Re: [PATCH] cpm2: Round the baud-rate clock divider to the nearest integer.

[Laurent Pinchart <laurentp@cse-semaphore.com>]

Hi Scott,

On Thursday 20 November 2008, Scott Wood wrote:
> On Thu, Nov 20, 2008 at 01:32:23PM +0100, Laurent Pinchart wrote:
> > Instead of rounding the divider down, improve the baud-rate generators
> > accuracy by rounding to the nearest integer.
> >
> > Signed-off-by: Laurent Pinchart <laurentp@cse-semaphore.com>
>
> This makes things worse on mpc8272ads -- the result of the division is
> 13.56, but I only get error-free serial output when rounding down.  I
> don't think the remote end has timing problems, as I don't see this with
> any other board.

I get exactly the opposite here with an mpc8248 base board. Rounding down 
makes 115200bds completely unreliable :-/

Could you measure the serial baudrate with and without the patch using an 
oscilloscope ?

> Possibly the crystal is a little off,

That's always possible.

> or maybe it's just better to be a little too fast than a little too slow for
> some reason? 
>
> Maybe we should change CPM_CLK to be MAIN_CLK/4 rather than MAIN_CLK/16
> when the BRG clock is below a certain frequency.

Isn't that the job of the boot loader ? Accuracy would be improved, but power 
consumption will raise as well :-/

Best regards,

Laurent Pinchart