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MikeValencia
Joined: 04 Aug 2004
Posts: 238
Location: Chicago
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| MCLR/VPP/RE3 pin termination... |
 Posted: Tue Aug 29, 2006 12:10 am |
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I am currently using a PIC18F458 whose VPP pin was designated as "VPP/MCLR" and could NOT be used as an i/o pin. Then came the next generation of PICs: PIC18F4580, where the pin can now be designated as an general purpose I/O so long as you configured it as such in the #fuse settings.
So the old way of doing things was to tie the VPP/MCLR line to +5V so the PIC wouldn't reset, and also connect this line to pin 1 of the ICD.
With these newer PICs, can I keep this pin as an unused dangling output, which i'll shoot a '1' out of it? What happens then when an ICD attempts to re-program it? Won't I exceed currents and fry something? Or is there some mechanism in the PIC that safely recognizes a high voltage on that pin as a programming voltage, and i won't fry it? |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Tue Aug 29, 2006 12:56 am |
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To solve this problem on a 16F628 design (in which the NOMCLR fuse
was used), I kept the MCLR pin as an input, and put an external 47K
pull-down resistor on it. There's no problem with the initial programming
or with re-programming. An ICD2 is used for ICSP programming.
This is a +5v design, running on an external 4 MHz crystal. |
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MikeValencia
Joined: 04 Aug 2004
Posts: 238
Location: Chicago
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| Posted: Tue Aug 29, 2006 8:50 am |
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Thanks PCM.
Looks like the only time that i can configure RE3/VPP as an output is when I am NOT using ICSP!
I managed to dig up an old email from an Arrow FAE on some VPP issues:
| Quote: | Mike,
If operating at 5 volts, a 1K pull up and 4.7 K pull down allows the pin to swing from 0 to 4.1 volts which works regardless of the design note says.
If you would prefer to use a pull down, I now not use less than 1K since it would require a fairly high source current to support a logic high. A 4.7K to 10K pull down would be preferable.
Remember, the Vpp - MCLR pin will have a high voltage (greater than 10 volts) applied when programming. Take care that it is not connected to anything else on your board that can not take the 10+ volts.
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