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barkerben
Joined: 19 Jan 2006
Posts: 22
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| ICSP 3.3v / 5v possible workaround... |
 Posted: Wed Jun 07, 2006 5:12 pm |
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The following occured to me:
ICSP is limmited at 3.3v
I need to run at 3.3v
If I connect both my Vdd pins on my PIC together, then wire them to a diode with its cathode facing the pins, and the anode wired up to the main 3.3v supply rail for the board, then under normal operation everything works fine asuming a small voltage drop across the diode.
If I attach the Vcc from the ICSP header to the cathde side of the didoe, then the programmer can drive the PIC at 5v without damaging the rest of the crcuit
This depends on all the other pins on the PIC going to 0v, not 5v (which would be bad for the 3.3v devices they connect to), when Vpp is detected - which I /think/ they do...
Does the above sound correct?
Ben |
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kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
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| Re: ICSP 3.3v / 5v possible workaround... |
| Posted: Wed Jun 07, 2006 7:38 pm |
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| barkerben wrote: | | asuming a small voltage drop across the diode. |
You can find a Schottky diode with a forward drop of about 0.4V, which is better than 0.7V for the silicon diodes.
| barkerben wrote: | | This depends on all the other pins on the PIC going to 0v, not 5v (which would be bad for the 3.3v devices they connect to), when Vpp is detected - which I /think/ they do... |
When the PIC is powered, the pins are in the high-Z state, and they don't source any current. |
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