Matt,

You are right, I must disable TX interrupts (I did) AND check TXIF flag
last because it will be almost always set.

Joan

> -----Mensaje original-----
> De: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]En nombre de Matt Pobursky
> Enviado el: viernes, 03 de mayo de 2002 2:02
> Para: PICLIST@MITVMA.MIT.EDU
> Asunto: Re: [PIC]: Weird 16F877 USART behaviour
>
>
> On Thu, 2 May 2002 23:37:28 +0200, Joan Ilari wrote:
> >Hi, Piclisters !
> >
> >1- Introduction :
> >
> >I am developing a project where a 16F877 has to communicate with
> >an external processor through a serial line by means of its
> >USART. The TX and Rx routines are interrupt-driven.
> >
> >2- The (checked) fact :
> >
> >If in the interrupt routine I check TXIF flag before RCIF, when
> >a byte is received, I find that : TXIF is set => TX routine is
> >called instead of RX one => RCREG register is not read => RX
> >interrupt is not reset => interrupt program is continuously
> >called => watchdog fires, etc ... etc...
> >
>
> The TXIF flag is a transmitter empty flag. It's set any time the
> transmitter register is empty, so it will be set most of the
> time. You need to disable TX interrupts until you load the first
> character to be sent into the TXREG. Once you write the TXREG,
> the TXIF flag will be cleared automatically. Then enable your TX
> interrupt, when the character is sent the TXIF flag will be set
> again and your TX interrupt will occur. After your last character
> to be sent is loaded into the TXREG, disable TX interrupts so you
> won't be interrupted after the last character is sent.
>
> This one bites a lot of people.
>
> Matt Pobursky
> Maximum Performance Systems
>
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