SIGNAL(UART0_TRANSMIT_INTERRUPT)
/*************************************************************************
Function: UART Data Register Empty interrupt
Purpose:  called when the UART is ready to transmit the next byte
**************************************************************************/
{
    unsigned char tmptail;

    
    if ( UART_TxHead != UART_TxTail) {
        /* calculate and store new buffer index */
        tmptail = (UART_TxTail + 1) & UART_TX_BUFFER_MASK;
        UART_TxTail = tmptail;
        /* get one byte from buffer and write it to UART */
        UART0_DATA = UART_TxBuf[tmptail];  /* start transmission */
    }else{
        /* tx buffer empty, disable UDRE interrupt */
        UART0_CONTROL &= ~_BV(UART0_UDRIE);
    }
}


void uart_putc(unsigned char data)
{
    unsigned char tmphead;

    
    tmphead  = (UART_TxHead + 1) & UART_TX_BUFFER_MASK;
    
    while ( tmphead == UART_TxTail ){
        ;/* wait for free space in buffer */
    }
    
    UART_TxBuf[tmphead] = data;
    UART_TxHead = tmphead;

    /* enable UDRE interrupt */
    UART0_CONTROL    |= _BV(UART0_UDRIE);

}/* uart_putc */