Re: [avr-gcc-list] interrupt-driven buffered UART receiver

[Michael Jin]

Lorne Gutz wrote:
> On Tuesday 11 March 2003 06:20, Olaf Zenker wrote:
>
> > address@hidden
>
>
>
> Hi folks,
>
>      To date I have not seen any driver written in C.
>
> I will send some that I have used in the past.   This
> code has been written to produce small code, and
> should be compiled with -Os.
>
> cheers
> Lorne
[snip]
> // UART Transmitter buffer
> static uint8_t      tx_buff[16];
> static uint8_t      tx_wr_index,tx_rd_index;
> volatile uint8_t    tx_counter;
>
> // UART Transmitter interrupt service routine
> SIGNAL( SIG_UART_DATA )
> {
>     uint8_t     tx_buffer_size = 16;
>
>     if( tx_counter != 0 ){
>         tx_counter--;
>
>         tx_wr_index++;
>         if( tx_wr_index > tx_buffer_size )
>             tx_wr_index = 0;
>
>         outp( tx_buff[ tx_wr_index ], UDR );
>
>     }
> }
>
>
> // Write a character to the UART Transmitter buffer
>
> void    putchar( uint8_t c )
> {
>     uint8_t     tx_buffer_size = 16;
>
>     while( tx_counter >= tx_buffer_size );
>
>     if( ( tx_counter == 0 ) && ( bit_is_clear( USR, UDRE )))
>         outp( c, UDR );
>
>     else{
>
>         tx_counter++;
>         if( tx_rd_index >= tx_buffer_size )
>             tx_rd_index = 0;
>
>         tx_buff[ tx_rd_index ] = c;
>
>         cli();
>         tx_counter++;
>         sei();
>     }
> }
I don't quite understand this piece of code, can you explain why the 
tx_rd_index never increased while the tx_counter did twice in the above 
function please?
TIA.

Michael