[En-Nut-Discussion] Proposal to change baudrate setting method using U2Xn

[Henrik Maier]

Hello fellow Developers,

I am currently working on the port to AT90CAN128. Usage of the CAN 
controller requires the change of the MCU crystal to either 12MHz or 
16MHz, otherwise standard CAN baud rates cannot be met.

These two clock speeds have the drawback that the accuracy of the 115 
KBit speed is less than 3% if ones does not use the "Double the USART 
Transmission Speed" feature of ATmega128 and AT90CAN128. This feature is 
enabled by setting U2Xn to 1.

As this does not have any influence on the accuracy when using the 
standard clock speed of 14.756MHz, I suggest to change the baud rate 
configuration for all advanced AVR architectures in such a manner as to 
set U2Xn = 1.

If this proposal finds general acceptance I am willing to commit the 
code to the CVS.

As an example the code for setting the baud rate for debug0 would then 
look like:

static int DebugIOCtl(NUTDEVICE * dev, int req, void *conf)
{
     if(req == UART_SETSPEED) {
    /*
     * For hardware supporting doubling the USARTn transmission speed
     * we do this as it gives better accuracy for 12MHz and 16MHz systems.
     */
#ifdef __AVR_ENHANCED__
         u_short sv;

         sv = (u_short) ((((2UL * NutGetCpuClock()) / (*((u_long *)conf) 
* 8UL)) + 1UL) / 2UL) - 1;
         sbi(UCSR0A, U2X0); /* double UART clock speed */
         UBRR0L = (u_char) sv;
         UBRR0H = (u_char) (sv >> 8);
#else
         outb(UBRR, (u_char) ((((2UL * NutGetCpuClock()) / (*((u_long 
*)conf) * 16UL)) + 1UL) / 2UL) - 1UL);
#endif
         return 0;
     }
     return -1;
}


Regards

Henrik Maier