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RS232 interrupt problem

 
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rhaguiuda



Joined: 05 Sep 2007
Posts: 46
Location: Londrina - Brazil

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RS232 interrupt problem
PostPosted: Thu Nov 20, 2008 5:00 am     Reply with quote
Hi.

I'm using a PIC18LF1220, internal oscillator in 8MHz and PCH version 4.057.

My very simple program only blinks a LED at a pin, and it is configured to interrupt on RS232 reception. The problem is the PIC when running only keeps interrupting, even with nothing connected to PIN_B4 (RX).

Below is the code I'm using.

Code:

#include <18F1220.h>
#fuses HS, NOWDT, PUT, NOMCLR
#use delay(internal=8M)
#use rs232(baud=9600, xmit=PIN_B1, rcv=PIN_B4, errors)

#define Led             PIN_B0

#INT_RDA
void rda_isr()
{
   printf ("Interrupt");
}

void main()
{
   enable_interrupts (int_rda);
   enable_interrupts (global);

  do
  {
      output_high (Led);
      delay_ms (1000);
      output_low (Led);
      delay_ms (1000);
  }
   while (1);
}

}


When this program is running, it only prints "Interrupt", and never blinks the led. So it always interrupted by RS232 rx.
Why my PIC only gets interrupted even with nothing connected to usart pins?

Thanks for any help.
Ttelmah
Guest
PostPosted: Thu Nov 20, 2008 5:19 am     Reply with quote
The 'idle' state for TTL RS232, is with the signal _high_ (5v). A disconnected pin, is probably floating _low_. This is seen as continuous RS232 data.
Then, the second reason, is your interrupt code. Once even one character is seen, it'll trigger for ever. When the RS232 interrupt occurs, it is saying 'I have a character that _needs_ reception'. Your routine does not retrieve the character, so it'll trigger again (and again, and again......).
So, do two things:
1) Add something to pull the RX pin hgh (a 100K resistor to +5v, or connect th line to the output of a MAX232 which will idle high - however you will still probably see one garbage character, as the chip wakes up).
2) Add a 'getc' to your interrupt.

Best Wishes
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