projects for many years and done plenty of stupid things with them and never fried one.· I guess I was due for a problem.· I don't blame the SX for the fried·pin (I did a stupid and paid the price.)·· I am just curious as to what leads to this Vdd + 0.6V limitation.· Does it reverse bias something which ends up causing a short?

Comments

  • BeanBean Posts: 8,129
    edited 2005-03-30 13:55
    The I/O pins have protection diodes to Vdd and GND, that's why the 0.6V. As long as the current is limited the diode will protect the pin. But if you connect a supply directly to the pin with no current limiting resistor, POOF.
    Bean.

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  • allanlane5allanlane5 Posts: 3,815
    edited 2005-03-30 19:26
    This is why the 220 ohm serial resistors on each I/O pin are recommended. If you'd connected 6 volts to a 220 ohm resistor whose other end was connected to the pin, the pin would not have been damaged.

    This is also how an I/O pin with a 22 Kohm resistor can serve as an RS-232 reciever. The 22 Kohm resistor limits the current, so the protection diodes can clip the +- 12 volt signal to something the SX reads as 0v or +5v.
  • Stanley FredericksonStanley Frederickson Posts: 19
    edited 2005-03-31 10:37
    Ahh, that makes sense.

    Thanks
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