How to prevent a PIC from running when Vcc not connected

Skirmitt · Joined: 19 May 2009 Posts: 60

I noticed on a project that my PIC started running when I applied a signal on an input when Vcc was not connected. I suppose that there is a small current that is leaking inside the PIC that starts the PIC.

How can I prevent this in a future design ?

Ttelmah · Joined: 11 Mar 2010 Posts: 19537

Current limiting resistors in the lines connected to the PIC....
The input pins of the PIC have (effectively) diodes to the supply rail. These provide the 'protection', clamping the signals from going more than a fraction of a volt above the rail. If one or more signals remain powered, when the PIC has no power, these diodes will tend to raise the supply line, and power the chip.
It is poor design to have other circuitry powered attached to parts that are not. Current limiting resistors are the most 'basic' defence if this happens, but is is better to ensure that the attached circuitry has it's outputs disabled, or is actually turned off.

Best Wishes

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Also see page 9 of Microchip AN607 on Power-up Trouble Shooting:
http://ww1.microchip.com/downloads/en/AppNotes/00607b.pdf

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

I often use a MAX809 or MAX810 chip to control the PIC Reset line. Among other things it prevents the PIC from running until the VCC is at the correct level. Stray current from an input will likely be at a lower voltage and the MAX chips will hold the PIC in Reset.
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Skirmitt · Joined: 19 May 2009 Posts: 60

Current limiting resistors would be the easiest to implement. What value are we talking about ? 10K or 100K ?