Re: [avr-chat] ATMega UART question

[Preston Wilson]

First, I have to take back what I said about not seeing a zero byte
transmitted.  I simply may not have noticed that behavior.  In one project I
have framing around the data I send and receive, and I just ignore data
received outside of a frame.  In the another project, the uart is
interacting with a modem, and at startup a zero byte will not cause any
problems.  It could be happening, and I just did not notice.

Neither project using these routines is on my bench, or I would check to see
if I get the zero byte on init.  If I get time during the holidays, I will
check it out.

> This is the code I have:
> UBRRL = (XTAL / (16L * BAUDRATE)) - 1;      //set baud rate;

Does that work?  I am not looking at the datasheet, but here is how I setup
the baudrate:

In my case, baudrate is assigned like you have the UBBRL assignment when
U2X0 is not used.  Swap out the 16L for an 8L when enabling U2X0.

UART_U2X is #defined to 0x02
flags is of type uint8_t

if (flags & UART_U2X) {
  UCSR0A = (1<<U2X0);
}

UBRR0H = (uint8_t)(baudrate >> 8);
UBRR0L = (uint8_t)(baudrate & 0xff);

followed by the code I gave before.

You will have a problem if the upper bits of baudrate are not zero.

> UCSRB = 0x18;                     // Rx enable Tx Enable
> UCSRC = 0x06;                     // Asyn,NoParity,1StopBit,8Bit
> 
> 
> As soon as the RX, Tx enable line executes I see a character sent, the PC
> listening to that UART also receives a byte of zeroes,


-Preston