>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 19 July 2005 09:30
>To: piclist@mit.edu
>Subject: [PIC] 16F628 @ 3.3 volts (or a bridge?)
>
>
>Greetings,
>
>I have used the great piclist resources and archive to get 
>myself going on several pic projects, but this is my first 
>post to the mailing list. 
>Thanks to everyone for their contributions to this resource.
>
>I am trying to interface the Usart of a PIC16F628 to another 
>device that runs at 3.3 volts.
>
>One solutions seems to be to run the PIC at 3.3 volts. The 
>datasheet for the 16F628 gives the power requirements as 
>"flexible", between 2 and 5 volts.  The datasheet also has a 
>graph of speed vs voltage, suggesting that I should not try to 
>exceed 4 MHz at 3 volts.  I have always worked at 5v, so I 
>wrote a simple LED blinking program to test the pic at 3.3 
>volts.  I used the same oscillator as always (a 3.579545MHz 
>crystal with 2 22pF capacitors).  The program works great at 5 
>volts, but I get nothing at 3.3volts. Does anyone have 
>experience working with PICs under 5 volts?  Is there 
>something obvious I have missed?
>

You probably have the brown out reset enabled in the configuration fuses.  The 16F628 has only one voltage setting which is 4volts, so it cannot be enabled for operation under this voltage.  If you want to run at 3.3 volts then either use an external reset controler (Microchip sell them cheaply) or move to one of the newer 18F devices that have a user defineable brown out voltage.

Regards

Mike

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