Re: [avr-gcc-list] Problems with ATMEGA8 USART

[David Kelly]

On Aug 16, 2005, at 2:48 AM, Nigel Winterbottom wrote:

> -----Original Message-----
>
> The one clumsy part of code was in my_putchar() which had to reach
> directly into the hardware for the specific port to restart the Tx
> IRQ if the buffer had been empty, which causes my Tx IRQ routine to
> shut itself off. Also there was a possible race condition.
>
> --
> David Kelly N4HHE, address@hidden
> ====================================================================== 
> ==
> Whom computers would destroy, they must first drive mad.
>
> You've missed a trick, restarting USART comms is a simple as re- 
> enabling
> the interrupt which is safe to do as many times as you like and  
> whenever
> you like.

Thats not quite what I meant. I do enable the IRQ for each character.  
The issue was in finding which UART needed to be enabled without  
enabling one which would then serve an empty event. Here is the exact  
code I wrote back in December. Today I'd probably bracket the  
insertion and masking with cli()/sei().

int
uputc( int c, Uart *fp )
{
        //  We don't inc tx_head until after storing c and
        //  possibly wrapping tx_head to prevent the IRQ
        //  routine from seeing the character until after wrap.
        //  fp->tx_head++ is tempting.

        fp->tbuf[fp->tx_head] = c;

        //  Increment and wrap using a mask
        fp->tx_head = (fp->tx_head + 1) & (TX_MAX_SIZE-1);

        //  If the service routine delivers the char above and
        //  clears UDRIE before we make sure its set below then
        //  below will cause an IRQ with tbuf empty. The service
        //  routine will disable UDRIE again, no harm done.

        //  This is a tacky hack to find the right UART.
        if( fp == &UART0 )
                UCSR0B |= (1<<UDRIE);   // ensure IRQ is enabled
        else
                UCSR1B |= (1<<UDRIE1);  // ensure IRQ is enabled

        return(c);
}


--
David Kelly N4HHE, address@hidden
========================================================================
Whom computers would destroy, they must first drive mad.