How to clear the buffer TX in #use rs232

[pilar]

Hi, I'm making a program to communicate the UART of Pic18f452 with four ports RS232 Multiplexed. The problem is that when switching from channel to channel and sending a control word garbage characters are introduced, but when I send the same command for the second time all are fine. I imagine that the problem is in the transmission buffer of the UART. How I can clear this buffer?

[FvM]

Clearing of TX buf may be necessary, if you want to switch while a character is transmitted. This would cause garbage at one peer anyway.

I rather guess, that you don't set a correct idle level or cause glitches during switchover.

[pilar]

Precisely to avoid the glitches during switchover, I introduced a delay(5000) and disable_interrupts(GLOBAL) and disable_interrupts(INT_RDA);

[Alan]

Had a similar problem, you have to wait for the last character to be out of the TX buffer before disabling the int. Maybe wait a second after putting the last character in the buffer and then disable

[ttelmah]

This is what the TRMT bit is for.....

Basically, the transmit system, has up to two characters of buffering. There is the transmit register, and the output shift register. You get the TXIF interrupt, when the transmit register empties, but there is still potentially the 'just transferred' character in the shift register, still being sent.
So, when performing serial switching, you have to disable the transmit interrupt, on the last character, then wait for the TRMT bit to go high, and say that the shift register is empty, before turning the bus around.
Since you are the person loading the buffer, presumably you do want the character to be sent. Otherwise, don't load it!.....

Best Wishes

[pilar]

[dyeatman]

Technically yes, your code should work. However, two issues need to be considered:

1. When you disable INT_RDA for the receive in the middle of a reception characters will be lost.

2. In place of the delay, if you enter a loop and poll the TRMT bit as Ttelmah suggests you don't need the 2 second delay and can turn around the switchover in as little as 10ms or less.
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[pilar]

Please, could you give a example code ??

[pilar]

[asmboy]

I actually suspect the circuitry that you are using to "switch" the output.

- hint each switched output needs a pullup or other means to assure that there is no accidental negative glitch during transitions.

The normal idle state of the PIC TX pin is HIGH and is pulled DOWN to
output chars - if your switching circuit does not handle this seamlessly - you will always generate glitches in the stream.

Care to post a link to your SCHEMATIC for the switch ??