Re: [avr-gcc-list] UART transmitting problem

[Neil Johnson]

Hi,

On Fri, 28 Nov 2003, Anderas Strodl wrote:

> Laurence Anderson wrote:
> > #define UART_BAUD_SELECT (F_CPU/(UART_BAUD_RATE*16l)-1)
>                                                     ^
> Are you sure this is correct? As far as I see it should be:
>   #define UART_BAUD_SELECT (F_CPU/(UART_BAUD_RATE*16)-1)

The "l" tells the preprocessor to treat the 16 as a long int, rather than
just a plain int.  This then forces the type of the result of the
multiplication to a long int, and then the type of the divide as long int.
Hopefully this should prevent overflows.

I'd like to suggest a slight change in the above macro (note: this is
untested at the moment).  When doing integer divide it is usually better
to round to the nearest integer, rather than to the lowest (which the
above expression does).

A better expression would add 0.5 (i.e. half the value of the denominator)
to the numerator before the division.  This can be achieved in the
following way:

#define UART_BAUD_SELECT ((F_CPU + UART_BAUD_RATE * 8L) /  \
                             (UART_BAUD_RATE * 16L)     - 1)

Thus a numerical result of, say, 12.7 would be rounded up to 13, while
12.2 would be rounded down to 12.

Anyone care to give this a try?  I don't have my dev. tools to hand at the
moment, so am unable to verify this myself (althought have written such
code many times in the past).

Comments?

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
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