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USART is not working. Please Help!!

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PCM programmer

Joined: 06 Sep 2003
Posts: 21708

Posted: Sun Apr 09, 2006 10:23 pm

Quote:

There is an option called "INVERT" that will allow your PIC to talk
directly with a generic COM port (in this case on the PC) without
any true RS-232 level convertors.

If you do this, you should put a resistor between the Tx pin of the
DB-9 on the PC (pin 3) and the Rx pin on your PIC. That's because
the RS232 driver in your PC puts out levels of about +/- 10v.
Your PIC has clamp diodes on the i/o pins to protect against voltages
that go below ground or above Vdd. The resistor will limit the amount
of current going through the input clamp diodes on the PIC to a
safe level, so they don't burn up. If they do burn up, at a minimum
the PIC input will likely be destroyed.

Look at the schematic on this page:
http://www.rentron.com/PIC16F84.htm
(Scroll down a bit to see it)
Notice the 22K resistor between the RS232 connector on the PIC and
the Rx pin (RA4 in this case) on the PIC.

UFAnders

Joined: 13 Apr 2005
Posts: 36
Location: Michigan


Posted: Sun Apr 09, 2006 10:31 pm

Yikes! You learn something new everyday - Guess I'll be putting a few resistors in my boards...

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